CH-26 Amniote Origins and Nonavian Reptiles – Integrated Principles of Zoology : Chapter Notes

This document contains Chapters 26 to 27 CHAPTER 26 AMNIOTE ORIGINS AND NONAVIAN REPTILES CHAPTER OUTLINE 26.1. Diversity A. Enclosing the Pond 1. Any animal with a shell-less egg remains tied to water. 2. The development of a shelled egg freed the reptilian groups to exploit land. 3. Extraembryonic membranes from previous evolutionary aquatic stages are maintained. 4. The allantois serves as a respiratory surface and a chamber to store nitrogenous wastes. 5. The chorion allows oxygen and carbon dioxide to freely pass. 6. Surrounding the whole young organism is a porous, parchment-like or leathery shell. B. Amniotes 1. Amniota is a single, monophyletic lineage of Paleozoic tetrapods with this developmental pattern. 2. Before the end of the Paleozoic Era, amniotes diverged into nonavian reptiles, birds and mammals. C. Diversity 1. Nearly 9500 species of paraphyletic class Reptilia; ~ 320 occur in the United States and Canada. 3. The Age of Reptiles lasted over 165 million years and included the dinosaurs. 4. Modern reptiles represent surviving lineages of a mass extinction at the end of the Mesozoic Era. 5. The tuatara is the sole survivor of a group that otherwise disappeared 100 million years ago. 6. Lizards and snakes radiated into diverse and abundant groups. 7. Ichthyosaurs, represented by extinct, aquatic dolphin-like forms comprise the fourth subgroup. 26.2. Origin and Early Evolution of Amniotes (Figures 26.1, 26.2) A. History 1. Amniotes arose from amphibian-like tetrapods, the anthracosaurs, during the Carboniferous. 2. By the late Carboniferous, amniotes had separated into three patterns of holes (fenestra) in the temporal region of the skull. a. Anapsids have a skull with no temporal opening behind the orbits.. b. Diapsids gave rise to all other traditional “reptiles” (except turtles) and to the birds. 1) The diapsid skull has two temporal openings; one below each cheeks and another above. 2) Lepidosaurs include ichthyosaurs and living reptiles, including lizards and snakes. 3) The more derived archosaurs included dinosaurs, living crocodilians, and birds. 4) Sauropterygians included extinct aquatic groups including the long-necked plesiosaurs. c. Synapsids are mammal-like reptiles with a single pair of temporal openings. 3. These openings are occupied by large muscles that elevate the lower jaw. a. Changes in jaw musculature might reflect a shift from suction feeding in aquatic vertebrates to terrestrial feeding that required larger muscles to produce static pressure. b. Amniotes exhibit more variation in feeding biology than ananmiotes and herbivory is common in amniote lineages. c. Functional significance of the evolution of temporal openings in amniotes isn’t fully understood. B. Adaptations of Amniotes 1. Amniotic egg (Figure 26.3) a. All amniotes have eggs with four extraembryonic membranes: the amnion, allantois, chorion, and yolk sac. 1) The amnion encloses the embryo in fluid, cushioning the embryo and providing an aqueous medium for growth. 2) The allantois stores metabolic wastes. 3) The chorion surrounds the contents of the egg and is highly vascularized. 4) The allantois and chorion form an efficient respiratory organ. 5) The yolk sac functions in nutrient storage. b. A possible advantage of the amniotic egg is that it permitted development of a larger, faster- growing embryo. c. One hypothesis suggests that a first step in the evolution of the amniotic egg was replacement of the jelly layer with a shell, which provided better support and movement of oxygen; the shell could also be broken down to provide calcium to growing skeletal structures. d. All amniotes lack gilled larvae and have internal fertilization; these characteristics eliminated the need for aquatic environments. e. A penis is the most common copulatory organ and is derived from the cloacal wall, appears to be another amniote innovation. 2. Thicker and more waterproof skin (Figure 26.4) a. Amniote skin is thick and tends to be more keratinized and less permeable to water. b. A variety of structures composed of keratin such as scales, hair, feathers, and claws project from the skin. c. Keratin protects the skin from physical trauma, and lipids prevent water loss. d. Few amniotes use their skin as a primary respiratory organ because keratin and lipids limit exchange of gases. e. Amphibians’ thin, moist skin permits gas exchange; however, this skin also makes them vulnerable to dehydration. f. The skin of reptiles has an epidermis of varying thickness and a thick, collagen-rich dermis. g. The dermis has chromatophores that are color-bearing cells that give many lizards and snakes their colorful hues. h. Resistance to desiccation is provided primarily by hydrophobic lipids in the epidermis. i. The scales of nonavian reptiles are formed mostly of beta keratin and provide protection against wear in terrestrial environments. j. In crocodilians, scales remain throughout life; in other nonavian reptiles such as lizards and snakes, new keratinized epidermis replaces the old epidermis which is shed. k. Turtles have scutes which are platelike modified scales. 3. Rib ventilation of the lungs a. Amniotes draw air into their lungs (aspiration) by expanding the thoracic cavity using costal muscles or pulling the liver posterior. b. The shift to negative ventilation probably influenced the evolution of amniotic limbs. c. Early tetrapods used their rib muscles to make lateral undulations causing a wriggling motion. 4. Stronger jaws a. Unlike fish that utilize suction-feeding, early tetrapods had expanded jaw musculature as well as a muscular tongue for mastication and swallowing. 5. High-pressure cardiovascular systems a. In amniotes, the right atrium is partitioned from the left atrium; in some, the right ventricle is partitioned from the left ventricle. b. There is either limited, or no mixing of oxygen-rich and oxygen-poor blood within the heart. Consequently, the systemic blood flow is considerably higher in amniotes. 6. Water-conserving nitrogen excretion a. In contrast to ammonia, which is toxic and must be eliminated with considerable water, many amniotes remove nitrogenous wastes as either urea or uric acid, which require more energy to produce but do not need to be dissolved in water. b. In some amniotes, excess salts can be removed through other structures such as the nose, eyes, or tongue. 7. Expanded brain and sensory organs a. Both the cerebrum and cerebellum are larger in amniotes. b. Many amniotes have excellent senses of vision and olfaction C. Changes in Traditional Classification of Reptiles (Figures 26.1, 26.2) 1. Crocodilians and birds are sister groups because they are more recently descended from a common ancestor. 2. Clade Archosauria includes the birds, crocodilians, and the extinct dinosaurs and pterosaurs. 3. Archosaurs along with their sister group the lepidosaurs (lizards and snakes), and turtles form a monophyletic groups that cladists call Reptilia. 4. The term “nonavian reptiles” refers to a paraphyletic group that includes the living turtles, lizards, snakes, tuataras, and crocodilians, and a number of extinct groups including plesiosaurs, ichthyosaurs, pterosaurs, and dinosaurs. 26.3. Characteristics and Natural History of Reptilian Orders A. Order Testudines (Chelonia): Turtles (Figures 26.6–26.11) 1. Fossils appear in the Upper Triassic, 220 million years ago, and they have occurred ever since. 2. The shell has an outer horny layer of keratin and an inner layer of bone, and consists of the dorsal carapace and the ventral plastron. 3. The carapace of the turtle shell is partly formed from ribs that enclose the scapula (Figures 26.7, 26.8) 4. The bony layer is a fusion of ribs, vertebrae, and dermally-ossified elements. 5. Unique among vertebrates, the limbs and limb girdles are located deep to the ribs. 6. Turtles lack teeth and have tough, horny plates for gripping food. 7. Breathing a. One consequence of having a rigid shell is that the turtle cannot expand its chest to breathe. b. Turtles solve this problem by using abdominal and pectoral muscles as a “diaphragm.” c. Air is drawn in by contraction of the limb flank muscles, increasing abdominal cavity volume. d. Exhalation is accomplished by drawing back the shoulder girdle to compress the viscera. e. These actions are visible as bellows-like movements at the turtle’s “limb pockets.” f. Movement of limbs during walking also helps ventilate the lungs. g. Some aquatic turtles gain sufficient oxygen by pumping water in and out of the mouth cavity. 8. Nervous System and Senses a. The turtle brain is small, less than one percent of its body weight. b. The cerebrum is larger than that of amphibians; some turtles can learn a path through a maze. c. Turtles have a middle and an inner ear but sound perception is poor; they make few sounds aside from those made during mating. d. Turtles have a good sense of smell, acute vision, and color perception about equal to humans. 9. Reproduction and Development (Figures 26.9, 26.10) a. Turtles are oviparous; fertilization is internal and all turtles bury their eggs in the ground. b. Once the female lays her eggs in a nest, she deserts them. c. In some turtle families, as in crocodilians and some lizards, nest temperature determines the sex of hatchlings; low temperatures produce males and high temperatures produce females. 10. Giant Turtles and Tortoises a. Buoyed by the water, marine turtles may reach two meters long and 725 kilograms in weight (Leatherback turtles). b. Giant land tortoises, such as those on the Galápagos Islands, weigh several hundred kilograms. c. Low metabolic activity may explain their longevity, believed to exceed 150 years. 11. Box Tortoises a. The shell is an effective coat of armor. b. The box turtle has a plastron that is hinged; it pulls the plastron up to fully enclose the body. 12. Snapping Turtles (Figure 26.6) a. These turtles have a reduced shell that does not permit full withdrawal of the body. b. Their jaws are adequate defense; the alligator snapper even has a worm-like bait in its mouth. c. They are entirely carnivorous and can eat fish, frogs, waterfowl, etc. d. Snappers are wholly aquatic but must come ashore to lay eggs. B. Order Squamata: Lizards and Snakes (Figures 26.13-26.27) 1. Diversity a. Squamates are the most recent and diverse of diaspids; comprising 95% of living reptiles. b. Lizards appeared in the fossil record in the Permian but did not radiate until the Cretaceous. c. Snakes appeared during late Cretaceous from group whose descendants include monitor lizards. d. Snakes gained specializations for losing their legs and therefore for engulfing large prey. e. Diapsid skulls have lost dermal bone ventral and posterior to the lower temporal opening. f. This allowed evolution in lizards of a mobile skull with movable joints, a kinetic skull. g. The quadrate, fused to the skull in other nonavian reptiles, has a joint at the dorsal end and articulates with the lower jaw. h. Joints in the palate and across the roof of the skull allow the snout to be tilted up. i. This allows squamates to seize and manipulate prey, and effectively close the jaw with force. j. Exceptional skull mobility of snakes is considered a major factor in their diversification. k. Viviparity 1) Viviparity in reptiles is limited to squamates. 