CH-28 Mammals – Integrated Principles of Zoology : Chapter Notes

This document contains Chapters 28 to 29 CHAPTER 28 MAMMALS CHAPTER OUTLINE 28.1. Features and Diversity A. Overview 1. Hair is a critical sign of being a mammal. 2. A few mammals, especially aquatic forms, may have very few hairs but they are still present. 3. Hair serves many functions: protection, concealment, waterproofing and buoyancy, signaling, sensitive vibrissae and especially thermal insulation. 4. Mammals have other characteristic features. a. Most have a specialized placenta to feed the embryo. b. Most have specialized teeth and jaws for processing diverse foods. c. The mammal nervous system is more advanced than in other animal groups. d. Mammary glands nourish the newborn. e. Convoluted turbinate bones in the nasal cavity provide a high surface area for warming and moistening inspired air and for reducing moisture loss during exhalation. B. Diversity 1. About 5700 living species are known. 2. Nevertheless they are among the most highly differentiated groups in the animal kingdom. 3. They have been domesticated for use as food, clothing, pets, beasts of burden and in research. 4. Exotic mammal introductions have usually disrupted the ecology. 5. In 2012, 643 species were listed as “critically endangered” or “endangered”. 28.2. Origin and Evolution of Mammals (Figures 28.1 − 28.4) A. History (Figure 28.1) 1. The evolution of mammals from their earliest amniote ancestors is very fully documented. 2. Over the last 150 million years, small, ectothermic, hairless ancestors evolved into today’s endothermic, furry mammals. 3. Skull structures, especially teeth, provide abundant evidence of the evolutionary descent (Figure 28.2). 4. The skull roof separates amniotes into synapsids, anapsids and diapsids. a. Mammals derive from the synapsids that have a pair of temporal openings in the skull. b. The synapsids were also the first amniotes to radiate widely into terrestrial habitats. c. The anapsids have solids skulls and include turtles and their ancestors. d. The diapsids have two pairs of openings in the skull roof and include dinosaurs, lizards, etc. 5. Therapsids Lineage a. The earliest synapsids radiated into diverse herbivorous and carnivorous pelycosaurs. b. From one of the early carnivorous synapsids arose the therapsids. c. The therapsids were the only synapsid group to survive beyond the Paleozoic. d. Therapsids were the first to have an efficient erect gait with upright limbs beneath the body. e. This reduced stability required the muscular coordination center of the brain to have an expanded role in muscle coordination. f. Most of the variety of herbivores and carnivores disappeared in the Permian extinction. g. Only the last therapsids subgroup to evolve, the cynodonts, survived into the Mesozoic. 6. Cynodonts (Figures 28.3, 28.4) a. This group evolved a high metabolic rate that supported a more active life. b. They have more jaw musculature and skeletal changes for greater agility. c. A secondary bony palate allowed them to breathe while holding prey or chewing food. d. This secondary palate would be important later to mammal evolution by allowing young to breathe while suckling. e. Heterodont teeth permitted better food processing and use of more diverse foods. f. Turbinate bones in the nasal cavity aided in the retention of heat. g. The number of ribs was reduced, making the spinal column more flexible. h. Within the cynodont clade, a small carnivorous group, called trithelodontids, resembles mammals. i. Loss of lumbar ribs in cynodonts is correlated with the evolution of a diaphragm and also may have provided greater dorso-ventral flexibility of the spinal column. 7. Early Mammals of the Triassic Period a. The earliest mammals of the late Triassic were small and mouse- or shrew-sized. b. They were diphyodonts; teeth were replaced only once as deciduous and permanent teeth. c. They were almost certainly endothermic although cooler than modern placental mammals. d. Hair was essential and also indicates that sebaceous and sweat glands were present. e. There is no fossil evidence, but mammary glands must have evolved before the end of the Triassic. f. Young early mammals must have hatched from eggs and relied on maternal milk. g. Mammals, having developed in the mid-Triassic, had to wait 150 million years to diversify. h. All non-mammalian synapsid groups became extinct when the dinosaurs became abundant. 8. Cenozoic Radiation of Mammals a. Mammals survived first as shrew-like nocturnal animals, then in a radiation in the Eocene Epoch. b. The radiation is attributed to the many habitats vacated by extinction of many amniote groups at the end of the Cretaceous. c. Mammals were agile, endothermic, intelligent, adaptable, and gave birth to young they protected. d. The transformation of three middle ear bones, the malleus, incus, and stapes occurred. 1) The stapes is homologous to the columella or hyomandibula of other vertebrates and already functioned in hearing in early synapsids. 2) The malleus and incus originated from the articular and quadrate bones that previously served as the jaw joint but became reduced in size to better transmit sound vibrations. 3) A new jaw joint was formed between the dentary and squamosal bones; this joint is the defining characteristic for fossil mammals. 28.3. Structural and Functional Adaptations of Mammals A. Integument and Its Derivatives 1. Skin (Figure 28.5) a. Mammal’s skin is generally thicker than in other classes of vertebrates. b. As with all vertebrates, skin is made of epidermis and dermis. c. In mammals, the dermis becomes much thicker than the epidermis. d. The epidermis is thinner and well protected by hair. e. In places that are subject to abrasion, the outer layers become thicker and cornified with keratin. 2. Hair (Figures 28.6–28.8) a. Hair is characteristic of mammals; it is reduced on humans and exists as a few bristles on whales. b. The hair follicle is an epidermal structure, but is sunk into the dermis of the skin. c. A hair grows continuously by rapid proliferation of cells in the follicle. d. Cells in the hair shaft are carried upward away from their source of nourishment and die. e. The dense protein keratin is the same protein as is found in nails, claws, hooves and feathers. f. Dense and soft underhair serves for insulation by trapping a layer of air. g. Coarse and longer guard hairs protect against wear and provide coloration. h. Hair consists of three layers. 1) The medulla or pith is in the center of the hair. 2) The cortex with pigment granules is next to the medulla. 3) The outer cuticle is composed of imbricated scales. i. Different mammals have unique hair structure. 1) The brittle hairs of deer are deficient in cortex. 2) The hollow, air-filled hairs of the wolverine are deficient in medulla. 3) The hairs of rabbits and others are scaled to interlock when pressed together. 4) The curly hair of sheep grows from curved follicles. j. Hair stops growing at a certain length; it remains in the follicle until new growth pushes it out. k. In most mammals, there are periodic molts of the entire coat. 1) Foxes and seals shed once every summer. 2) Most mammals molt twice, in the spring and in the fall, with the winter coat much heavier. 3) Some have white winter coats for camouflage and brown summer coats. 4) Arctic mammals are not genetically albino where eye and skin pigments are also missing. 5) White winter fur is leukemism; they have dark eyes, dark-colored ear tips, noses, etc. l. Patterns including spots, stripes, salt-and-pepper, etc. are disruptive and conceal the animal. m. Vibrissae or “whiskers” are sensory hairs; they provide a tactile sense to nocturnal mammals. n. Porcupine, hedgehog, and echidna quills are barbed and break off easily. 3. Horns and Antlers (Figures 28.9, 28.16A) a. True Horns 1) Horns are found in ruminants such as sheep and cattle. 2) Horns are hollow sheaths of keratinized epidermis. 3) They embrace a core of bone rising from the skull. 4) They are not normally shed and are not usually branched, but may be greatly curved. 5) Horns grow continuously and occur in both sexes, although they may be longer in males. b. Antlers 1) Antlers are formed in the deer family. 2) Antlers are composed of solid bone when mature. 3) Antlers develop beneath an annual spring covering of highly vascular soft skin or “velvet.” 4) Except for caribou, only males produce antlers. 5) When growth is complete just before breeding season, the blood vessels constrict and the stag removes the velvet by rubbing it against trees. 6) Antlers are shed after the breeding season and a new bud appears for the next growth. 7) Each year, the new pair of antlers is larger than the previous set. 8) Growing antlers may require a moose or elk to accumulate over 50 pounds of calcium salts. c. Rhinoceros Horn 1) Hairlike keratinized filaments arise from dermal papillae and are cemented together. 2) These structures, however, are not attached to the skull. 4. Glands (Figure 28.5) a. Mammals have the greatest variety of integumentary glands; all are derived from the epidermis. b. Sweat glands are tubular, highly coiled glands found in mammals but never in other vertebrates. c. Eccrine Sweat Glands 1) Eccrine glands secrete a watery fluid that draws heat away from the skin surface. 2) They are found in hairless regions such as footpads. 3) Eccrine sweat glands are reduced or absent in rodents, rabbits and whales. d. Apocrine Sweat Glands 1) Apocrine sweat glands are larger than eccrine glands and have more convoluted ducts. 2) The glands have a secretory coil in the dermis that extends into the hypodermis. 3) Apocrine glands open into a hair follicle. 4) In humans, they develop near puberty and are restricted to armpits, external ear canals, etc. 5) In contrast to watery secretions of eccrine glands, apocrine secretions form a film on the skin. 6) Apocrine glands are unrelated to heat regulation and are correlated with reproductive function. e. Scent Glands 1) Present in nearly all mammals, they vary greatly in location and function. 2) They communicate with members of the same species: mark territory, warning and defense. 3) Scent-producing glands are located in many different regions in different mammals. 4) The scent glands of skunks, minks and weasels open into the anus and are very odoriferous. 5) Many mammals give off strong scents during the mating season to attract the opposite sex. f. Sebaceous Glands 1) Most are associated with hair follicles although some open directly onto the surface. 2) Cells in the cellular lining accumulate fats, then die and are expelled to form oily sebum. 3) It does not turn rancid but serves as a dressing to keep the skin and hair pliable and glossy. 4) Most mammals have sebaceous glands over the entire body. g. Mammary Glands 1) Mammary glands are probably modified apocrine glands. 2) They are rudimentary in males and occur on all female mammals. 3) The epidermis thickens to form a milk line along which mammae appear. 4) Human females develop mammary glands at puberty with fat accumulation; additional development occurs during pregnancy. 5) Other mammals have swollen mammae periodically when pregnant or nursing. 6) Mammary glands increase their size at maturity (in most mammals milk is secreted from mammary glands via nipples or teats, but monotremes lack nipples and simply secrete milk into a depression on the mother’s belly where it is lapped up by the young). B. Food and Feeding 1. Mammals exploit a wide variety of food sources; some are specialists and others are generalists. 2. Mammal structures are closely associated with adaptations for food finding or capturing. 3. Teeth (Figure 28.10) a. Structure of teeth reveals the life habits of a mammal. b. Reptiles had homodont dentition or uniform tooth patterns. c. Differentiation of teeth for cutting, seizing, gnawing, etc. resulted in heterodont dentition. d. Types 1) Incisors have sharp edges for snipping or biting. 2) Canines are specialized for piercing. 3) Premolars have compressed crowns with one or two cusps for shearing and slicing. 4) Molars have larger bodies and variable cusp arrangements for crushing and grinding. e. A primitive mammal tooth formula is three incisors, one canine, four premolars and three molars. f. Mammals do not continually replace teeth; they have one deciduous set and a permanent set. g. Generally, the incisors, canines and premolars are deciduous; molars are a single permanent set. 4. Feeding Specializations (Figures 28.10–28.13) a. Insectivores 1) Shrews, moles, anteaters and most bats are insectivores. 2) They eat little fibrous vegetable matter so their digestive tract is short. 3) Because many omnivores also consume insects, the insectivorous diet is distinctive because of its lack of plant material and vertebrates. b. Herbivores 1) Browsers and grazers include horses, deer, antelope, cattle, sheep and goats. 2) Gnawers include rodents, rabbits and hares. 3) Herbivores have reduced or absent canines, but molars are broad and high-crowned. 4) Rodents have chisel-shaped incisors that grow throughout life. 5) Cellulose is a chain of glucose molecules, but the chemical bonds are difficult to break. 6) Herbivores use anaerobic fermentation chambers so microorganisms can metabolize cellulose. 7) A side pocket or cecum may also serve as a fermentation chamber and absorptive area. 8) Rodents eat fecal pellets in order to provide additional fermentation. 9) Ruminants have a huge four-chambered stomach. 10) Food is regurgitated, re-chewed, and passed to the rumen, reticulum, omasum and abomasum. 11) Herbivores generally have long digestive tracts for the prolonged time needed to digest fiber. c. Carnivores 1) Most carnivores feed on herbivores. 2) This requires specialization for killing the prey. 3) A high protein diet is easily digestible and therefore the digestive tract is shorter. 4) Carnivores do not have to continuously graze and they have more leisure time. 5) Capturing prey also requires more intelligence, stealth, and cunning. 6) In turn, this has driven herbivores to have keen senses and escape behaviors. 7) Some herbivores use size (i.e., rhinos, elephants) or defensive group behaviors. 8) Humans have exterminated many carnivores from some areas. 9) Production of crops and elimination of carnivores has in turn benefited small rodent pests. d. Omnivores 1) Omnivores feed on both plant and animal tissues. 2) Examples include pigs, raccoons, rats, bears and most primates including humans. 3) Many carnivores will switch to fruits, berries, etc. when normal food is scarce. 4) Food supplies in temperate regions vary by season; migration and hibernation are solutions. 5) Some mammals cache food stores during times of plenty, a common behavior of rodents. 5. Body Weight and Food Consumption (Figures 28.14, 28.15) a. The smaller the animal, the greater is its metabolic rate and the more it must eat per unit size. b. The amount of food varies in proportion to the body surface area rather than the body weight. 1) Surface area is proportional to about 0.7 power of body weight. 2) Amount of food a mammal or bird eats is also about 0.7 power of body weight. 3) A 3 gram mouse will consume per gram of body weight five times more food than does a 10 kilogram dog and about 30 times more food than does a 50,000 kilogram elephant. c. Small mammals must spend much more time hunting and eating food than do large mammals. d. A small shrew weighing 2 grams must eat more than its body weight each day and will starve if deprived of food for a few hours. e. In contrast, a mountain lion may kill an average of one deer a week. C. Migration (Figures 28.16, 28.17) 1. Few terrestrial mammals make regular seasonal migrations; most remain in a home range. 2. There are some striking animal migrators; more are in North America than any other continent. 3. The barren-ground caribou migrates twice each year, spanning 160–1100 kilometers (100–700 miles). 4. Gray whales migrate 18,000 kilometers (11,250 miles) between Alaska and Baja, Mexico annually. 5. The fur seal breeds on the Pribilof Islands and then journeys to wintering grounds off the southern California coast. 6. Although bats can fly and could migrate similar to birds, most bats hibernate in winter. D. Flight and Echolocation (Figures 28.18, 28.19) 1. Mammals have not exploited the skies extensively; bats can fly and some mammals glide from trees. 2. Bats are nocturnal or crepuscular (active at twilight). 3. Echolocation, along with flight, allows bats to navigate and eat insects in total darkness at night. 4. They can also inhabit totally dark deep caves, a habitat ignored by other mammals and birds. 5. Bats use frequencies from 30,000 to 100,000 Hz (cycles per second), well beyond our hearing range. 6. Ten to 200 pulses of signals are sent to locate prey; an echo is received before the next pulse is sent. 7. Some moths have evolved ultrasonic detectors to detect and avoid approaching bats. 8. External ears of bats are large to focus on sound location. 9. Bat navigation may allow bats to build a mental image of surroundings similar to visual images. 10. All bats are nocturnal although fruit-eating bats use sight and olfaction to locate food. 11. Flowers that are evolved to utilize bats as pollinators have smelly white flowers that open at night. 12. The tropical vampire bat has razor-sharp incisors and anticoagulant saliva. E. Reproduction (Figure 28.20) 1. Reproductive Cycles a. Most mammals have mating seasons timed to coincide with most favorable time to birth and rear young. b. Female mammals usually restrict mating to a fertile period during the periodic estrus cycle. c. This time of female receptivity is known as heat or estrus. d. Stages of the Estrus Cycle 1) Proestrus is the period of preparation when new follicles grow. 2) Estrus is when mating occurs; this is timed to be simultaneous with ovulation. 3) If pregnancy does not occur, estrus is followed by metestrus, a period of repair. 4) During diestrus, the uterus becomes small and anemic until the cycle repeats. e. Some animals lengthen gestation period by delayed implantation; the blastocyst remains dormant while its implantation in the uterine wall is postponed to align birth with a favorable season. f. Animals with only one breeding season a year are monestrous; recurrent breeding is polyestrous. g. Menstrual Cycle 1) Old World monkeys and humans have a cycle terminated by menstruation. 2) Menstruation involves shedding of the endometrium or lining of the uterus. 2. Reproductive Patterns (Figures 28.21–28.23) a. Egg-Laying Monotremes 1) Monotremes, such as the duck-billed platypus, lay eggs with one breeding season per year. 2) Eggs are fertilized in the oviduct before albumin and a thin, leathery shell are added. 3) She lays eggs in a burrow nest where they are incubated for 12 days. 4) Similar to reptiles and birds, there is no gestation and the egg provides all nutrients. 5) However, after hatching, young suck milk from the mother’s fur near her mammary glands. b. Pouched Marsupials 1) Marsupials are pouched, viviparous mammals. 2) Although only eutherians are “placental mammals,” marsupials do have a primitive choriovitelline “placenta.” 3) The embryo is first encapsulated by shell membranes and floats free for several days. 4) After “hatching” from shell membranes, the embryo erodes a shallow depression in the uterine wall and absorbs nutrient secretions by a vascularized yolk sac. 5) Gestation is brief and marsupials give birth to tiny young that are still embryos. 6) Early birth is followed by a prolonged interval of lactation and parental care. 7) In red kangaroos, the first pregnancy is followed by a 33-day gestation and then birth. 8) The mother immediately becomes pregnant, but the presence of a suckling young arrests development of the new embryo at the 100-cell stage. 9) Such a period of arrest is called embryonic diapause. 10) It is possible to stairstep three young with one external, one suckling, and one embryonic. 11) With wide variation, marsupials have young born at extremely early stages of development. c. Placental Mammals 1) Eutherians are viviparous placental mammals. 2) They have an investment in a prolonged gestation in contrast to marsupials with an investment in prolonged lactation. 3) The embryo in the uterus is nourished through a chorioallantoic placenta. 4) Gestation is longer than in marsupials and is much longer for large mammals. 5) Gestation and body size are loosely correlated because there is variation in maturity at birth. 6) Humans are slower developing than any other mammal; this contributes to our uniqueness. d. Patterns 1) The ultimate number of young produced per year also depends on mortality rate. 2) Small rodents that are prey for carnivores usually produce more than one litter each season. 3) Meadow mice can produce up to 17 litters of four to nine young each year! 4) At the other extreme, an elephant produces on average four calves during her 50-year life. 5) Although placentals have the advantage of higher reproductive rates, the marsupial mode of reproduction may be advantageous in highly unpredictable climates. F. Territory and Home Range (Figures 28.24, 28.25) 1. An animal may use a burrow or den as the center of its territory. 2. If it has no set address, the territory is marked, usually with scent glands. 3. A grizzly bear may have a territory of several square kilometers that it defends against other grizzlies. 