2) It has evolved at least 100 separate times. 3) It is associated with cold climates. 4) Viviparity involves increasing the length of time eggs are kept in the oviduct. 5) Developing young respire through extraembryonic membranes. 6) Young obtain nutrition from yolk sacs or via the mother, or a combination of both. 2. Suborder Sauria: Lizards (Figures 26.14 − 26.19) a. Lizards are a diverse group with terrestrial, burrowing, aquatic, arboreal, and some aerial members. b. Geckos are small, agile, nocturnal forms; adhesive toe pads allow them to walk on ceilings. c. Iguanids include many New World lizards as well as the marine iguana of the Galápagos. d. Chameleons are arboreal lizards of Africa and Madagascar; many have an extendible tongue. e. Some lizards have degenerate limbs; the glass lizards are nearly limbless. f. Lizards have movable eyelids whereas snakes have a transparent covering. g. Nocturnal geckos have retinas with only rods; day-active lizards have both rods and cones. h. Lizards have an external ear that snakes lack. i. Geckos use vocal signals to announce territory and drive away males. j. Some lizards survive well in hot and dry regions. k. Lizards conserve water by producing semisolid urine with a high content of crystalline uric acid. The also possess lipids within the skin that minimize water loss. l. Some lizards can store fat in their tails to provide energy and metabolic water during drought. m. The Gila monster and beaded lizard are the only lizards capable of a venomous bite. n. Lizards keep their body temperature relatively constant by behavioral thermoregulation, although they are ectotherms. o. Ectothermy is a successful strategy in ecosystems with low productivity and warm climates, such as tropical deserts and grasslands. p. The amphisbaenians or “worm lizards” are lizards highly specialized for a fossorial (burrowing life). (Figure 26.19) 1) Until recently, they were placed in a separate suborder, Amphisbaenia because they appeared to be so different from other lizards. 2) Morphological and molecular data show they are highly modified lizards. 3) Amphisbaenians have elongate, cylindrical bodies of nearly uniform diameter. 4) They lack any trace of limbs. 5) Eyes are usually hidden below the skin and there are no external ear openings. 6) Their skull is conical or spade-shaped to assist in tunneling. 7) The skin is formed in independently-moving rings which can grip the soil; this creates a movement similar to earthworms. 8) They are found in South America and tropical Africa; in the U.S., one species occurs in Florida called the “graveyard snake.” 3. Suborder Serpentes: Snakes (Figures 26.20–26.27) a. Snakes are limbless and have lost the pectoral and pelvic girdles (except in pythons). b. The many vertebrae are shorter and wider than in other tetrapods, allowing undulation. c. Elevation of the neural spine gives the musculature more leverage. d. The feeding apparatus allows them to eat prey several times their own diameter. e. The two halves of the lower jaw are loosely joined, allowing them to spread apart. f. The skull bones also are loosely articulated so the mouth can accommodate a large prey. g. To keep breathing during the slow process of swallowing, the tracheal opening is extended. h. Eyeballs have reduced mobility and a permanent corneal membrane for protection. i. Most have poor vision, but arboreal snakes in tropical forests have highly developed vision. j. Snakes lack external ears and do not respond to most aerial sounds. k. Snakes can feel vibrations at low frequencies, especially vibrations carried in the ground. l. Chemical senses rather than vision or hearing are the main senses snakes use to hunt prey. m. Jacobson’s organs are a pair of pits in the roof of the mouth; they are lined with olfactory epithelium and the forked tongue picks up scent particles and conveys them past this organ. n. A snake’s skin is infolded between scales; when stretched by a big meal, the skin is unfolded. o. Snake Locomotion (Fig. 26.24) 1) Lateral undulation: S-shaped movement that pushes against rough ground and water. 2) Concertina movement: extension of S-shaped loops to strike or to climb trees. 3) Rectilinear movement: straight movement using minute lifting of consecutive ribs. 4) Sidewinding: sideways looping by desert vipers that “walks” them across loose sand. p. Pit vipers, such as rattlesnakes, have “pits” with nerve endings sensitive to heat emitted by warm-bodied birds and mammals. q. Viper fangs are hollow and hinged to inject venom when the snake strikes. r. Of an average of 7,000 snake bites each year in the U.S., only about 5 result in death. s. Nonvenomous snakes kill prey by constriction or by biting and swallowing. t. Venomous Snakes 1) Family Viperidae includes New World and Old World vipers with and without pits. 2) Family Elapidae includes cobras, mambas, coral snakes, and kraits. 3) Family Hydrophiidae includes the specialized sea snakes. 4) Most members of the family Colubridae are non-venomous; several including the African boomslang and African twig snake are rear-fanged and their bite can be fatal to humans. u. Snakebite and Toxicity 1) Saliva of harmless snakes contains limited toxins; this provided a basis for natural selection of venom. 2) Most snake venoms are a complex combination of venom types. 3) Neurotoxins act on the nervous system, causing blindness or stopping respiration. 4) Hemorrhagin type venoms broke down blood vessels; blood is leaked into tissue spaces. 5) Toxicity is measured by the median lethal dose on laboratory animals, called the LD50. 6) Sea snakes and the Australian tiger snake have the most deadly venom per unit. 7) Large venomous snakes deliver more venom; a king cobra may be the most dangerous. 8) In India and Burma, dense populations and poor footwear contribute to 200,000 bites/year. 9) Approximately 7,000 snakebites/year in United States, resulting in only about 5 deaths. 10) The world total for deaths from snakebite is about 50,000 to 60,000 each year, with most deaths occurring in India, Pakistan, Myanmar, and nearby countries where poorly shod people frequently come into contact with venomous snakes or do not get immediate medical attention once bit. 11) Less than 20 percent of all snakes are venomous, although venomous species outnumber nonvenomous species by 4 to 1 in Australia. v. Reproduction 1) Most snakes are oviparous and lay shelled eggs under logs or rocks or in holes in the ground. 2) Others, including pit vipers, are ovoviviparous. 3) A few snakes are viviparous, having a primitive placenta to exchange nutrients with the young. 4) Snakes can store sperm and lay several clutches of fertile eggs long after a single mating. 4. Order Sphenodonta: The Tuatara (Figure 26.28) a. Only two living species in New Zealand represent this ancient lineage. b. Sphenodontids radiated modestly in the early Mesozoic but then declined. c. Once widespread across New Zealand, the two species are now restricted to small islands. d. Loss of the tuatara populations was caused by human introduction of nonnative species which preyed upon the tuatara; the tuatara are vulnerable because they have slow growth and reproductive rates. e. They are lizard-like and live in burrows often shared with petrels. f. The tuatara is slow growing and may live very long (one captive male lived to 114 years old). g. Its skull is nearly identical to diapsid skulls of 200 million years ago. h. It also has a well-developed median parietal eye (a “third eye”) buried beneath its skin. i. Sphenodon represents one of the slowest rates of evolution known among vertebrates. 5. Order Crocodilia: Crocodiles, Alligators, Caimans, and Gharials (Figure 26.29) a. Modern crocodilians are the only surviving reptiles of the archosaurian lineage. b. This clade gave rise to the Mesozoic radiation of dinosaurs and to birds. c. Modern crocodilians differ little from primitive crocodilians of the early Mesozoic. d. Modern crocodilians are classified in three families. 1) Alligators and caimans are found primarily in the New World; they have a broader snout. 2) Crocodiles are widely distributed and include the huge saltwater crocodile. 3) One species of gavial occurs in India and Burma; it has a very narrow snout. e. All have a long, well-reinforced skull and jaw musculature for a powerful bite. f. A type of dentition called thecodont, where teeth are set in sockets, was typical of archosaurs. g. They have a complete secondary palate, a feature only shared with mammals. h. They also share a four-chambered heart with birds and mammals. i. The estuarine crocodile in southern Asia and the Nile crocodile are both very large. j. Alligators and crocodiles are oviparous; usually 20–50 eggs are laid in a mass of vegetation. k. Nests left unguarded are easily discovered and raided by predators. l. High nest temperatures produce males; low temperatures produce females and can result in females outnumbering males five to one. C. Classification Subclass Anapsida Order Captorhinida Subclass Diapsida Order Testudines Superorder Lepidosauria Order Squamata Suborder Lacertilia Suborder Serpentes Order Sphenodonta Superorder Ichthyosauria Superorder Sauropterygia Order Plesiosauria Superorder Archosauria Order Crocodilia Order Pterosauria Order Saurischia Suborder Sauropodomorpha Suborder Theropoda Order Ornithischia Subclass Synapsida Order Pelycosauria Order Therapsida Lecture Enrichment 1. Recent fossil discoveries, especially in China, have placed dinosaurs and early evolution of birds and mammals on the front pages of newspapers, National Geographic, etc. These occur often enough to provide a “current event” connection virtually every semester. 2. Dinosaurs are particularly fascinating for most students at all ages. Visuals are particularly important for the more unique features and dinosaurs themselves. Note that many visuals are heavily theory-laden in their interpretation of endothermy, thermoregulation, etc. and may convey more detail than the fossil evidence supports. 3. Reptilian evolution, including the origin of birds, is a particularly volatile topic and university authorities are currently in hot dispute over the degree of endothermy, glide-down versus running origins of bird flight, origin of feathers for insulation before flight, etc. This is a prime academic arena to illustrate the role of intellectual argument and competing theories. These topics show that science, far from being cold and objective, is laden with emotion and subjective interpretations. 