4. When an intruder knows it is in another’s territory, it usually flees upon an encounter. 5. A beaver represents a mammal that forms strong monogamous bonds that last a lifetime. 6. The male beaver, assisted by the female, expends substantial energy building dams and lodges. 7. A prairie dog is unusual in relinquishing its home to the young and moving to the edge of the “town.” 8. A home range of a mammal extends much further beyond the defended territory. G. Mammalian Populations (Figures 28.26, 28.27) 1. A population of animals includes all members of a species that can potentially interbreed in a region. 2. All mammals live in ecological communities with other animal and plant species. 3. Populations obviously expand after young are born and are lowest before breeding season. 4. Density-independent factors (e.g., fires, severe weather, etc.) affect animals regardless of their density. 5. Density-dependent factors (e.g., infectious diseases) are related to crowding of populations. 6. Lemmings have dramatic cycles of abundance forcing them to migrate. 7. The snowshoe hare cycles in population size, apparently due to density-related psychogenic causes. 28.4. Humans and Mammals (Figures 28.28, 28.29) A. Domesticated Animals 1. Dogs were probably the first domesticated animals, being an adaptable offspring of social wolves. 2. The domestic cat is probably derived from an African race of wildcat. 3. Nomadic people probably subdued horses, camels, oxen and llamas. 4. Some totally domesticated animals no longer exist as wild species (e.g., dromedary camel, llama). 5. Many have been selectively bred to yield characteristics desirable for human purposes. 6. Animals range from fully dependent to reindeer and elephants that do not breed in captivity. B. Mammals, Crop Damage and Human Disease 1. Rodents and rabbits are major pests of growing crops and stored foods. 2. Human monocultures and the elimination of predators have made this a more severe problem. 3. Many rodents carry diseases. a. Norway rats and prairie dogs carry bubonic plague and typhus. b. Tularemia is transmitted to humans by wood ticks and carried by rabbits and other rodents. c. Rocky Mountain spotted fever is carried to humans by ticks from ground squirrels and dogs. d. Ticks from white-tailed deer transmit Lyme disease. e. Trichina worms and tapeworms are acquired by humans who eat meat of infected mammals. 28.5. Human Evolution A. History 1. Darwin devoted the book The Descent of Man and Selection in Relation to Sex to human evolution. 2. At that time there was essentially no fossil evidence linking apes and humans. 3. Darwin’s evidence was based on anatomical comparisons. 4. Two skeletons of Neanderthals were collected in the 1880s. 5. In 1891, Eugene DuBois discovered Java man, Homo erectus. 6. Major finds in Africa, between 1967and 1977, provided many intermediates. 7. Modern biochemical studies have also shown humans and chimpanzees to be genetically similar. 8. There is no search for the “missing link” since the lineage is now fairly well understood. B. Evolutionary Diversification of the Primates (Figures 28.30–28.32) 1. As primates, humans have grasping fingers, flat fingernails, and forward-pointing eyes. 2. Ancestral primates split into two major lineages: one that gave rise to lemurs and lorises (traditionally called prosimians) and the other that gave rise to tarsiers, monkeys and apes (traditionally called simians or anthropods). 3. Both were probably arboreal; this promoted intelligence, grasping limbs and tool use. 4. Highly developed sense organs aided vision and coordination; all depended on a large cerebral cortex. 5. The earliest simian fossils are from Africa about 40 million years ago. 6. Some of these primates became day-active and vision became the dominant sense with color vision. 7. There are three major simian groups. a. The ceboids are New World monkeys of South America. b. Cercopithecoids are Old World monkeys including the baboon, mandrill and colobus. c. Anthropoid apes include apes and humans. d. Old World monkeys and anthropoid apes form the sister group of New World monkeys. e. Old World monkeys lack a grasping tail, have close-set nostrils, opposable thumbs, and derived teeth. f. Humans, orangutans, gorillas, and chimpanzees are now recognized to belong to a single family, Hominidae, and are referred to as hominids. 8. Apes first appear in 25-million-year-old fossils. C. The First Humans and the Origin of Bipedalism (Figures 28.33–28.35) 1. Skeletal differences between humans and other hominids are associated with a more omnivorous diet and an upright, bipedal posture. 2. Genetic evidence indicate that humans diverged from chimpanzees 5-7 million years ago. 3. Earliest well-known human believed to be Ethiopean Ardipithecus ramidus (4.4 million years ago). D. Early Homo: Tool-Making and Migration Out of Africa 1. Three species of Homo shared the African landscape with australopithecines. 2. Homo habilis was a fully erect hominid that used stone and bone tools; it appeared about 2 million years ago and survived about 500,000 years. 3. About 1.5 million years ago, the larger Homo erectus appeared with larger height and brain size. 4. Homo erectus had a complex culture and spread throughout the tropical and temperate Old World. E. Modern Humans 1. Modern humans diverged from African Homo erectus around 800,000 years ago. 2. Neanderthals a. Among the many early subcultures of Homo sapiens, the Neanderthals emerged about 150,000 years ago and occupied most of Europe and the Middle East. b. They were proficient hunters and tool-users. c. Their robust, heavily muscled bodies allowed them to survive the cold climates of the Ice Age. d. About 30,000 years ago, they were replaced and perhaps exterminated by modern humans. 3. Modern humans were tall people with a culture different from the Neanderthals. 4. Speech and language may have evolved with the Homo sapiens lineage a. Muscle movements required for speech appear to coincide with changes in FOXP2 gene about 200,000 years ago. b. Hyoid and larynx moved into positions that could support speech more recently. 5. Although the “Multiregional Hypothesis” (that the Homo lineage consists of a single, unbranched lineage that evolved into various species in various regions) is supported by some genetic studies, it is possible that the evolution of humans was not via a single, unbranched lineage (i.e., the “Out of Africa Hyptothesis”). F. Unique Human Position 1. Homo sapiens is a product of the same processes that have affected evolution from life’s origin. 2. Mutation, isolation, genetic drift and natural selection affect human populations. 3. Only humans, however, have a non-genetic cultural evolution that provides constant feedback between our past and future experience. 4. Symbolic languages, conceptual thought, knowledge of history and an ability to manipulate our environment emerge from this cultural endowment. 5. The arboreal ancestry provided much of the intellectual equipment; evolution through other lineages would have posed major impediments. 6. Recent molecular genetic studies indicate that human populations have formed a single evolutionary lineage for the past 1.7 million years. 7. The earliest human remains originally classified as Homo sapiens from 500,000 to 300,000 years ago, now are identified by anthropologists as Homo heidelbergensis. 8. Fossil and mtDNA evidence indicates that characteristics of Homo sapiens, as presently defined, arose in Africa about 200,000 years ago. 9. All mtDNA can be traced to a single female, “Eve” who lived in Africa approximately 170,000 years age. 10. Neanderthals and remaining Homo erectus disappeared approximately 10,000 years after the first appearance of Homo sapiens in Europe and Eastern Asia. 28.6. Classification of the Class Mammalia (Figures 28.37–28.42) Subclass Prototheria Infraclass Ornithodelphia Order Monotremata Subclass Theria Infraclass Metatheria Order Didelphimorphia Order Paucituberculata Order Microbiotheria Order Dasyuromorphia Order Peramelemorphia Order Notoryctemorphia Order Diprotodontia Infraclass Eutheria Order Afrosoricida Order Macroscelidea Order Tubilidentata Order Proboscidea Order Hyracoidea Order Sirenia Order Cingulata Order Pilosa Order Dermoptera Order Scandentia Order Primates Suborder Strepsirhini Suborder Haplorhini Family Callitrichidae Family Cebidea Family Cercopithecoidea Family Hylobatidae Family Hominoidea Order Lagomorpha Order Rodentia Order Soricomorpha Order Erinaceomorpha Order Chiroptera Order Pholidota Order Carnivora Order Perissodactyla Order Artiodactyla Order Cetacea Lecture Enrichment 1. The textbook sequence of explanation of monotremes and marsupials shows the logical and appropriately detailed sequence of changes that occur in moving from oviparity to full placental viviparity. 2. The dog as the first domestic animal is no accident. Students, as a group, may be able to identify the various social behaviors of wolves in wolf packs that pre-adapt them to associating with us. 3. Some students may have read literature portraying pet ownership and domestication as “slavery”; note the examples of animal species that, if abandoned, would go extinct. The commercial silkworm is also totally dependent upon humans to keep it alive. 4. The rapidly growing numbers of early hominid fossil varieties, and their sometimes rapidly revised names, can be confusing to students, but it is nevertheless an important reflection of our view of their evolutionary position. This is also an example of how “all facts are theory-laden” and “all names are theory-laden.” Commentary/Lesson Plan Background: Rural students and pet owners should have some understanding of the nature of the estrus cycle and the compulsive mating behavior of animals based on odors rather than mental imaging found in humans. Misconceptions: There is a tendency to expect the evolution of the next “higher” group to originate from the most recently evolved of the previous ancestral group. The text is very clear in showing that mammals originated from within an early reptilian lineage, and that primates likewise diversified from early mammalian ancestors. Schedule: HOUR 1 28.1. Features and Diversity A. Overview B. Diversity 28.2. Origin and Evolution of Mammals A. History 28.3. Structural and Functional Adaptations of Mammals A. Integument and Its Derivatives B. Food and Feeding C. Migration D. Flight and Echolocation E. Reproduction F. Territory and Home Range G. Mammalian Populations 28.4. Humans and Mammals A. Domesticated Animals B. Mammals, Crop Damage and Human Disease 28.5. Human Evolution A. History B. Evolutionary Radiation of the Primates C. Early Humans D. Emergence of Homo E. Homo sapiens: Modern Hominids F. Unique Human Position 28.6. Classification of the Class Mammalia ADVANCED CLASS QUESTIONS: 1. Only mammals have lips because only mammals need to form suction to nurse. Why does a duckbill platypus with a horny “duckbill” violate this principle and not have lips? Answer: The notion that only mammals have lips solely for nursing is an oversimplification. While lips are indeed highly developed in mammals and play a crucial role in suckling during nursing, their functions extend beyond just feeding. In mammals, lips serve several important functions: 1. Feeding: Lips aid in forming a seal around the nipple during nursing, facilitating the creation of suction for suckling milk. This is particularly important for newborn mammals that rely on their mother's milk for nutrition. 