4. The methods of snake locomotion, except for sidewinding, are readily demonstrated by a tame kingsnake. This can be demonstrated in class. If the floor is well waxed, the lateral undulatory movement will not advance the animal until you place a finger behind a loop for it to “push off.” It will use rectilinear movement to cross over your finger or arm, and perhaps concertina out to another hand. Commentary/Lesson Plan Background: While the backgrounds of students may vary widely, and many students from rural areas will have firsthand experience with reptiles and especially snakes, most students will carry in overriding social attitudes of unmerited fear, “the only good snake is a dead snake,” etc. Nevertheless, the group is intrinsically fascinating and there may be ample opportunities to “play off” some students’ experiences for the benefit of others. Misconceptions: Most people think reptiles always lay eggs (oviparity); the lack of understanding of ovoviviparity likely gave rise to the myth that garter snakes swallow their young to protect them when it was observed that cutting a gravid female released its maturing young. Most students assume that all animals use XX-XY chromosomes for sex determination; there are many exceptions and the temperature factor of lizards is a major variant. Recent movies have portrayed dinosaurs from a Jurassic time that were actually from other time periods; the brief but careful summary of dinosaur radiation in the text should help clarify which dinosaurs existed during the Triassic, Jurassic, and Cretaceous. Students often assume that at 50-50 ratio of males to females is “normal” and “best.” Note that only one male is necessary to fertilize a large number of females in herds of cattle, etc. The even ratio may reflect the species being “caught” in a mechanism that provides this sex ratio and does not necessarily need all of the excess portable sperm delivery units (males). Schedule: HOUR 1 26.1. Diversity A. Amniotic “Pond” B. Amniotes C. Diversity 26.2. Origin and Early Evolution of Amniotes A. History B. Derived Characters of Amniotes C. Changes in Traditional Classification of Reptiles 26.3. Characteristics of Nonavian Reptiles that distinguish them from Amphibians A. Lungs B. Skin C. Amniotic eggs D. Jaws E. Circulatory System Modifications F. Water Conservation G. Nervous System HOUR 2 26.4. Characteristics and Natural History of Reptilian Orders A. Order Testudines (Chelonia): Turtles B. Order Squamata: Lizards and Snakes C. Classification ADVANCED CLASS QUESTIONS: 1. Is it logical that birds would have evolved from pterodactyls? Why or why not? Answer:Birds did not evolve directly from pterodactyls. Pterodactyls, or more accurately, pterosaurs, were a group of flying reptiles that lived during the Mesozoic Era, alongside dinosaurs. Birds, on the other hand, are descendants of theropod dinosaurs, specifically a group known as maniraptoran dinosaurs. While both pterosaurs and birds were flying vertebrates, they evolved separately from different ancestors and belong to distinct evolutionary lineages. Here's why: 1. Anatomical Differences: Pterosaurs and birds have distinct anatomical features that indicate separate evolutionary origins. Pterosaurs had elongated fourth fingers that supported their wing membranes, while birds have a unique skeletal structure, including fused wrist bones (carpometacarpus) and a keeled sternum, adapted for powered flight. Additionally, birds possess feathers, a trait absent in pterosaurs. 2. Evolutionary Lineages: Pterosaurs belong to the archosaur lineage, which includes dinosaurs, crocodilians, and other reptiles. Birds, on the other hand, belong to the dinosaur lineage, specifically the clade Dinosauria, within which they are classified as avian dinosaurs. Both pterosaurs and birds evolved flight independently within their respective lineages. 3. Temporal Separation: Pterosaurs were dominant flying reptiles during the Mesozoic Era, particularly during the Jurassic and Cretaceous periods. Birds, on the other hand, evolved from theropod dinosaurs in the Late Jurassic period, after the divergence of pterosaurs. The earliest known bird, Archaeopteryx, lived around 150 million years ago, several million years after the decline of pterosaurs. 4. Ecological Niches: Pterosaurs occupied diverse ecological niches, including gliding, soaring, and powered flight, and ranged in size from small insectivores to large apex predators. Birds evolved to fill ecological niches previously occupied by non-avian dinosaurs, developing a wide range of adaptations for various lifestyles, including flight, terrestrial locomotion, and aquatic habits. In summary, while both pterosaurs and birds were capable of flight, they evolved from different ancestral lineages and possess distinct anatomical and evolutionary characteristics. Birds evolved from theropod dinosaurs, not from pterosaurs, and their evolutionary pathways are separate and distinct. 2. If both pit vipers and boa constrictors have heat receptor pits, why do we not consider this a feature that links them together for classification? Answer: The presence of heat-sensing pits in both pit vipers and boa constrictors is indeed a fascinating example of convergent evolution, where unrelated organisms develop similar traits independently in response to similar environmental pressures. While these heat-sensing pits serve a similar function in detecting infrared radiation emitted by warm-blooded prey, they are not considered a feature that links these two groups together for classification for several reasons: 1. Evolutionary History: Pit vipers belong to the family Viperidae, while boa constrictors belong to the family Boidae. These two groups are evolutionarily distinct and belong to different suborders within the order Squamata (the scaled reptiles). Pit vipers are members of the suborder Serpentes (snakes), while boa constrictors belong to the suborder Alethinophidia (advanced snakes). Despite both having heat-sensing pits, they evolved these structures independently. 2. Anatomical Differences: While both pit vipers and boa constrictors possess heat-sensing pits, there are significant anatomical differences between them. In pit vipers, these pits are located between the nostrils and the eyes, while in boa constrictors, they are located along the upper and lower jaws. Additionally, the morphology of the pits and their innervation may differ between the two groups. 3. Other Morphological and Molecular Traits: Classification of organisms takes into account a wide range of morphological, molecular, ecological, and behavioral traits, in addition to specialized sensory structures like heat-sensing pits. Pit vipers and boa constrictors exhibit numerous differences in these other traits, including body shape, scale patterns, reproductive biology, and venom composition (in the case of pit vipers). 4. Phylogenetic Relationships: Classification schemes aim to reflect the evolutionary relationships between organisms. Phylogenetic analyses, which reconstruct evolutionary relationships based on genetic data, morphology, and other evidence, place pit vipers and boa constrictors in separate branches of the snake evolutionary tree, indicating their independent evolutionary histories. In summary, while heat-sensing pits are a striking example of convergent evolution shared by pit vipers and boa constrictors, they are not sufficient to group these organisms together for classification. Classification considers a broader range of characteristics and reflects the evolutionary relationships between organisms, which in this case, indicates that pit vipers and boa constrictors are separate evolutionary lineages within the broader group of snakes. 3. Why is it not possible to merely make two types of antivenin to counteract the two general types of snake venoms? Why do we have to make antivenin fairly specific for different species or groups of snakes? Answer:Creating antivenom that targets specific species or groups of snakes is necessary due to the complex nature of snake venoms and the variability between species. Here are several reasons why: 1. Variability in Venom Composition: Snake venoms vary greatly in their composition, even among species within the same family or genus. Venoms can contain a mixture of enzymes, toxins, proteins, and other bioactive molecules, each with specific effects on the body. These components can differ in structure, function, potency, and immunogenicity, requiring tailored antivenoms to effectively neutralize their effects. 2. Cross-Reactivity: While some components of snake venoms may be similar across species or genera, there can also be significant differences. Attempting to create a broad-spectrum antivenom that targets multiple types of venoms may result in reduced efficacy due to inadequate neutralization of specific venom components. Additionally, there is a risk of cross-reactivity, where antibodies raised against one venom component may bind to similar components in other venoms, leading to adverse reactions or reduced efficacy. 3. Geographic Variation: Snake venoms can vary geographically, with different populations of the same species producing venoms with distinct compositions and effects. Antivenoms must be tailored to the venoms of specific snake populations found in particular regions to ensure effectiveness against the local snakebite cases. 4. Species-Specific Effects: Some snake species have venom components that are unique to them or have particularly potent effects on certain physiological systems. Developing species-specific antivenoms allows for targeted neutralization of these venom components, improving treatment outcomes for snakebites from those species. 5. Safety and Efficacy: Creating antivenom that is specific to the venoms of particular snake species or groups maximizes both safety and efficacy. By focusing on specific venoms, antivenom production can be optimized to ensure high neutralizing potency while minimizing the risk of adverse reactions or side effects associated with non-specific binding of antibodies to unrelated venom components. In summary, the complexity and variability of snake venoms necessitate the development of species-specific or group-specific antivenoms to ensure optimal efficacy and safety in treating snakebite envenomation. A one-size-fits-all approach would likely result in suboptimal treatment outcomes due to differences in venom composition, geographic variation, and species-specific effects. 4. Before the vaccine for polio was developed, researchers studied the effect of cobra venom in an attempt to find a cure for children with polio. Why? Answer:The study of cobra venom in relation to polio research was based on the theory that certain toxins, including those found in snake venoms, might have therapeutic properties against viral infections. Here's why researchers explored this avenue: 1. Antiviral Properties: Some components of snake venoms, particularly certain enzymes and proteins, were believed to possess antiviral properties. Researchers hypothesized that these compounds might be able to inhibit the replication of viruses, including the poliovirus, and thus potentially serve as therapeutic agents. 