2. Manipulation of Food: Lips help in grasping, manipulating, and guiding food into the mouth during feeding. They allow for precise control over the movement and placement of food within the oral cavity, aiding in chewing and swallowing. 3. Sensory Perception: Lips are highly sensitive to touch, temperature, and texture, allowing mammals to explore and evaluate food items before ingestion. This sensory feedback is important for food selection and detection of potential threats or contaminants. Now, regarding the duck-billed platypus: The platypus, despite being a mammal, possesses a unique anatomical feature—the duck-like bill—that lacks distinct lips as seen in many other mammals. However, this doesn't necessarily violate the principle that only mammals have lips. Instead, it highlights the diversity of adaptations among mammalian species. The duck-billed platypus is a semi-aquatic mammal native to Australia. Its bill, which resembles that of a duck, serves several purposes: 1. Sensory Detection: The bill of the platypus is covered in specialized receptors known as electroreceptors, which allow it to detect electrical signals produced by the muscular contractions of its prey underwater. This adaptation is particularly useful for locating prey in murky water. 2. Feeding: While the platypus doesn't suckle milk like other mammals, its bill is adapted for feeding on aquatic invertebrates such as insects, larvae, and small crustaceans. The bill's shape and structure enable the platypus to capture, manipulate, and crush its prey effectively. 3. Protection: The bill also serves as a tool for digging and excavating burrows along riverbanks, where the platypus constructs its nesting chambers and shelters. While the duck-billed platypus lacks distinct lips, its bill is a highly specialized structure that fulfills functions analogous to those served by lips in other mammals. Therefore, the absence of lips in the platypus does not necessarily contradict the principle that only mammals have lips, but rather showcases the diversity of adaptations among mammalian species in response to their unique ecological niches and feeding behaviors. 2. We conduct drug tests because so many candidate pharmaceuticals are toxic or may not be effective. Why do we begin testing drugs intended for humans with mice, rather than on grasshoppers, for instance? Answer: Testing drugs on mice, or other mammalian models, rather than on organisms like grasshoppers, is primarily because mice share more physiological and genetic similarities with humans. This similarity makes them more suitable for predicting how a drug might behave in the human body. Here's why mice are commonly used in drug testing: 1. Genetic Similarity: Mice and humans share a significant portion of their genetic makeup. Many of the genes involved in fundamental biological processes, disease pathways, and drug metabolism are conserved between mice and humans. This genetic similarity allows researchers to study how drugs interact with specific molecular targets and pathways relevant to human health and disease. 2. Physiological Similarity: Mice have organ systems and physiological processes that closely resemble those of humans. For example, they have similar cardiovascular, nervous, immune, and endocrine systems. This similarity enables researchers to assess how drugs are absorbed, distributed, metabolized, and excreted in a mammalian system that shares key physiological characteristics with humans. 3. Complexity of Biological Systems: Mice are more complex organisms than grasshoppers, with a greater degree of biological complexity, including multiple organ systems and tissue types. Testing drugs in a more complex biological system allows researchers to better understand potential effects on various organs and tissues and assess for unintended side effects or toxicities. 4. Predictive Value: The results obtained from preclinical studies using mice are often more predictive of human responses compared to simpler organisms like grasshoppers. This predictive value increases the likelihood of identifying promising drug candidates and avoiding those with potential safety concerns before advancing to human clinical trials. 5. Ethical Considerations: There are ethical considerations involved in using animals for research, including considerations of animal welfare and minimizing harm. While mice are still used in research, their use is generally considered more ethically justifiable compared to other animals due to their small size, ease of handling, and the availability of established ethical guidelines and regulations for their care and use in research. Overall, while grasshoppers and other simpler organisms may offer advantages in certain types of research, such as basic biological studies or ecological research, their use in drug testing is limited due to the significant physiological and genetic differences compared to humans. Mice provide a more relevant and reliable model for studying the safety and efficacy of pharmaceuticals intended for human use. 3. Why are so many more mammals hosts to human disease agents, rather than reptiles or birds? Answer: The prevalence of mammals as hosts to human disease agents compared to reptiles or birds can be attributed to several factors related to their biology, ecology, and evolutionary history: 1. Genetic Similarity to Humans: Mammals, including humans, share more genetic and physiological similarities with each other compared to reptiles or birds. This genetic similarity makes it easier for pathogens to adapt to mammalian hosts and overcome their immune defenses. Additionally, the physiological mechanisms underlying disease transmission, such as receptor recognition and immune evasion strategies, may be more conserved between mammals and their pathogens. 2. Ecological Factors: Mammals often share habitats and interact closely with humans, increasing the opportunities for disease transmission. Many mammalian species are synanthropic, meaning they live in close association with human populations, either in urban environments or as domesticated animals. This proximity increases the likelihood of zoonotic diseases, where pathogens can jump from animals to humans. 3. Body Temperature and Metabolism: Mammals, including humans, typically maintain relatively high body temperatures and have faster metabolic rates compared to reptiles or birds. These factors can influence the replication rates and survival of pathogens within the host's body. Many pathogens are adapted to thrive in the warm, nutrient-rich environments provided by mammalian hosts. 4. Social Behavior and Population Density: Mammals often exhibit complex social behaviors and live in densely populated communities, which can facilitate the spread of infectious diseases. Social interactions, such as grooming, mating, and territorial behavior, provide opportunities for direct contact and transmission of pathogens between individuals. Higher population densities increase the likelihood of disease outbreaks and epidemics. 5. Immune System Evolution: Mammals have evolved sophisticated immune systems capable of mounting complex immune responses to combat pathogens. However, this evolutionary arms race between hosts and pathogens has led to the emergence of new diseases and the evolution of virulent pathogens capable of evading host defenses. The dynamic interplay between host immunity and pathogen virulence contributes to the ongoing threat of infectious diseases in mammalian populations. While reptiles and birds can also serve as hosts for human disease agents, the prevalence of mammalian hosts is often higher due to the combination of genetic, ecological, and physiological factors that make mammals particularly susceptible to pathogen transmission and establishment. However, it's essential to recognize that disease dynamics are complex and can vary widely depending on the specific pathogen, host species, and environmental factors involved. 4. How could Darwin anticipate the ape-man lineage if there were no transitional fossils at the time he wrote? Answer: Darwin didn't specifically anticipate the existence of a direct ape-to-human lineage in his writings. His theory of evolution by natural selection provided a framework for understanding how species change over time, but he didn't have access to the extensive fossil record and genetic evidence that we have today. However, Darwin did propose that humans shared a common ancestor with other primates based on comparative anatomy and embryology. He argued that similarities in the anatomical structures and developmental processes among humans, apes, and other mammals suggested a common ancestry. Darwin's ideas were revolutionary because they challenged prevailing beliefs about the origin of species and the uniqueness of humans. While he didn't have direct evidence of transitional fossils linking humans to other primates, his theory provided a framework for understanding the patterns of evolution and the relationships between different species. It wasn't until later, with the discovery of fossil hominins (early human ancestors) and advances in genetics and molecular biology, that scientists were able to gather more direct evidence supporting the idea of a shared ancestry between humans and other primates. Transitional fossils such as Ardipithecus, Australopithecus, and various species of Homo have since been discovered, providing insights into the evolutionary history of humans and their relationships with other primates. In summary, while Darwin's theory laid the groundwork for understanding human evolution, it wasn't until later discoveries and advancements in scientific techniques that more direct evidence of transitional fossils and genetic similarities between humans and other primates emerged. PART IV ACTIVITY OF LIFE 29 Support, Protection, and Movement 30 Homeostasis 31 Internal Fluids and Respiration 32 Digestion and Nutrition 33 Nervous Coordination 34 Chemical Coordination 35 Immunity 36 Animal Behavior CHAPTER 29 SUPPORT, PROTECTION, AND MOVEMENT CHAPTER OUTLINE 29.1. Integument A. General 1. The integument is the outer covering that includes skin, hair, setae, scales, feathers and horns. 