2. Limited Treatment Options: Before the development of the polio vaccine, there were few effective treatments for polio. Medical researchers were exploring various avenues to find potential treatments or cures for the disease, including investigating natural substances with potential antiviral properties. 3. Historical Context: In the early to mid-20th century, there was significant interest in the therapeutic potential of natural products, including plant extracts, animal venoms, and other biological substances. This interest was driven by the belief that nature held many untapped sources of medicinal compounds. 4. Experimental Approach: Researchers often employed an experimental approach in which they tested various substances, including cobra venom, in laboratory settings and animal models to assess their effects on viral infections. While the rationale for studying cobra venom in particular may not have been well-founded scientifically, it was part of a broader effort to explore diverse avenues for combating polio. Ultimately, while the exploration of cobra venom in polio research did not lead to a breakthrough treatment, it reflects the experimental and exploratory nature of scientific inquiry, particularly in the search for treatments or cures for diseases with limited therapeutic options. 5. Generally, a pet dog or cat recognizes its specific owner. But a pet snake or iguana often does not respond any differently to its owner than to a stranger. Why? Answer:Dogs and cats have evolved in close association with humans over thousands of years, leading to strong social bonds and behaviors that facilitate communication and interaction with their owners. This evolutionary history has resulted in dogs and cats being highly attuned to human cues, including vocalizations, gestures, and facial expressions, allowing them to recognize and respond to their owners in distinctive ways. In contrast, snakes and iguanas, as reptiles, have different evolutionary histories and exhibit different social behaviors compared to mammals like dogs and cats. Here are some reasons why snakes and iguanas may not exhibit the same level of recognition or responsiveness to their owners: 1. Social Behavior: Dogs and cats are social animals that engage in complex social behaviors, including forming social hierarchies, cooperating with conspecifics, and displaying affiliative behaviors towards humans. In contrast, most snakes and iguanas are solitary animals that do not engage in social behaviors beyond mating or territorial interactions. 2. Communication: Dogs and cats have evolved sophisticated vocalizations, body language, and facial expressions that facilitate communication with humans and other animals. They can interpret and respond to human cues, such as tone of voice and body posture. Snakes and iguanas, however, lack these communication mechanisms and may not perceive or respond to human cues in the same way. 3. Cognitive Abilities: Dogs and cats possess relatively advanced cognitive abilities compared to snakes and iguanas. They can learn and remember complex tasks, solve problems, and exhibit emotional responses to their environment and interactions with humans. Snakes and iguanas have simpler cognitive capabilities and may not form the same type of attachments or recognize specific individuals in the same manner. 4. Sensory Perception: Dogs and cats have well-developed senses of smell, hearing, and vision, which they use to perceive and interpret their environment, including the presence of familiar individuals such as their owners. While snakes and iguanas also have sensory systems, their perception of their environment may be more focused on detecting prey, predators, or environmental cues rather than recognizing specific individuals. Overall, the differences in social behavior, communication, cognitive abilities, and sensory perception between dogs and cats on one hand, and snakes and iguanas on the other, contribute to the variations in how they interact with and recognize their owners. While dogs and cats often form strong bonds with their owners, snakes and iguanas may not exhibit the same level of recognition or responsiveness due to their evolutionary history and behavioral characteristics. 6. If some snakes lay eggs while others are ovoviviparous, what are the biological “trade-offs? If one method is more advantageous, why didn’t all reptiles evolve to that system? Answer:The differences between laying eggs (oviparity) and giving birth to live young (ovoviviparity) in snakes involve various biological trade-offs influenced by factors such as energy allocation, reproductive success, and environmental conditions. Here are some considerations regarding these reproductive strategies: 1. Energetic Investment: Oviparous species invest energy in producing and laying eggs but do not have to allocate resources for sustaining developing embryos internally. Ovoviviparous species invest more energy in carrying and nourishing embryos internally but avoid the risks associated with egg predation and environmental fluctuations. 2. Reproductive Output: Oviparous species typically produce more offspring per reproductive event compared to ovoviviparous species. However, ovoviviparous species often have higher offspring survival rates because embryos receive direct maternal nourishment and protection until birth. 3. Habitat and Climate: Oviparity may be advantageous in environments with predictable seasonal changes, as females can synchronize egg laying with favorable conditions for embryo development. In contrast, ovoviviparity may be favored in habitats with unstable or extreme environmental conditions, where protection of embryos from external threats is critical for survival. 4. Parental Care: Oviparous species generally exhibit minimal parental care beyond egg deposition, whereas ovoviviparous species may provide extended maternal care, such as thermoregulation or protection of offspring, which can enhance offspring survival in certain environments. 5. Evolutionary History: The evolution of reproductive strategies is influenced by ancestral traits, environmental pressures, and reproductive success. Oviparity is the ancestral condition for most reptiles, including snakes. Ovoviviparity may have evolved independently in certain lineages in response to specific ecological or physiological factors. 6. Trade-Offs and Flexibility: Both oviparity and ovoviviparity have their advantages and limitations, depending on ecological factors and species-specific characteristics. The diversity of reproductive strategies among snakes reflects trade-offs between reproductive output, offspring survival, and parental investment. The presence of both reproductive modes in snakes suggests that each strategy may be advantageous under different circumstances, leading to their coexistence within snake populations and diversity of reproductive behaviors among species. In summary, the evolution of oviparity and ovoviviparity in snakes represents trade-offs between reproductive investment, offspring survival, and environmental adaptation. The coexistence of these reproductive strategies within snake populations reflects the flexibility of reptiles to adapt to diverse ecological conditions and reproductive challenges. 7. Why do we know that snakes evolved from legged ancestors and not that lizards evolved from snakes? Answer:The evidence supporting the idea that snakes evolved from legged ancestors rather than lizards evolving from snakes is based on several lines of anatomical, developmental, and molecular evidence: 1. Fossil Record: Fossil evidence provides crucial insights into the evolutionary history of snakes. Fossilized remains of early snake ancestors, such as Najash rionegrina and Haasiophis terrasanctus, exhibit characteristics transitional between lizards and modern snakes. These fossils show features like elongated bodies, reduced limbs, and adaptations for burrowing or aquatic lifestyles, suggesting a gradual transition from legged ancestors to limb-reduced snakes. 2. Embryological Development: Studies of embryonic development in snakes reveal vestigial structures associated with limb formation, such as the presence of limb buds in early snake embryos. These limb buds develop and then regress during embryogenesis, indicating that snakes have retained genetic information for limb development inherited from their legged ancestors. 3. Molecular Phylogenetics: Molecular phylogenetic analyses, which compare genetic sequences among different species to infer evolutionary relationships, support the idea that snakes evolved from legged ancestors. These analyses consistently place snakes within the clade Squamata (scaled reptiles), which also includes lizards. Within Squamata, snakes are nested within the clade Serpentes, indicating their evolutionary relationship to other reptiles with limbs. 4. Comparative Anatomy: Comparative anatomical studies reveal similarities between snakes and other squamate reptiles, particularly in skeletal structure, internal organs, and developmental processes. Despite the absence of limbs in snakes, they share many anatomical features with their legged relatives, further supporting their evolutionary connection. Overall, the weight of evidence from the fossil record, embryological development, molecular phylogenetics, and comparative anatomy strongly supports the conclusion that snakes evolved from legged ancestors within the broader group of squamate reptiles. While the possibility of convergent evolution cannot be completely ruled out, the cumulative evidence overwhelmingly supports the scenario of snakes evolving from legged ancestors rather than lizards evolving from snakes. CHAPTER 27 BIRDS CHAPTER OUTLINE 27.1. Diversity A. Profile 1. Over 10,400 species have been described worldwide; only fishes have more species among vertebrates. 2. Birds live in all terrestrial and aquatic environments, from the North to the South Pole. 3. Some live in dark caves, and some dive to 45 meters depth. 4. The “bee” hummingbird is one of the smallest vertebrate endotherms. 5. The feather is the unique and essential feature or hallmark of birds; however, feathers were also present in some theropod dinosaurs, although these feathers were not capable of supporting flight and obviously served in other capacities such as thermoregulation or mating behavior. 6. Uniformity in Structure a. Despite 150 million years of evolution, birds are still readily recognized. b. Forelimbs are modified as wings, although not all are capable of flight. c. Hindlimbs are adapted for walking, swimming or perching. d. All birds have horny, keratinized beaks. e. All birds lay eggs. f. The driving force for this uniformity appears to be the adaptations necessary for flight. 1) Wings are present for support and propulsion. 2) The respiratory system must meet high oxygen demands and cool the body. 3) Bones must provide a light but rigid airframe. 4) Digestion and circulation must meet the high-energy demands of flight. 5) And the nervous system must have superb sensory systems for high-velocity flight. 27.2. Origin and Relationships (Figures 27.1-27.4) A. History 1. The discovery of the fossil of Archaeopteryx lithographica in 1861 linked birds and dinosaurs. a. The skull resembled modern birds but it had teeth rather than a beak. b. The skeleton was reptilian with clawed fingers, abdominal ribs and a long bony tail. c. Feathers were unmistakably imprinted along the wings. 2. Zoologists had long recognized that birds and reptiles shared many similarities. a. Both have skulls that abut the first neck vertebra by a single ball-and-socket joint. b. Both have a single middle ear bone, the stapes. c. The lower jaw in both is composed of five or six bones; in mammals there is one mandibular bone. d. Both birds and reptiles excrete nitrogenous wastes as uric acid; mammals excrete urea. e. Both lay similar yolked eggs; the embryo develops on the surface by shallow cleavage patterns. 3. Thomas Henry Huxley classified birds with theropod dinosaurs. a. This group of dinosaurs has a long, mobile, S-shaped neck. b. Theropods belong to the lineage of diaspid reptiles, the archosaurians, which includes crocodiles. c. Fossil evidence from Spain, China, etc. is accumulating that Huxley’s theory is correct. d. Dromeosaurs, a group of theropods that includes Velociraptor, share many additional derived characteristics with birds, including a furcula (fused clavicles) and lunate wrist bones that permit swiveling motions used in flight. e. Additional evidence linking birds to dromeosaurs comes from recently described fossils from late Jurassic and early Cretaceous deposits in China f. These fossils, including Proachaepteryx and Caudipteryx, are dromeosaurs-like theropods, but with feathers. g. The feathers of dromeosaurs could not have been used for powered flight, but may have been used in social displays. h. Additional theropod dinosaurs recently unearthed in China, such as Sinosauropteryx, are covered with filaments that appear to be homologous with feathers. B. Relationships 1. Modern birds include Paleognathae with a flat sternum and Neognathae with a keeled sternum. 2. Original theories were based on the Paleognathae, or ratite lineage, never having attained flight. 3. This is now rejected; flightlessness has evolved many times among many bird groups. 4. Smaller birds can revert to flightlessness on islands that lack terrestrial predators. 5. Larger flightless birds such as the ostrich and emu can outrun predators. 6. Flightless birds are free from the weight restrictions of flight and some evolved to very large sizes. 27.3. Structural and Functional Adaptations for Flight A. Feathers 1. Structure (Figure 27.5) a. The feather is a special bird adaptation that contributes to more power or less weight. b. The hollow quill emerges from the skin follicle and continues as a shaft or rachis. c. The rachis bears numerous barbs. d. Up to several hundred barbs are arranged to form a flat, webbed surface, the vane. e. Each barb resembles a miniature feather; numerous parallel filaments or barbules spread laterally. f. With up to 600 barbules in each side of a barb, there may be over one million in the whole feather. g. Barbules from two neighboring barbs overlap; they “zip” together with tiny hooks. h. When separated, they are “zipped” back together by preening. 2. Types of Feathers a. Contour feathers provide the form of the bird; flight feathers extend off the wing in flight. b. Down feathers are under contour feathers; their barbules lack hooks and function as insulation. c. Filoplume feathers are hairlike, degenerate feathers with a weak shaft and tuft of short barbs. d. Powder-down feathers on herons and their relatives disintegrate and release a talc-like powder to waterproof feathers. 3. Origin and Development a. The bird feather is homologous to the reptile scale. b. The feather develops from an epidermal elevation over a nourishing dermal core. c. Rather than flattening, the feather bud rolls into a cylinder. d. During growth, pigments are added to the epidermal cells. e. Near the end of its growth, the soft rachis and barbs transform into hard structures of keratin. f. When the protective sheath splits apart, the feather protrudes and the barbs unfold. 4. Molting (Figure 27.6) a. The fully-grown feather is a dead structure; shedding or molting is an orderly process. b. Except in penguins, molting is a gradual process that avoids leaving bare spots. c. Flight and tail feathers are lost in pairs, one on each side, to maintain balance. d. In some species, replacement is continuous; therefore flight is unimpaired. e. In many water birds, primary feathers are molted all at once and the birds are temporarily grounded. f. Most birds molt once a year, usually in late summer after the nesting season. 5. Color a. Feather color may be due to pigments or to structural color. b. Pigments, or lipochromes, color red, orange and yellow feathers. c. Black, brown, red-brown, and gray colors are from the pigment melanin. d. The blue color of the blue jay, indigo bunting and bluebird is from scattering of light by structure. B. Skeleton (Figures 27.7, 27.8) 1. Bone Weight a. Compared with the Archeopteryx, modern birds have light, delicate bones laced with air cavities. b. These are termed pneumatized bones; they are nevertheless strong. c. The total weight of a bird’s feathers may outweigh its skeleton. 2. Bird Skull a. As archosaurs, birds evolved from ancestors with diapsid skulls. b. Bird skulls are so specialized that it is difficult to see the diapsid condition. c. The skull is fused into one piece; the braincase and orbits are large to hold a larger brain and eyes. d. While the skull is lighter, the legs are heavier than in mammals; this lowers the center of gravity. 3. Jaws a. In Archeopteryx, both jaws contained teeth set in sockets. b. Modern birds have a horny keratinous beak molded around bony jaws. c. Most birds have kinetic skulls; in some, the upper jaw is hinged to the skull. 4. Vertebral Column and Appendages a. The bird vertebral column is very rigid; vertebrae are fused except for the cervical vertebrae. b. Additional bony structures called uncinate processes are fused with the pelvic girdle to support legs and provide rigidity for flight. c. Ribs are mostly fused with the vertebrae, pectoral girdle and sternum. d. Except in flightless birds, the sternum bears a large keel for anchorage of flight muscles. e. Bones of forelimbs are highly modified for flight, with some bones reduced in number or fused. f. All of the elements of the basic vertebrate limb are represented in modified form. g. The bird’s legs have undergone less modification since their function remains walking, etc. h. Most caudal vertebrae are fused into a pygostyle. i. Fused clavicles form an elastic furcula that apparently stores energy as it flexes during wing beats. C. Muscular System (Figures 27.9, 27.10) 1. The pectoralis muscles depress the wing in flight and are attached to the keel. 2. The supracoracoideus muscle raises the wing, is also attached to the keel, lays under the pectoralis muscles, and pulls the wing up from below by way of a “rope-and-pulley” type of arrangement. 3. Having both muscles low in the body provides aerodynamic stability. 4. The main leg muscle mass is in the thigh with connections by long tendons to the feet and toes. 5. A toe-locking mechanism prevents a perching bird from falling off a branch while asleep. 6. Birds have lost the long reptilian tail and substituted a muscle mound where tail feathers are rooted. 7. As many as 1000 muscles may control the tail feathers for steering in flight. 8. The neck is thoroughly interwoven with stringy muscles to provide great flexibility. D. Food, Feeding and Digestion (Figure 27.11) 1. Insect Eaters a. In their early evolution, birds were carnivorous, primarily feeding on the great variety of insects. b. Modern birds have specialized to hunt nearly all types of insects in most habitats. 2. Other Diets a. Other animals joined the diet of birds, including worms, molluscs, crustaceans, fish, frogs, etc. b. Nearly one-fifth of birds feed on nectar. c. Euryphagous species eat a wide variety of items and can switch to whatever is seasonally abundant. d. Stenophagous species are specialists but are vulnerable if their food source is jeopardized. e. The beaks of birds often reveal their food habits and vary between seed-eaters, insect-eaters, etc. f. A woodpecker has a straight, hard, chisel-like beak to expose insect burrows; its long, flexible, barbed tongue seeks out the insects in the wood galleries. 3. Food Quantity a. Contrary to the saying “to eat like a bird” meaning “to eat little,” birds are voracious feeders. b. Birds have a high metabolic rate and small birds need even more food per body mass. c. A hummingbird uses oxygen 12 times faster than a pigeon and 25 times that of a chicken. d. A hummingbird eats 100% of its body weight each day, a blue tit about 30% and a chicken, 3.4%. e. Birds have rapid and efficient digestive systems. 1) A shrike can digest a mouse in three hours. 2) A thrush will pass berries through the tract in just 30 minutes. f. Because birds lack teeth, foods that require grinding are cut apart in the gizzard. g. Salivary glands are poorly developed, but lubricate both the food and the slender tongue. h. There are few taste buds, but birds can taste to some extent. i. A long, muscular esophagus extends from pharynx to stomach. j. Many birds have a crop that serves to store food at the lower end of the esophagus. k. The crop of pigeons, doves and some parrots, also produces a lipid- and protein-rich “milk.” l. The stomach consists of a proventriculus that secretes gastric juice and a gizzard that grinds food. m. Birds may also swallow pebbles or grit to assist in grinding in the gizzard. n. Birds of prey such as owls form a pellet of indigestible material in the proventriculus and eject it. o. Paired ceca are at the junction of the intestine and rectum; they serve as fermentation chambers. p. The end of the digestive system is the cloaca, which also receives the products from the genital ducts and ureters. E. Circulatory System 1. The four-chambered heart is large, with strong ventricular walls. 2. Birds share with mammals a complete separation of respiratory and systemic circulations. 3. The right aortic arch, instead of the left as in mammals, leads to the dorsal aorta. 4. The two jugular veins in the neck have a cross vein shunt to continue circulation as the head rotates. 5. The brachial and pectoral arteries to the wings and breast are unusually large. 6. The heartbeat is relatively fast compared to mammals and is inversely proportional to size. a. A turkey heart beats 93 times per minute. b. A chicken heart beats 250 times per minute. c. A small black-capped chickadee heart beats 500 times per minute. 7. Bird red blood cells (erythrocytes) are nucleated and biconvex. 