2. It is usually tough and pliable, providing mechanical protection against abrasion. 3. It may also provide moisture proofing for sealing water in or out. 4. It is a vital barrier against invasion by bacteria. 5. Skin also protects underlying cells against damage by ultraviolet sunlight. 6. In endothermic animals, it is concerned with insulation and cooling. 7. Skin contains sensory receptors. 8. It has excretory functions and sometimes respiratory functions as well. 9. Skin pigmentation may assist in camouflage and signaling. 10. Skin secretions may make the animal sexually attractive or influence other animals’ behaviors. B. Invertebrate Integument (Figure 29.1A) 1. Protozoa have plasma membranes or a protective pellicle. 2. Most multicellular invertebrates have complex tissue coverings. 3. Some secrete a noncellular cuticle over the epidermis. 4. A molluscan epidermis is soft and contains mucous glands; some secrete the calcium carbonate shell. 5. Cephalopods have a more complex integument of cuticle, simple epidermis, connective tissue, reflecting cells, and a thicker layer of connective tissue. 6. Arthropod Exoskeleton a. The firmness of the exoskeleton, and jointed appendages, allows for muscle attachment. b. A single-layered epidermis or hypodermis secretes a cuticle with two zones. c. The thicker, inner procuticle is made of protein and chitin layers. d. The thin epicuticle is nonchitinous, made of proteins and lipids, and provides moisture-proofing. e. Arthropod cuticle may remain soft and flexible as in small crustaceans and insect larvae. f. Decapod cuticle is hardened by deposition of calcium carbonate in the procuticle (calcification). g. Insect cuticle hardens by sclerotization, formation of cross-linkages between procuticle lamellae. h. Arthropod cuticle is one of the toughest animal materials, yet it is very light. 7. Molting a. The epidermal cells divide by mitosis. b. Enzymes secreted by the epidermis digest most of the procuticle; digested materials are absorbed. c. In the space beneath the old cuticle, a new epicuticle and procuticle are formed. d. After the old cuticle is shed, the new cuticle is thickened and calcified or sclerotized. C. Vertebrate Integument and Derivatives (Figures 29.1B-C, 29.2, 29.3) 1. Structure a. An example of vertebrate skin is frog or human skin. b. The thin, outer stratified epithelial layer is epidermis derived from ectoderm. c. The inner, thicker layer is dermis derived from mesoderm. d. The dermis contains blood vessels, collagenous fibers, nerves, pigment cells, fat cells and connective tissue cells, all to support and nourish the epidermis. e. The epidermis is stratified squamous epithelium. f. The basal part is made of cells that undergo frequent mitosis to renew the layers above it. g. As outer cells are displaced upward, fibrous protein keratin accumulates inside the cells. h. Cells die as the keratin accumulates to replace the metabolically active cytoplasm. i. Cornified, or dead transformed cells, are shed as household dust and dandruff. j. While still on the body, the cornified cells are resistant to abrasion and are the stratum corneum. k. This area thickens when exposed to constant pressure, such as calluses and footpads of mammals. l. Dermis supports the epidermis; any bony structures derive from dermis tissue. m. Scales of contemporary fishes are bony dermal structures evolved from bony armor of early fishes (Figure 29.2). n. Most amphibians lack dermal bones in their skin. o. In reptiles, dermal bones form the armor of crocodilians, the beaded skin of lizards, and contribute to the shell of turtles. p. Claws, beaks, nails and horns are made up of combinations of epidermal and dermal components (Figure 29.3). 2. Animal Coloration (Figure 29.4) a. Structural colors are produced by the physical structure of the surface tissues. b. More common are pigments, varied molecules that reflect specific light rays. c. In crustaceans and ectothermic vertebrates, these pigments are in large cells with branching processes called chromatophores. d. Pigments may concentrate in the center of a cell or disperse throughout the cell for display. e. Cephalopod chromatophores are different; each small sac-like cell with pigment is surrounded by muscle cells that can stretch the cell into a pigmented sheet—a rapid response system. f. Melanins are black or brown polymers often secreted within cells called melanophores responsible for earth-colored shades. g. Carotenoid pigments provide yellow and red colors often contained inside xanthophores. h. Ommochromes and pteridines are responsible for yellow pigments of molluscs and arthropods. i. Green is usually produced by yellow pigment overlying blue structural color. j. Iridophores contain crystals of guanine or another purine rather than pigment; they produce silvery or metallic colors. k. Mammals are relatively uncolorful, a fact related to their being mostly colorblind. l. Primates are an exception, and the brilliant skin patches of baboons and mandrills reflect this. m. Dermal melanophores deposit melanin in growing hair of mammals, providing the muted color. 3. Injurious Effects of Sunlight a. Human sunburn is one demonstration of the damaging effect of ultraviolet radiation on cells. b. Protozoans or flatworms exposed to sun in shallow water are damaged or killed. c. Arthropod cuticle and scales, feathers and fur of the various vertebrates provide protection. d. Sunburn is caused by dermal and epidermal cells releasing histamines causing vessels to enlarge. e. A suntan is due to melanin pigment built up in deeper epidermis, and pigment darkening. f. Sunlight causes about one million new cases of skin cancer annually in the United States. g. High doses of sunlight in childhood cause genetic mutations that cause skin cancer when older. 29.2. Skeletal Systems A. Hydrostatic Skeletons (Figures 29.5, 29.6) 1. Many invertebrates use their body fluids as an internal hydrostatic skeleton. 2. Muscles in the body wall of earthworms contract against the coelomic fluids that are incompressible. 3. In a body compartmentalized by septa, each surviving part can still develop pressure and move. 4. The elephant trunk, tongues of mammals and reptiles, and tentacles of cephalopods are examples of muscular hydrostats based on muscles arranged in cross-wise patterns. B. Rigid Skeletons 1. Rigid skeletons provide rigid elements to which muscles can attach. 2. Muscles can only contract; to be lengthened, they must be extended by pull of an antagonistic muscle. 3 The exoskeleton is a protective skeleton that often must be molted to allow growth; in the case of molluscs, the shell grows with the animal. 4. The endoskeleton is found in echinoderms and vertebrates. 5. The vertebrate endoskeleton is composed of bone and cartilage; it protects and supports, is the major reservoir for calcium and phosphorus, and is the origin of blood cells. C. Notochord and Cartilage 1. The notochord is a semirigid supportive axial rod of the protochordates and all vertebrate larvae. 2. Except in jawless vertebrates, a notochord is surrounded or replaced by backbone during development. 3. Cartilage is a major skeletal element of jawless fishes and elasmobranchs; the cartilage is a derived feature since their Paleozoic ancestors had bony skeletons. 4. Hyaline cartilage is the basic form of cartilage made of cartilage cells surrounded by firm complex protein gel interlaced with a meshwork of collagenous fibers. 5. Blood vessels are missing in cartilage; this is the reason many sport injuries heal poorly. 6. Hyaline cartilage also makes up articulating surfaces of bone joints of adult vertebrates, as well as supporting tracheal, laryngeal and bronchial rings. 7. In some invertebrates, cartilage occurs in the radula of gastropods and lophophore of brachiopods. 8. Cephalopods have cartilage with long, branching processes that resemble cells of vertebrate bone. D. Bone (Figure 29.7) 1. Bone Types a. Bone is living tissue with significant deposits of calcium salts in an extracellular matrix composed of collagenous fibers in a protein-carbohydrate gel. b. Bone is nearly as strong as cast iron, yet only one-third as heavy. c. Bone is always laid down in a “replacement area” formed by some connective tissue. d. Most bone develops from cartilage and is called endochondral or replacement bone. e. Embryonic cartilage is eroded, leaving honeycomb spaces that are invaded by bone-forming cells. f. Bone-forming cells deposit extracellular matrix that becomes calcified around the strandlike remnants of the cartilage. g. Intramembranous bone develops from sheets of embryonic cells, mainly the face and cranium. h. Cancellous, or spongy, bone has an open, interlacing framework of bony tissue. i. Some bones proceed to add additional matrix to become compact bone. 2. Microscopic Structure of Bone a. Compact bone has a calcified bone matrix arranged in concentric rings. b. The rings contain cavities (lacunae) filled with bone cells (osteocytes). c. Lacunae are interconnected with canaliculi passages that allow communication between bone cells and distribute nutrients. d. The whole cylindrical structure is an osteon or Haversian system. e. Osteoclasts slowly resorb bone while osteoblasts deposit additional bone. f. These simultaneous processes allow the growth of a rigid structure without any weakening. g. Parathyroid hormone stimulates bone resorption; calcitonin from thyroid gland (or the ultimobrachial gland in nonmammal vertebrates) inhibits bone resorption. h. Both hormones, together with a derivative of vitamin D, maintain constant blood calcium levels. i. Bone growth also responds to usage; astronauts living without gravity suffer bone loss. 3. Plan of the Vertebrate Skeleton (Figures 29.8, 28.9) a. The vertebrate skeleton is composed of the axial and appendicular skeleton. 1) The axial skeleton includes skull, vertebral column, sternum and ribs. 2) The appendicular skeleton includes limbs and pectoral and pelvic girdles. b. Skull 1) Movement from water to land forced dramatic changes in body form. 2) Increased cephalization made the skull the most intricate part of the skeleton. 3) Some early fishes had 180 skull bones. 4) Over time, many skull bones were lost or fused. 5) Amphibians have from 50 to 95, mammals have 35 or fewer and humans have 29. c. Vertebral Column 1) The vertebral column is the main stiffening axis and serves the same function as a notochord. 2) Movement from water to land resulted in selection for withstanding new vertebral stresses from the two pair of appendages. 