8. Mobile phagocytes are active and efficient in repairing wounds and destroying microbes. F. Respiratory System (Figure 27.12) 1. The bird respiratory system differs radically from the lungs of both reptiles and mammals. 2. Bird Lungs a. The finest branches of the bronchi do not terminate in alveoli but are tube-like parabronchi. b. Air sacs extend into the thorax, abdomen, and even the long bones. c. A large portion of the air bypasses the lungs and flows directly to the air sacs on inspiration. d. On expiration, this oxygenated air flows through the lungs; therefore there is continuous air flow. e. Thus it takes two respiratory cycles for a single breath of air to pass through the system. f. This is the most efficient respiratory system of any vertebrate. 3. An air sac system helps cool a bird during vigorous exercise when up to 27 times more heat is produced. 4. The air sacs extend into bones, legs and wings, providing considerable buoyancy to the bird. G. Excretory System (Figure 27.13) 1. A pair of large metanephric kidneys is composed of many thousands of nephrons. 2. Each nephron has a renal corpuscle and a nephric tubule. 3. Birds use the vertebrate pattern of glomerular filtration and selective resorption. 4. Urine flows through ureters to the cloaca. 5. Uric Acid a. Birds also use the reptilian adaptation of excreting nitrogenous wastes as uric acid. b. In shelled eggs, all excretory products remain within the eggshell; uric acid is stored harmlessly. c. Since uric acid has low solubility, a bird can use far less water to excrete wastes. d. Concentration of uric acid occurs almost entirely in the cloaca where water is absorbed. e. A bird kidney is less efficient than a mammal kidney in removing ions of sodium, etc. f. Mammal kidneys can concentrate solutes to 4–25 times that of the blood; avian kidneys concentrate solutes only a little greater than the blood concentration. g. Marine birds must excrete larger salt loads due to the food they eat and seawater they drink; salt glands located above each eye excrete highly concentrated solutions. h. Salt solution runs out the nostrils; thus gulls and other sea birds have a perpetual “runny nose.” H. Nervous and Sensory Systems (Figures 27.14, 27.15) 1. A bird’s nervous and sensory system must accommodate the problems of flight and a visual lifestyle. 2. The bird’s brain has well-developed cerebral hemispheres, cerebellum and midbrain tectum. 3. The cerebral cortex, a chief coordinating center in mammals, is thin, unfissured and poorly developed. 4. The core of the cerebrum, the corpus striatum, is enlarged into the principal integrating center. 5. The size of the cerebral hemisphere is directly related to the intelligence of the bird. 6. The cerebellum is where muscle-position sense (proprioception), equilibrium sense and visual cues are assembled. 7. The optic lobes bulge to each side of the midbrain and form a visual association apparatus. 8. Sense of smell is poorly developed except in flightless birds, ducks and vultures. 9. Birds have good hearing and superb vision, the best in the animal kingdom. 10. The bird ear is similar to the ear of mammals. a. The external ear canal leads to an eardrum. b. The middle ear contains a rod-like columella that transmits vibrations to the inner ear. c. An inner ear has a short cochlea; it allows birds to hear about the same range of sound as humans. d. Bird ears do not hear as high a frequency as do humans, but surpass us in ability to distinguish differences in pitch and intensities. 11. The bird eye is similar to the mammal eye, but it is relatively larger for a given body size. a. A bird eye is less spherical and almost immobile; a bird turns its head rather than its eyes. b. The light-sensitive retina has both rods and cones. c. Diurnal birds have more cones; nocturnal birds have more rods. d. A pecten is a highly vascularized organ attached to the retina and it juts into the vitreous humor; it may provide oxygen and nutrients to the eye. e. Herbivores must avoid predators and they have eyes placed to each side to view all directions. f. Birds of prey have eyes directed forward to provide better depth perception. g. Many birds have two foveae or regions of detailed vision; this provides both sharp monocular and binocular vision. h. A hawk has eight times the visual acuity of a human and can see a rabbit over a kilometer away. i. An owl’s ability to see in dim light is more than ten times that of a human. j. Many birds can see partially into the ultraviolet spectrum, seeing flower nectar guides. I. Flight 1. History a. The early airspace was an unexploited habitat with flying insects for food. b. Flight also provided rapid escape from predators and ability to travel to better environments. c. There are two hypotheses on the evolution of bird flight. 1) The “ground-up” (cursorial) hypothesis is based on running birds with primitive wings to snare insects. 2) The “trees-down” (arboreal) hypothesis has birds passing through tree-climbing, leaping, parachuting, gliding, and finally powered flight. d. Feathers preceded flight and arose for thermoregulatory purposes. e. There is no evidence for bird ancestors first being membrane-winged. f. The debate about the origin of flight has not been settled. 2. Bird Wing as a Lift Device (Figure 27.16) a. The modified hand bones with attached primary feathers provide the propulsion. b. Lift is provided by the more medial part of the wing and secondary feathers of the forearm. c. A wing is streamlined with a concave lower surface. d. The leading edge of the wing has small tight-fitting feathers. e. Over two-thirds of the total lift comes from negative pressure from the airstream flowing a longer distance over the top of the wing, the convex surface. f. Lift-to-drag ratio is determined by the angle of tilt and the airspeed. g. At high speeds, sufficient lift is generated so that the wing is held at a low angle of attack, creating less drag. h. At a point near 15o, the angle of attack becomes too steep and stalling occurs. i. Stalling is delayed or prevented by a wing slot along the leading edge to direct rapidly moving air across the leading surface. 1) In some birds the alula, or group of small feathers on the “thumb,” provides a midwing slot. 2) Slotting between the primary feathers provides a wing-tip slot. 3. Flapping Flight (Figures 27.17, 27.18) a. Flapping flight requires a vertical lifting force and a horizontal thrusting force. b. Thrust is provided by primaries at the wing tips and lift is provided by the secondaries. c. Greatest power is provided by the downstroke. d. Primary feathers are bent upward and twist to a steep angle of attack. e. On the upstroke, the primary feathers bend so that their upper surfaces twist to produce thrust. f. The powered upstroke is essential for hovering and fast, steep takeoffs. 4. Wing Dynamics at Low and High Speeds (Figure 27.19) a. The lift-to-drag ratio is determined by the angle of attack (angle of tilt) and airspeed. b. At high speeds, the wing is held at low angle of attack, creating less drag. c. As speed decreases, lift is generated by increasing the angle of attack, but this also increases drag. d. Stalling occurs when the angle of attack is too steep (around 15 degrees) because of turbulence on the upper surface. e. Stalling can be delayed or prevented by wing slots, which directs a layer of rapidly moving air across the upper surface of the wing (Figure 27.19C) f. In birds, two types of wing slots occur: alula, a group of small feathers on the thumb providing a mid-wing slot; and gaps between primary feathers, causing wing-tip slots. g. Wing-tip vortexes, eddies of air at the tips of wings, are problematic at high speeds because they create drag (Figure 27.19D). h. The effects of wing-tip vortexes are reduced in wings with pointed tips and in long wings with widely separated tips (high-aspect ratio wings). 5. Basic Forms of Bird Wings (Figures 27.20) a. Elliptical Wings 1) Birds that must maneuver in forested habitats have elliptical wings. 2) Elliptical wings are slotted between primary feathers to prevent stalling at low speeds, etc. 3) The small chickadee can change its course within 0.03 seconds. b. High-Speed Wings 1) Birds that feed on the wing or make long migrations have high-speed wings. 2) These wings sweep back and taper to a slender tip; this reduces “tip vortex” turbulence. 3) They are flat in section and lack wing-tip slotting. c. Dynamic Soaring wings 1) Albatrosses, gannets and other oceanic soaring birds have wings with long, narrow wings. 2) The high-aspect ratio of long, narrow wings lack wing slots and allow high speed, high lift and dynamic soaring. 3) They have the highest aerodynamic efficiency of any design, but are less maneuverable. 4) These birds exploit the highly reliable sea winds and air currents of different velocities. d. High-Lift Wings 1) Vultures, hawks, eagles, owls and other birds of prey that carry heavy loads have wings with slotting, alulas and pronounced camber. 2) This produces high lift at slow speed. 3) Wings of these birds have an aspect ratio intermediate between elliptical wings and high aspect ratio wings. 4) Many are land soarers; their broad, slotted wings allow sensitive response for static soaring. 27.4. Migration and Navigation (Figure 27.21) A. Migration 1. About half of all bird species migrate. 2. They can move between southern wintering regions and northern summer breeding regions. 3. They can exploit seasonal changes in abundance of insects and avoid bird predators. 4. Appearing one time a year prevents buildup of specialized predators. 5. Migration also expands living space and reduces aggressive territorial behavior. 6. Migration favors homeostasis, allowing birds to avoid climatic extremes and food shortages. B. Migration Routes 1. Most migratory birds follow established north-south routes. 2. Some use different routes in the fall and spring. 3. Some aquatic species make rapid journeys; others such as warblers take 50–60 days to migrate. 4. Smaller species migrate at night and feed by day; others are daytime migrants. 5. Many birds follow landmarks; some fly over large bodies of water. 6. Some have very narrow migration lanes; others have wide migration lanes. 7. The Arctic tern circles from North America to coastlines of Europe and Africa to winter quarters, a total of 18,000 kilometers (11,200 miles). C. Stimulus for Migration 1. The long days of late winter and early spring stimulate development of gonads and fat. 2. Long day length stimulates the anterior lobe of the pituitary. 3. Release of pituitary gonadotropic hormone sets in motion a complex series of physiological and behavioral changes resulting in gonadal growth, fat deposition, migration, courtship, mating behavior and care of young. D. Direction Finding in Migration (Figure 27.22) 1. Experiments suggest birds navigate chiefly by sight. 2. Birds recognize topographical landmarks and follow familiar migratory routes. 3. This pools navigational resources and also experience of older birds. 4. Birds have a highly accurate sense of time. 5. Research indicates they can navigate by the earth’s magnetic field; this may be related to magnetite found in the neck musculature of pigeons. 6. Sun-azimuth Orientation a. German ornithologists used special cages to show birds navigate by sun at day and stars at night. b. Planetarium experiments revealed they use the sun as a compass; an internal clock tracks position. c. These experiments suggest use of the North Star as an axis at night. 7. Migration involves a combination of environmental and innate cues. 8. Natural selection culls individuals that make errors; only the best navigators leave offspring. 27.5. Social Behavior and Reproduction (Figures 27.23, 27.24) A. Cooperative Behavior 1. Sea birds often gather in huge colonies to nest and to rear young. 2. Land birds, except for birds such as starlings and rooks, tend to seek isolation for rearing their brood. 3. Birds that isolate during breeding may congregate for migration or feeding. 4. There are many advantages for flocking together: a. mutual protection from enemies, b. greater ease in finding mates, c. less opportunity for an individual straying during migration and d. mass huddling for protection against low night temperatures during migration. 5. Pelicans use organized cooperative behavior to feed. 6. Organized social interactions of birds are most noticeable during breeding season; they stake out territory, select mates, build nests, incubate and hatch eggs, and rear young. B. Reproductive System (Figures 27.25, 27.26) 1. Bird testes are very small until the approach of the breeding season, when they may enlarge 300 times. 2. Before discharge, sperm are stored in a greatly enlarged seminal vesicle. 3. Males of most species lack a penis; mating involves bringing cloacal surfaces in contact. 4. In most birds, the left ovary and oviduct develop and the right ovary and oviduct degenerate. 5. The expanded end of the oviduct, the infundibulum, receives the discharged eggs. 6. Special glands add albumin or egg white to the egg as it passes down the oviduct. 7. Farther down the oviduct, the shell membrane, shell, and shell pigments are also secreted. 8. Fertilization must therefore take place in the upper oviduct before albumin and shell are added. 9. Sperm remain alive in the oviduct for many days after a single mating. C. Mating Systems (Figures 27.27) 1. Over 90% of bird species are monogamous; they only mate with one partner each breeding season. 2. Monogamy: In a few species, such as swans and geese, partners are chosen for life. 3. A smaller number are polygamous; individuals mate with two or more partners each breeding season. 4. Because of their high monogamy rate, compared to mammals, birds lack a built-in food supply and require the parental care from both parents to provision the young. 5. Females enforce monogamy by selecting males that will not divide their time with another female. 6. Bird Territories a. A male sings often to announce his presence to females and drive away males. b. Females wander about to select a male that offers the best chance of reproductive success. c. Usually a male can defend an area that provides just enough resources for one nesting female. 7. Polygamy: a. The most common form of polygamy is polygyny, where one male mates with many females. b. Male grouse collect at a lek where each has a small territory to display to males. c. The male grouse does not care for young. d. Competition for females is intense and females appear to choose the dominant male for mating. e. Recent DNA analyses have shown many passerine species frequently are “unfaithful,” engaging in extra-pair copulations; nests of many of these species may contain 30 percent of young with fathers other than the attendant male. f. In many monogamous birds both male and female birds are equally adept at most aspects of parental care. g. Polyandry in which a female mates with several males and the male incubates the eggs, is relatively rare in birds. D. Nesting and Care of Young (Figures 27.28, 27.29) 1. Nearly all birds lay eggs that must be incubated by one or both parents. 2. Eggs of most songbirds require 14 days for hatching; those of ducks and geese may require a month. 3. Often the female performs most of the duties of incubation; rarely the male has equal or sole duties. 4. Some birds merely lay eggs on bare ground or rocks. 5. Others build elaborate nests using mud, lichens, brush, etc. 6. Nests are often carefully concealed from enemies. 7. Woodpeckers, chickadees, bluebirds and others nest in tree hollows and other cavities. 8. Cuckoos and cowbirds are nest parasites; they lay eggs in other bird’s nests. 9. Precocial birds are able to feed and run or swim as soon as they are hatched. 10. Altricial birds are naked and helpless at birth and must be fed in the nest for a week or more. 11. Nesting success in altricial birds is very low; sometimes barely 20% of nests produce young. 12. Causes of nesting failure include predators, nest parasites and other factors. 27.6. Bird Populations and Their Conservation (Figures 27.30, 27.31) A. Factors 1. Bird populations vary in size from year to year. 2. Birds of prey may cycle with the food supply; for example, snowy owl populations vary with the rodents they eat. 3. When food supplies crash, the birds may move elsewhere to locate alternative food supplies. 4. Humans have introduced birds to new regions; the starling and the house sparrow are both abundant now in the United States. 5. Since the dodo went extinct in 1695, more than 80 bird species have also become extinct due to human influence. 6. Causes of bird extinction include habitat destruction and hunting. 7. Modern hunting interests have helped recover wetlands; no legally hunted birds are endangered. 8. Recent Decline of Songbirds a. Some songbird species that were abundant 40 years ago are in decline. b. Agriculture has utilized once-fallow fields. c. Fragmentation of forests in the United States exposes nests to nest predators. d. House cats are formidable predators that kill many songbirds. e. The loss of tropical forests also deprives about 250 migratory songbirds of their wintering homes. f. Birds stressed in their wintering grounds are therefore in poor condition to make northward migrations. g. Some species are adversely affected by deforestation: others such as robins, sparrows and starlings can accommodate these changes. 27.7. Classification of Class Aves (Figures 27.32–27.35) Superorder Paleognathae Order Struthioniformes Order Rheiformes Order Casuariiformes Order Apterygiformes Order Tinamiformes Superorder Neognathae Order Anseriformes Order Galliformes Order Sphenisciformes Order Gaviiformes Order Podicipediformes Order Phoenicopteriformes Order Procellariiformes Order Pelicaniformes Order Phaethontiformes Order Suliformes Order Ciconiiformes Order Accipitriformes Order Falconiformes Order Otidiformes Order Mesitornithiformes Order Cariamiformes Order Eurpygiformes Order Gruiformes Order Charadriiformes Order Pterocliformes Order Columbiformes Order Psittaciformes Order Opisthocomiformes Order Musophagiformes Order Cuculiformes Order Strigiformes Order Caprimulgiformes Order Apodiformes Order Coliiformes Order Trogoniformes Order Leptosomiformes Order Coraciiformes Order Bucerotiformes Order Piciformes Order Passeriformes Lecture Enrichment 1. One way to illustrate the principle of lift from negative pressure due to air flowing over a convex surface is to blow over a piece of paper held between your hands with the trailing edge drooping away. As you blow over the top, the paper will rise. 2. Provide feathers for students to “unzip” and “re-zip.” Nothing abstract or visual can replace the “feel” of these million barbule structures and this also provides the basis for understanding preening. Note that about 80% of songbirds are protected under the Migratory Bird Treaty Act, and this includes feathers, active nests, and eggs. Therefore it is best to utilize feathers from chickens, city pigeons, and other species designated as unprotected by your state’s wildlife laws. 3. The mathematical complexity of flight often exceeds the students’ background in basic physics; some comparisons can be made to fixed-wing aircraft with broad wings for slow flight and narrow wings for fast flight. The U-2 plane is an albatross design. Some students will have observed the slots in jetliner wings used upon landing. 4. Early bird evolution is becoming more well-known as fossils are found, especially in China. This generates considerable news coverage and controversy and is another opportunity for incorporating current events. 5. Audubon artwork and early narratives can be used to portray the abundance of the passenger pigeon and other birds. Commentary/Lesson Plan Background: Most students, unless vegetarian, have eaten chicken and steaks and will recognize the much lighter weight of the chicken bones. They should recognize the keel as the tapering T-shaped portion under the white meat of the breast, and the white meat itself should be identifiable as pectoralis and supracaracoideus. Birdhouses, birdfeeders and birdwatching may still be among some students’ background; they may be able to relate identification and behavioral features described in this chapter. Some students may come from a hunting background and be able to relate the life history of quail or prairie chicken, or the wildlife preservation activities of some organizations. Students with scouting background may have extensive birding experience. Oddly, fewer children “play” with bird feathers and some students may lack the experience of feeling a feather. Misconceptions: The chapter speaks directly to the misconception of “eating like a bird.” Some students will interpret the bird’s ability to navigate by stars or sun as a complex process similar to our envisioning a map; it is not of this level of reasoning and closer to a complex reflex. The text also defuses the harmless image of outdoor cats; they are effective bird predators and feral or wild cats often outnumber the tame pets. There is a great tendency to think of humans as the pinnacle of evolution; that is, we do everything best, but this is far from the case as can be seen in birds’ visual acuity and high lung efficiency. There is likewise the adaptationist error of assuming that if something such as a parabronchus is highly efficient, then all other breathing organisms would evolve toward this design—it is only an advantage if the organism has a high oxygen demand. Schedule: HOUR 1 27.1. Diversity A. Profile 27.2. Origin and Relationships A. History B. Relationships 27.3. Structural and Functional Adaptations for Flight A. Feathers B. Skeleton C. Muscular System HOUR 2 D. Food, Feeding and Digestion E. Circulatory System F. Respiratory System G. Excretory System H. Nervous and Sensory Systems I. Flight 27.4. Migration and Navigation A. Migration B. Migration Routes C. Stimulus for Migration D. Direction Finding in Migration 27.5. Social Behavior and Reproduction A. Cooperative Behavior B. Reproductive System C. Mating Systems D. Nesting and Care of Young 27.6. Bird Populations and Their Conservation A. Factors 27.7. Classification of Class Aves ADVANCED CLASS QUESTIONS: 1. Birdwatchers who use a feeder to attract birds often are plagued with squirrels that raid the feeder. It is now possible to buy birdfeed laced with red pepper; this stops the squirrels from eating the birdseed and yet the birds are unaffected. Why? Answer: Red pepper, specifically capsaicin, is a common ingredient in birdseed meant to deter squirrels. Birds are generally not affected by capsaicin because they lack the receptors that detect its spicy heat. However, squirrels are sensitive to capsaicin and find it unpleasant, causing them to avoid the birdseed laced with red pepper. So, while the birds continue to enjoy the birdseed, the squirrels are deterred, solving the problem of raiding feeders. 