3) Vertebrae are separated into cervical thoracic, lumbar, sacral and caudal. 4) In birds and humans, the caudal vertebrae are reduced in size and number and sacral vertebrae are fused. 5) The python has over 400 vertebrae. 6) A young child has 33 vertebrae; in adults, 5 fuse to form sacrum and 4 form the coccyx. 7) Humans have 7 cervical or neck vertebrae, 12 thoracic, and five lumbar or back vertebrae. 8) The number of cervical vertebrae is seven; this is constant in nearly all mammals. 9) The first cervical vertebra is the atlas; it supports the skull and allows it to pivot. 10) The second cervical vertebra is the axis; it allows the head to turn side-to-side. d. Ribs 1) Fishes have a pair of ribs for every vertebra. 2) They serve as stiffening rods and improve effectiveness of muscle contractions. 3) Some vertebrates have reduced ribs; the leopard frog has no ribs at all. 4) In mammals, the ribs from the thoracic basket prevent collapse of the lungs. 5) Sloths have 24 pairs of ribs; horses possess 18 pairs. 6) Primates other than humans have 13 pairs of ribs; humans have 12 pairs, rarely a 13th pair. e. Appendages 1) Most vertebrates have paired appendages. 2) Fishes, except agnathans and some eels, have pectoral and pelvic girdles supporting pectoral and pelvic fins. 3) Tetrapods, unless they are limbless, have two pairs of pentadactyl limbs. 4) Even when highly modified, living and fossil tetrapods have the same homologous limb. 5) Horses have gained speed by evolution of a longer third toe. 6) Bird embryos demonstrate 13 distinct wrist and hand bones that reduce to three in adults. 7) In tetrapods, the pelvic girdle is firmly attached to transmit force. 8) The pectoral girdle is more loosely attached to allow greater freedom for manipulation. 4. Effect of Body Size on Bone Stress (Figure 29.10) a. Cross-section-to-volume-ratio 1) Consider one animal twice as long, wide and tall as a second animal. 2) The larger animal is eight times the volume and eight times the weight. 3) However, the cross-sectional area of bones, tendons and muscles is four times greater. 4) Therefore, eight times the weight is to be carried by four times the strength. 5) Mammalian bone is uniform per cross-sectional area and this places an upper limit on size. 6) Bone shape does not change much; mammals adapted by shifting posture and alignment. 7) Mechanical advantage can be gained by aligning weight with ground reaction forces, such as in an upright horse; beyond the size of a horse, not much advantage can be gained. 8) Elephants and large dinosaurs had thick and robust bones but this decreases running speed. 29.3. Animal Movement A. Mechanism 1. Animal movement is an important characteristic of animals, compared to plants. 2. Movements include streaming of cytoplasm and massive muscle movements. 3. Most animal movement relies on a single fundamental mechanism: contractile proteins. 4. Contractile machinery is composed of ultrafine fibrils: fine filaments, striated fibrils or tubular fibrils. 5. All are arranged to relax or contract when powered by ATP. 6. The most important protein contractile system is composed of actin and myosin. 7. The acto-myosin system is almost universal and is found from protozoa to vertebrates. 8. However, cilia and flagella are composed of proteins other than actin and myosin. B. Ameboid Movement 1. Ameboid movement is found not only in amebas, but also in wandering cells of metazoans. 2. Ameboid cells change shape by extending and withdrawing pseudopodia on any cell surface. 3. Under the plasmalemma is a non-granular gel-like ectoplasm that encloses a more liquid endoplasm. 4. Under one model, as the pseudopod extends, hydrostatic pressure forces actin subunits into the pseudopod where they assemble into a network to form a gel. 5. At the trailing edge of the gel, the network disassembles and freed actin and actin-binding proteins interact with myosin to create a contractile force that pulls the cell along. 6. Locomotion is assisted by membrane-adhesion proteins that attach to the substrate to provide traction. C. Ciliary and Flagellar Movement (Figures 29.11, 29.12) 1. Cilia are minute, hairlike, motile processes that extend from the surfaces of many animal cells. 2. It is a distinctive feature of ciliate protozoans. 3. Except for nematodes, where they are absent, they are found in all major animal groups. 4. Cilia function to move whole unicellular organisms and ctenophores. 5. Cilia also propel fluids and materials across epithelial surfaces in larger animals. 6. Everywhere cilia are found, they have a uniform diameter of 0.2 to 0.5 micrometers. 7. Each cilium contains a peripheral circle of nine double microtubules around two central microtubules. 8. Each microtubule has a spiral array of protein subunits called tubulin. 9. The microtubule doublets around the periphery are connected to each other and the central pair by a system of microtubule-associated proteins. 10. Extending from each doublet is a pair of arms composed of the protein dynein. 11. Dynein arms act as cross bridges between doublets and produce a sliding force between microtubules. 12. The flagellum is whiplike, longer than a cilium and present in fewer numbers. 13. Flagella are found in flagellate protozoans, animal spermatozoa, and in sponges. 14. Flagella have the same basic internal structure as cilia although exceptions to this “9+2” arrangement are the “9+1” and “9+0” sperm tails of flatworms and mayflies. 15. Flagella differ more in their beating pattern than in structure. 16. A flagellum beats symmetrically with snakelike undulations to propel water parallel to the long axis. 17. A cilium beats asymmetrically with a fast power stroke in one direction followed by a slow recovery; water is propelled parallel to the ciliated surface. 18. During ciliary movement, the microtubules behave similar to the sliding microtubule hypothesis described for muscle cell movement. D. Muscular Movement 1. Contractile Tissue a. Contractile tissue is most highly developed in muscle cells, called fibers. b. During ciliary flexion, dynein arms link to adjacent microtubules, swivel and release in cycles. 2. Types of Vertebrate Muscle (Figures 29.13–29.15) a. Striated Muscle 1) Striated muscle is transversely striped with alternating dark and light bands. 2) Striated or skeletal muscle is organized into sturdy, compact bundles. 3) Skeletal muscles attach to skeletal elements and move the trunk, appendages, eyes, etc. 4) Skeletal muscle fibers are very long, cylindrical cells with many nuclei. 5) They are packed together in bundles called fascicles and are enclosed in connective tissue. 6) Fascicles are grouped into a discrete muscle enclosed in thin connective tissue. 7) Some muscles taper at ends as they connect by tendons to bone; others are flattened sheets. 8) Skeletal muscle contracts powerfully and quickly, but fatigues more rapidly than smooth muscle. 9) Skeletal muscle, called voluntary muscle, is stimulated by motor fibers under conscious control. 10) Antagonistic muscles are functional opposites that oppose the other’s action. b. Cardiac Muscle 1) Cardiac muscle has striations but is uninucleate with branching cells. 2) It is the muscle tissue of the vertebrate heart and is seemingly tireless. 3) It is fast acting and striated like skeletal muscle, but contraction is under autonomic control. 4) The heartbeat originates within the specialized muscle; the autonomic nerves merely speed up or slow down this rate. 5) Cardiac muscle is composed of closely opposed but separate uninucleate cell fibers joined by junctional complexes. c. Smooth Muscle 1) Smooth or visceral muscle lacks the alternating bands or striations. 2) Cells are much smaller, tapering strands, each containing a single nucleus. 3) Smooth muscle cells form sheets of muscle circling the walls of the alimentary canal, blood vessels, respiratory passages, and urinary and genital ducts. 4) Smooth muscle is slow acting; it maintains prolonged contractions using little energy. 5) Controlled by the autonomic nervous system, contractions are involuntary and unconscious. 6) Most smooth muscles push material in a tube or regulate tube diameter. 3. Types of Invertebrate Muscle a. Giant barnacles and Alaskan king crabs have giant muscle fibers 3 mm in diameter and 6 cm long. b. Such large cells are important in studying muscle physiology. c. There is a wide variety of invertebrate muscles; two are described here. d. Bivalve Molluscan Adductor Muscles 1) Scallops use “fast” striated muscle fibers to close their valves during their swimming actions. 2) A slower smooth muscle is able to keep up long-lasting contraction for hours or days. 3) Slow adductors use very little energy and require very little stimulation to keep contracted. 4) The contracted state resembles a “catch mechanism” with stable cross-linkages. e. Insect Flight Muscles 1) Some small flies can beat their wings faster than 1000 beats per second. 2) This fibrillar muscle contracts at frequencies much faster than any vertebrate muscle. 3) A wing leverage system is arranged so the muscles shorten very little during each downbeat. 4) Muscles and wings operate as a rapidly oscillating system in an elastic thorax. 5) Recoiling elastically and activated by stretch, they do not need one impulse per contraction. 6) One reinforcement impulse for every 20–30 contractions is enough to keep system active. 4. Structure of Striated Muscle a. Striated muscle is named for the periodic bands visible under the light microscope. b. Each cell or fiber is a multinucleated tube with many myofibrils packed together. c. The cell membrane folds in to form the sarcolemma. d. A myofibril contains thick filaments of protein myosin and thin filaments of protein actin. e. Thin filaments are held together by a dense structure called the Z line. f. The sarcomere extends between successive Z lines. g. Repeated, high-intensity, short exercises cause synthesis of additional actin and myosin. h. Endurance exercise develops more mitochondria, myoglobin and capillaries. i. Thick Filaments 1) Each thick filament is made of myosin molecules packed together in a bundle. 2) Each myosin molecule has two polypeptide chains, each having a club-shaped head. 3) The double heads of each myosin molecule face outward from the center of the filament. 4) The heads act as molecular cross bridges that interact with the thin filaments in contraction. j. Thin Filaments 1) Thin filaments are composed of three different proteins. 2) The backbone is a double strand of actin twisted into a double helix. 3) Surrounding the actin filament are two strands of tropomyosin that lie in the grooves of actin. 4) Each tropomyosin is a double helix. 