2. If soaring wings have the highest efficiency and allow high speed and high lift, why don’t all birds have this design? Answer: While soaring wings indeed offer high efficiency and are well-suited for high-speed flight and efficient gliding, not all birds have this wing design for several reasons: 1. Specialization: Birds have evolved diverse wing shapes and sizes based on their specific ecological niches and behaviors. Birds that rely on soaring, such as eagles and albatrosses, have long, broad wings ideal for gliding on air currents. However, birds with different lifestyles, such as those that require maneuverability in dense forests or rapid take-offs and landings, have evolved wings suited to those needs. For instance, birds like hummingbirds have short, rounded wings that enable agile flight. 2. Foraging Strategies: Birds' wing designs often correlate with their foraging strategies. Species that hunt in open spaces or cover long distances, such as raptors and seabirds, benefit from soaring wings for energy efficiency. Conversely, birds that hunt in cluttered environments or require quick bursts of speed have wings optimized for agility and rapid acceleration. 3. Habitat Variation: The environments in which birds live vary widely, from dense forests to open plains to mountainous regions. Wing shapes have evolved to accommodate these diverse habitats. For example, birds living in forested areas often have shorter, more rounded wings to maneuver through trees, while birds in open habitats may have longer, more tapered wings for efficient gliding. 4. Dietary Needs: Birds' wing designs may also be influenced by their dietary preferences. Birds that primarily feed on insects or small prey may have wings optimized for agile flight and rapid changes in direction to capture prey. In essence, the diversity of bird wing designs reflects the diverse ecological roles and evolutionary pressures that birds face in their respective habitats and lifestyles. 3. How can we design an experiment to determine what cues are used by a migratory bird? Answer: Designing an experiment to determine the cues used by migratory birds involves careful consideration of their behaviors, environmental factors, and potential cues they may rely on. Here's a basic outline for such an experiment: 1. Selecting Study Species: Choose a migratory bird species known for its well-documented migratory behaviors and routes. Common choices include songbirds like warblers or shorebirds like sandpipers. 2. Habitat Selection: Identify a suitable study area along the migratory route of the selected species. This area should provide opportunities for observation and experimentation while minimizing disturbances to the birds. 3. Experimental Setup: - Establish multiple experimental groups of birds. Each group should be exposed to a different set of potential cues or environmental conditions. - Manipulate the presence or absence of specific cues hypothesized to be important for migration. Cues might include magnetic fields, celestial cues (e.g., stars or the sun), landmarks, odors, or other environmental factors. - Control groups should be exposed to natural conditions without manipulation to serve as a baseline for comparison. 4. Data Collection: - Use tracking devices (such as GPS tags or geolocators) to monitor the migratory movements of the birds in each experimental group. - Record the departure times, migration routes, flight durations, and stopover locations of individual birds. - Conduct behavioral observations to document any changes in the birds' orientation, flocking behavior, or other migratory behaviors in response to the experimental manipulations. 5. Data Analysis: - Analyze the migratory data collected from each experimental group to determine whether the manipulated cues influenced the birds' migratory behavior. - Compare the migratory patterns and behaviors of experimental groups to those of control groups to assess the importance of specific cues. - Statistical analyses, such as regression models or ANOVA, can help identify significant relationships between cue manipulation and migratory behavior. 6. Interpretation and Conclusions: - Evaluate the results to determine which cues are most important for the migratory navigation of the studied species. - Consider potential interactions between different cues and how they may complement or override each other. - Draw conclusions about the mechanisms underlying avian migration and the cues birds rely on for navigation. By following this experimental framework, researchers can gain insights into the cues used by migratory birds and contribute to our understanding of avian navigation and migration ecology. 4. Why do we not see extensive seasonal migration among reptiles or mammals? Answer: Extensive seasonal migration, as observed in many bird species, is less common among reptiles and mammals for several reasons: 1. Physiological Constraints: Reptiles and mammals have different physiological constraints compared to birds that make long-distance migration less feasible. While birds have efficient respiratory systems and high metabolic rates that enable sustained flight over long distances, most reptiles and mammals lack these adaptations. Their physiology is generally less suited for sustained aerobic activity over long periods. 2. Energy Requirements: Migration is energetically costly, requiring significant energy reserves to fuel the journey. Birds have relatively low body weights compared to many mammals and reptiles, which reduces the energy required for flight. Additionally, birds can store large amounts of energy in the form of fat reserves to sustain them during migration. In contrast, many mammals and reptiles have higher body masses and lower energy storage capacities, making long-distance migration less practical from an energy standpoint. 3. Habitat Stability: Many reptiles and mammals have evolved to inhabit stable environments year-round, where resources are available consistently. Unlike birds, which often migrate to exploit seasonal fluctuations in resource availability (such as breeding grounds with abundant food), mammals and reptiles may have adapted to rely on local resources or hibernation/estivation to survive periods of resource scarcity. 4. Ecological Niches: The ecological niches occupied by reptiles and mammals may not necessitate extensive seasonal migration. While some mammal species, such as caribou or wildebeest, undertake long-distance seasonal movements in search of food or breeding grounds, these migrations are typically not as widespread or well-documented as those of birds. Many reptiles and mammals have evolved alternative strategies for coping with seasonal changes in their environments, such as hibernation, estivation, or local movements within their home ranges. 5. Physical Barriers: Unlike birds, which can often fly over barriers such as mountains or bodies of water, many terrestrial mammals and reptiles face physical barriers that impede long-distance movement. These barriers, such as mountain ranges, deserts, or urban development, can fragment habitats and limit the feasibility of extensive migrations. Overall, while some mammals and reptiles exhibit seasonal movements or migrations, they are generally less extensive and well-known compared to those of birds. The differences in physiology, energy requirements, habitat stability, ecological niches, and physical barriers all contribute to the relatively limited extent of seasonal migration in these groups. 5. Why are birds mostly monogamous? Answer: Birds exhibit a variety of mating systems, including monogamy, polygyny, polyandry, and promiscuity. However, monogamy is indeed a common mating strategy among birds for several reasons: 1. Parental Care: Monogamy is often associated with biparental care, where both male and female birds contribute to raising offspring. By forming monogamous pairs, birds can share the responsibilities of incubating eggs, feeding chicks, and defending the nest. This cooperative effort increases the likelihood of offspring survival and reproductive success. 2. Resource Defense: Monogamous pairs often defend territories that provide essential resources such as food, nesting sites, and suitable habitat. By forming stable pair bonds, birds can maintain exclusive access to these resources, enhancing their reproductive success. 3. Mate Assistance: Monogamous pairs can provide assistance to each other throughout the breeding season. This assistance can include shared incubation duties, cooperative feeding of offspring, and mutual defense against predators or rivals. By cooperating with a single mate, birds can increase their breeding success and improve their chances of raising healthy offspring. 4. Genetic Benefits: Monogamy allows birds to ensure paternity certainty, meaning that males can be confident that the offspring they help raise are genetically related to them. This increases the likelihood that males will invest resources in caring for their own offspring rather than those of other males. 5. Social Stability: Monogamous pair bonds can provide social stability within bird populations. By forming long-term partnerships, birds can reduce the uncertainty associated with finding and attracting new mates each breeding season. This stability can also facilitate cooperative behaviors and social interactions within breeding pairs and their offspring. 6. Ecological Factors: Environmental factors such as resource distribution, predation pressure, and breeding site availability can influence the prevalence of monogamy in bird species. In environments where resources are limited or unpredictable, forming stable pair bonds and sharing parental care responsibilities can be advantageous for maximizing reproductive success. Overall, monogamy in birds is a complex behavior influenced by a combination of ecological, social, and evolutionary factors. While many bird species exhibit monogamous mating systems, variations in mating strategies can be observed across different taxa and environmental conditions. Instructor Manual for Integrated Principles of Zoology Cleveland Hickman, Jr., Susan Keen, Allan Larson, David Eisenhour, Helen I'Anson, Larry Roberts 9780073524214