5) The third protein of the thin filament is troponin, a complex of three globular proteins. 6) Troponin is a calcium-dependent switch that acts as the control point in contraction. 5. Sliding Filament Model of Muscle Contraction (Figure 29.16) a. A sliding filament model was proposed independently in the 1950s by two English physiologists. b. According to this model, the thick and thin filaments link together by molecular cross bridges. c. They then act as levers to pull the filaments past each other. d. The cross bridges on the thick filaments snap rapidly back-and-forth, attaching and releasing from special receptor sites on the thin filaments. e. This ratchet action draws the Z lines together. f. All sarcomere units shorten together as the muscle contracts. g. Relaxation is passive; when the cross bridges between thick and thin filaments release, the sarcomeres are free to lengthen. h. This requires some force, usually supplied by recoil of elastic fibers within the muscle, and by antagonistic muscles or by gravity. 6. Control of Contraction a. Muscles contract in response to nerve stimulation. b. When a nerve to a muscle is severed, the muscle atrophies or wastes away. c. Skeletal muscles are innervated by motor neurons whose cell bodies are located in the central nervous system (brain and spinal cord). d. Each cell body leads to an axon that branches to many terminal points on a muscle. e. Each terminal branch innervates a single muscle fiber; a single motor axon may innervate a few fibers (for precise control) or up to 2000 muscle fibers (for general effect). f. The motor neuron and all of the muscle fibers it innervates are a motor unit. g. When a motor neuron fires, the action potential passes simultaneously to all motor units. h. The total force of the muscle contraction depends on the number of motor units activated. i. Precise control of movement requires varying the number of motor units activated at one time. j. Motor unit recruitment is a steady increase in muscle tension by increasing the motor units. 7. The Neuromuscular Junction (Figure 29.17) a. A motor axon terminates on a muscle fiber at the myoneural junction. b. At this junction is a tiny synaptic cleft. c. The neuron stores acetylcholine in small synaptic vesicles. d. When the nerve impulse reaches the cleft, the acetylcholine is released. e. Acetylcholine causes depolarization of the muscle fiber membrane by binding to the receptor sites. f. This depolarization spreads through the muscle fiber causing it to contract. g. The synapse provides a chemical bridge that couples the nerve impulse and muscle fibers. h. On the sarcolemma surface are numerous invaginations that project as tubules into a muscle fiber. i. This is called a T-system and it is continuous with the sarcoplasmic reticulum, fluid-filled channels that run parallel to the myofilaments. 8. Excitation-Contraction Coupling (Figure 29.18) a. In resting muscle, the tropomyosin strands block the myosin heads from attaching with actin. b. When muscle is stimulated, the electrical depolarization causes calcium ions to be released from the sarcoplasmic reticulum. c. When calcium binds to troponin, the troponin changes shape, which allows the tropomyosin to move out of its blocking position. d. This exposes the active sites on actin filaments and the myosin heads begin binding to them. e. Attach-Pull-Release Cycle 1) Release of bond energy from ATP activates the myosin head, which swings 45 degrees and releases a molecule of ADP. 2) This power stroke pulls the actin filament about 10 nanometers. 3) This pull comes to an end when another ATP molecule binds to the myosin head, inactivating the site. 4) Each cycle of attach-pull-release requires expenditure of energy in the form of ATP. f. Shortening continues as long as nerve stimulation keeps free calcium available. g. The cross-bridge cycling can repeat 50–100 times per second. h. While each sarcomere shortens a very small distance, the pull is magnified by the thousands of sarcomeres lying end to end in a muscle fiber. i. A strongly contracting muscle may shorten by one-third its resting length. j. When stimulation stops, calcium is pumped back into the sarcoplasmic reticulum. 9. Energy for Contraction a. Muscle contraction requires large amounts of energy. b. The ATP normally present will sustain contraction only a second or two. c. Creatine phosphate is a high-energy phosphate compound that stores bond energy while at rest. d. Creatine phosphate releases stored bond energy to convert ADP to ATP in the reaction: 1) Creatine phosphate + ADP –> ATP + Creatine e. Within a few to 30 seconds, the reserves of creatine phosphate are depleted. f. Glycogen is the third and largest store of energy. g. Glycogen is a chain of glucose molecules and is stored in the liver and in muscle. h. Glycogen is abundant, can be mobilized quickly, and provides energy under anoxic conditions. i. As creatine phosphate declines, enzymes convert glycogen into glucose-6-phosphate. j. This first stage of glycolysis proceeds into mitochondrial respiration and generates ATP. k. If muscle contraction is not too vigorous or prolonged, glucose is completely oxidized to carbon dioxide by aerobic respiration. l. During prolonged exercise, the blood cannot provide enough oxygen for complete oxidation. m. The contractile machinery must then receive energy from anaerobic glycolysis. n. Without anaerobic glycolysis, all heavy muscular exertion would be impossible. o. Fast glycolytic fibers rely almost exclusively on anaerobic glycolysis. p. Anaerobic glycolysis degrades glucose to lactic acid; energy released is used to resynthesize creatine phosphate. q. Lactic acid accumulates in the muscle and diffuses into general circulation. r. Continued muscular exertion causes a buildup of lactic acid that leads to fatigue. s. Recent evidence suggests that muscle fatigue in muscle types relying on creatine phosphate stores may be due to accumulation of inorganic phosphate. t. Muscles incur an oxygen debt because accumulated lactic acid must be oxidized by extra oxygen to become pyruvic acid. u. Oxygen consumption must remain elevated until all lactic acid has been oxidized. E. Muscle Performance 1. Fast and Slow Fibers a. Skeletal muscles have several types of fibers. b. Slow fibers are specialized for slow, sustained contractions without fatigue (e.g. posture). c. Slow fibers constitute red muscles because they contain a rich blood supply, a high density of mitochondria, and abundant stored myoglobin oxygen reserves. d. Two kinds of fast fibers provide fast, powerful contractions. e. One type of fast fiber lacks efficient blood supply and high density of mitochondria and myoglobin; they are pale in color and function anaerobically and fatigue rapidly. f. White meat of chicken is an example of this fast fiber muscle. g. Another kind of fast fiber has the extensive blood supply, mitochondria and myoglobin and functions aerobically and can sustain exercise for long periods of time. h. Geese, dogs and ungulates have limb muscles with a high percentage of fast aerobic fibers. i. The cat family has running muscles made up almost entirely of fast fibers that operate anaerobically. j. Such muscles rapidly build up an oxygen debt; cheetahs must rest 30–40 minutes after a chase. 2. Importance of Tendons in Energy Storage (Figure 29.19) a. During walking and running, kinetic energy is stored from step to step. b. A kangaroo also uses the recoil of energy in tendons to bound along; therefore each movement does not have to rely on alternate muscle contractions and relaxations. c. Elastic storage occurs in legs of grasshoppers and fleas, wing hinges of flying insects, hinge ligaments of bivalve molluscs, and the dorsal ligament that supports the head of hoofed mammals. Lecture Enrichment 1. A long balloon can be used to illustrate muscular hydrostats; when the “muscle cell” balloon shortens, it becomes fatter and when other muscles that wrap around it constrict, the “muscle cell” is elongated. 2. The continuous restructuring of bone during growth is a process that has no analogies in the human construction world; it is somewhat like expanding a small brick room into a large brick building without ever allowing the structure to become weak from removing bricks or wedging new ones in! Osteoblasts and osteoclasts can accomplish what we cannot. 3. This is a good place to begin the concept that the blood maintains a constant pool of nutrients (e.g. the amino acid pool, the calcium pool, etc.) by organs and tissues storing or releasing the nutrients based on hormonal responses. 4. Bears that hibernate are not using their bones; yet they do not suffer bone loss and we do not yet know the reason. Discovering this mechanism might provide a basis for astronauts having bone loss while in space. Commentary/Lesson Plan Background: Cartilage is rather commonly seen by anyone eating chicken breasts; the keel underneath the long spears of white meat tapers from bone into milky and bendable cartilage. Misconceptions: Few students realize that the surface of other people that we see is dead skin cells; to inspect living tissues directly, you would have to look into someone’s eye with a special scope. Students often conceptualize bone as relatively nonliving since it is “rock-hard”; in reality, bone is active tissue heavily infused with blood vessels and indeed, bone surgery is very bloody, while cartilage is the inactive and bloodless tissue. High school biology texts usually explain all muscles as antagonist pairs. Therefore in structures that lack bones, the fact that muscle cells never forcibly extend themselves is not intuitive and appears to contradict our ability to lick stamps, etc. The textbook explanation of muscular hydrostats will be new to many students. The reduction in the number of skull bones over time, and the loss of bones in limbs, are again cases where evolution proceeds to lose or fuse bones and reduce the number of structures, in contradiction to many students’ understanding of evolution as an increase in number and complexity. From general biology discussions which relate many traits to number of cells, students may incorrectly assume that bigger muscles are due to more cells. Schedule: HOUR 1 29.1. Integument A. General B. Invertebrate Integument C. Vertebrate Integument and Derivatives 29.2. Skeletal Systems A. Hydrostatic Skeletons B. Rigid Skeletons C. Notochord and Cartilage D. Bone HOUR 2 29.3. Animal Movement A. Mechanism B. Ameboid Movement C. Ciliary and Flagellar Movement D. Muscular Movement E. Muscle Performance ADVANCED CLASS QUESTIONS: 1. Why does a suntan soon wear off? If the melanin molecule is stable and does not break down, where does the melanin go? Answer: A suntan fades because your skin is constantly renewing itself. The outer layer of your skin, called the epidermis, sheds gradually over time, and as new cells replace old ones, the tan eventually fades away. Melanin, the pigment responsible for the tan, doesn't disappear; rather, it gets gradually broken down and recycled as the skin renews itself. So, while the melanin molecules themselves may be stable, they're eventually dispersed and eliminated as the skin naturally exfoliates. 2. Many katydids and long-horned grasshoppers are a strong green color when living, but after being dried in an insect collection, slowly “fade” to a yellow color. Why? Answer:The green coloration in katydids and long-horned grasshoppers is often due to pigments in their exoskeletons, such as chlorophyll or other compounds. When these insects are alive, these pigments contribute to their vibrant green appearance. However, when the insects are dried for preservation in insect collections, the pigments gradually break down or degrade due to exposure to light, air, and other environmental factors. This breakdown process can cause the green color to fade, resulting in a yellowish or brownish hue over time. Additionally, the drying process itself can alter the physical structure of the exoskeleton, further affecting the appearance of the insect. 3. When a mother is pregnant, the embryo or fetus has priority on nutrients such as calcium. Why is tooth and bone erosion a problem for pregnant women—detail exactly what sequences of physiological events transfer calcium from her teeth to the infant—and how does a physician prevent this loss? Answer: During pregnancy, the developing fetus requires a significant amount of calcium for the formation of bones, teeth, and other tissues. If the mother's diet does not provide enough calcium to meet the needs of both herself and the fetus, the body prioritizes the fetus' calcium requirements, often at the expense of the mother's own calcium reserves. This can lead to tooth and bone erosion in pregnant women. The sequence of physiological events that transfer calcium from the mother's teeth to the fetus involves several mechanisms: 1. Increased Demand: As the fetus grows, its demand for calcium increases, especially during the second and third trimesters when bone and tooth development peak. 2. Hormonal Changes: Pregnancy hormones, particularly estrogen, can affect calcium metabolism. Estrogen can enhance the efficiency of calcium absorption from the intestines but can also lead to increased calcium loss from the mother's bones and teeth. 3. Calcium Mobilization: If the mother's dietary intake of calcium is insufficient to meet the demands of the fetus, the body may mobilize calcium from the mother's bones and teeth to supply the growing fetus. 4. Demineralization: Calcium is released from the mother's bones and teeth through a process called demineralization, where calcium ions are dissolved and transferred into the bloodstream. In teeth, this process can lead to weakened enamel and increased susceptibility to tooth decay. To prevent tooth and bone erosion in pregnant women, physicians typically recommend the following measures: 1. Calcium Supplementation: Pregnant women are often advised to take calcium supplements to ensure an adequate intake of this mineral. These supplements can help meet the increased calcium demands of both the mother and the developing fetus. 2. Dietary Changes: A diet rich in calcium-containing foods such as dairy products, leafy green vegetables, nuts, and fortified foods can help support maternal and fetal calcium needs. 3. Regular Dental Care: Pregnant women should maintain good oral hygiene practices, including regular brushing, flossing, and dental check-ups. Dental cleanings and treatments for tooth decay should be addressed promptly to minimize the risk of dental erosion. 4. Monitoring: Physicians may monitor the mother's calcium levels through blood tests to ensure that both she and the fetus are receiving adequate amounts of this essential mineral. By addressing these preventive measures, healthcare providers can help minimize the risk of tooth and bone erosion in pregnant women while supporting the healthy development of the fetus. 4. When a person has big biceps, do they have more muscle cells or just larger cells? Why? Answer: When a person has big biceps, it's primarily due to an increase in the size of individual muscle cells, not necessarily an increase in the number of muscle cells. This phenomenon is known as hypertrophy. Here's why: 1. Hypertrophy: With regular resistance training, such as weightlifting, muscle cells undergo hypertrophy, which means they increase in size due to an increase in the volume of their cellular components, particularly the contractile proteins like actin and myosin. This hypertrophic response is a result of the muscle cells adapting to the stress placed upon them by resistance exercise. 2. Limited Hyperplasia: While muscle hypertrophy is the primary mechanism for muscle growth in response to exercise, there is limited evidence suggesting that new muscle cells (hyperplasia) can form in response to extreme training stimuli or injury. However, the extent to which hyperplasia contributes to overall muscle growth in humans is still a subject of debate among researchers. 3. Genetic Factors: The potential for muscle hypertrophy can vary among individuals due to genetic factors. Some people may have a greater propensity for muscle growth based on their genetic makeup and responsiveness to resistance training. So, while muscle cells can indeed increase in size (hypertrophy), the increase in muscle mass associated with big biceps is primarily due to the enlargement of existing muscle cells rather than the formation of new ones. 5. What is the physiological cause of some muscles having fine control and other muscles having very coarse control? Answer: The physiological cause of some muscles having fine control while others have coarse control lies in the organization and innervation of muscle fibers by motor neurons in the nervous system. 1. Muscle Fiber Types: Skeletal muscles consist of different types of muscle fibers, mainly categorized as slow-twitch (Type I) and fast-twitch (Type II) fibers. Slow-twitch fibers are more fatigue-resistant and are responsible for sustained, low-intensity contractions, providing fine control. Fast-twitch fibers, on the other hand, generate more force and are used for rapid, high-intensity movements, offering less precision but greater power. 2. Motor Unit Recruitment: Motor units are composed of a motor neuron and the muscle fibers it innervates. Fine control muscles typically have smaller motor units, with fewer muscle fibers per motor neuron. This allows for precise control of movement because individual motor neurons can selectively activate small groups of muscle fibers, facilitating fine adjustments. 3. Muscle Innervation: Muscles that require fine control are often innervated by a greater number of motor neurons, each controlling a smaller number of muscle fibers. This finer innervation allows for more precise control over the contraction and relaxation of muscle fibers, enabling intricate movements. 4. Muscle Fiber Diameter and Density: Muscles with fine control tend to have smaller muscle fiber diameter and higher fiber density. This structural organization allows for a greater number of muscle fibers to be packed into a given area, enhancing the precision of movement. 5. Neuromuscular Adaptations: With training and practice, the nervous system can refine motor control through neuromuscular adaptations. This includes improved coordination, synchronization of motor unit recruitment, and enhanced proprioception (awareness of body position and movement), all of which contribute to finer control over specific muscles. In contrast, muscles with coarse control, such as those involved in powerful movements like lifting heavy weights or jumping, typically have larger motor units with a greater number of muscle fibers per motor neuron. This allows for simultaneous activation of multiple muscle fibers, generating more force but sacrificing precision in movement control. 6. What is the physiological difference between a small muscle acting to pick up a pencil and the same muscle acting to lift a large weight? Answer: The physiological difference between a small muscle acting to pick up a pencil and the same muscle acting to lift a large weight lies in the recruitment of muscle fibers and the magnitude of force generated by those fibers. 1. Motor Unit Recruitment: When performing a task like picking up a pencil, which requires minimal force, only a small number of motor units are recruited within the muscle. These motor units consist of motor neurons and the muscle fibers they innervate. For fine, delicate movements, the nervous system selectively activates smaller motor units, which contain slow-twitch muscle fibers. These muscle fibers are fatigue-resistant and generate low levels of force but provide precise control over movement. 2. Force Production: In contrast, when lifting a large weight, the nervous system recruits a greater number of motor units within the muscle. This recruitment involves larger motor units, which contain fast-twitch muscle fibers. Fast-twitch fibers are capable of generating higher levels of force but fatigue more quickly compared to slow-twitch fibers. By activating more motor units and fast-twitch fibers, the muscle can generate the necessary force to lift the heavy weight. 3. Muscle Fiber Type Composition: Muscles involved in fine, precise movements like picking up a pencil are composed primarily of slow-twitch muscle fibers. These fibers are adapted for endurance and provide sustained, low-intensity contractions. In contrast, muscles involved in lifting heavy weights contain a higher proportion of fast-twitch muscle fibers, which are optimized for generating quick, powerful contractions. 4. Energy Requirements: The energy demands placed on the muscle differ depending on the task. Fine movements require relatively low levels of energy expenditure since they involve small, controlled contractions of muscle fibers. Lifting heavy weights, on the other hand, requires more energy due to the recruitment of a larger number of muscle fibers and the generation of higher force levels. Overall, the physiological difference between a small muscle acting to pick up a pencil and the same muscle acting to lift a large weight lies in the recruitment pattern of muscle fibers, the magnitude of force generated, and the energy requirements of the task. Instructor Manual for Integrated Principles of Zoology Cleveland Hickman, Jr., Susan Keen, Allan Larson, David Eisenhour, Helen I'Anson, Larry Roberts 9780073524214