Chapter 28. Features. Mammals. History. Diversity. Therapsids Lineage. Cynodonts

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1 Chapter 28 Mammals Features Hair is a critical sign of being a mammal. A few mammals, especially aquatic forms, may have very few hairs but they are still present. Hair serves many functions: protection, concealment, waterproofing and buoyancy, signaling, sensitive vibrissae and especially thermal insulation. Mammals have other characteristic features. Most have a specialized placenta to feed the embryo. The mammal nervous system is more advanced than in other animal groups. Mammary glands nourish the newborn. 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. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Diversity About 4600 living species are known. Nevertheless they are the most highly differentiated group in the animal kingdom. They have been domesticated for use as food, clothing, pets, beasts of burden and in research. Exotic mammal introductions have usually disrupted the ecology. Habitat destruction is a factor in 300 species and subspecies being considered endangered. History The evolution of mammals from their earliest amniote ancestors is very fully documented. Over the last 150 million years, small, ectothermic, hairless ancestors evolved into today s endothermic, furry mammals. Skull structures, especially teeth, provide abundant evidence of the evolutionary descent. The skull roof separates amniotes into synapsids, anapsids and diaspids. Mammals derive from the synapsids that have a pair of openings in the skull roof. The synapsids were also the first amniotes to radiate widely into terrestrial habitats. The earliest synapsids radiated into diverse herbivorous and carnivorous pelycosaurs. From one of the early carnivorous synapsids arose the therapsids. The therapsids were the only synapsid group to survive beyond the Paleozoic. Therapsids were the 1st to have an efficient erect gait with upright limbs beneath the body. This reduced stability required the cerebellum to have an expanded role in muscle coordination. Most of the variety of herbivores and carnivores disappeared in the Permian extinction. Only the last therapsids subgroup to evolve, the cynodonts, survived into the Mesozoic. Therapsids Lineage Cynodonts This group evolved a high metabolic rate that supported a more active life. They have more jaw musculature and skeletal changes for greater agility. A secondary bony palate allowed them to breathe while holding prey or chewing food. This secondary palate would be important later to mammal evolution by allowing young to breathe while suckling. Bones improved in biomechanics and developed processes for firmer muscle attachment. The number of ribs was reduced, making the spinal column more flexible. Within the cynodont clade, a small carnivorous group, called trithelodontids, resembles mammals. Loss of lumbar ribs in cynodonts is correlated with the evoluion of a diaphragm and also may have provided greater dorso-ventral flexibility of the spinal column. 1

2 Early Mammals of the Triassic Period Fig The earliest mammals of the late Triassic were small and mouse- or shrew-sized. They were diphyodonts; teeth were replaced only once as deciduous and permanent teeth. They were almost certainly endothermic although cooler than modern placental mammals. Hair was essential and also indicates that sebaceous and sweat glands were present. There is no fossil evidence, but mammary glands must have evolved before the end of the Triassic. Young early mammals must have hatched from eggs and relied on maternal milk. Mammals, having developed in the mid-triassic, had to wait 150 million years to diversify. All non-mammalian synapsid groups became extinct when the dinosaurs became abundant. Cenozoic Radiation of Mammals Mammals survived first as shrew-like nocturnal animals, then in a radiation in the Eocene Epoch. The radiation is attributed to the many habitats vacated by extinction of many amniote groups at the end of the Cretaceous. Mammals were agile, endothermic, intelligent, adaptable, and gave birth to young they protected. With the evolution of a new jaw joint between the dentary and squamosal (temporal) bones in mammals, bones of the previous jaw joint, the articular and quadrate, continued their gradual reduction in size and became relocated in the middle ear, where they became the malleus and incus, respectively. Skin Mammals skin is generally thicker than in other classes of vertebrates. As with all vertebrates, skin is made of epidermis and dermis. In mammals, the dermis becomes much thicker than the epidermis. The epidermis is thinner and well protected by hair. In places that are subject to abrasion, the outer layers become thicker and cornified with keratin. Hair Hair is characteristic of mammals; it is reduced on humans and exists as a few bristles on whales. The hair follicle is an epidermal structure, but is sunk into the dermis of the skin. A hair grows continuously by rapid proliferation of cells in the follicle. Cells in the hair shaft are carried upward away from their source of nourishment and die. The dense protein keratin is the same protein as is found in nails, claws, hooves and feathers. Dense and soft underhair serves for insulation by trapping a layer of air. Coarse and longer guard hairs protect against wear and provide coloration. Hair Hair consists of three layers The medulla or pith is in the center of the hair. The cortex with pigment granules is next to the medulla. The outer cuticle is composed of imbricated scales Different mammals have unique hair structure. Hair stops growing at a certain length; it remains in the follicle until new growth pushes it out. 2

3 Hair In most mammals, there are periodic molts of the entire coat. Foxes and seals shed once every summer. Most mammals molt twice, in the spring and in the fall, with the winter coat much heavier. Some have white winter coats for camouflage and brown summer coats. Arctic mammals are not genetically albino where eye and skin pigments are also missing. White winter fur is leukemism; they have dark eyes, dark-colored ear tips, noses, etc. Patterns including spots, stripes, salt-and-pepper, etc. are disruptive and conceal the animal. Vibrissae or whiskers are sensory hairs; they provide a tactile sense to nocturnal mammals. Porcupine, hedgehog, and echidna quills are barbed and break off easily. Glands Mammals have the greatest variety of integumentary glands; all are derived from the epidermis. Sweat glands are tubular, highly coiled glands found in mammals but never in other vertebrates Eccrine Sweat Glands Apocrine Sweat Glands Scent Glands Present in nearly all mammals, they vary greatly in location and function. They communicate with members of the same species: mark territory, warning and defense. Scent-producing glands are located in many different regions in different mammals. The scent glands of skunks, minks and weasels open into the anus and are very odoriferous. Many mammals give off strong scents during the mating season to attract the opposite sex. Sebaceous Glands Most are associated with hair follicles although some open directly onto the surface. Cells in the cellular lining accumulate fats, then die and are expelled to form oily sebum. It does not turn rancid but serves as a dressing to keep the skin and hair pliable and glossy. Most mammals have sebaceous glands over the entire body. Mammary Glands Mammary glands are probably modified apocrine glands. They are rudimentary in males and occur on all female mammals. The epidermis thickens to form a milk line along which mammae appear. Human females develop mammary glands at puberty with fat accumulation; additional development occurs during pregnancy. Other mammals have swollen mammae periodically when pregnant or nursing. 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). Food and Feeding Mammals exploit a wide variety of food sources; some are specialists and others are generalists. Mammal structures are closely associated with adaptations for food finding or capturing. 3

4 Teeth Structure or teeth reveal the life habits of a mammal. Reptiles had homodont dentition or uniform tooth patterns. Differentiation of teeth for cutting, seizing, gnawing, etc. resulted in heterodont dentition. Types of Teeth Incisors have sharp edges for snipping or biting. Canines are specialized for piercing. Premolars have compressed crowns with one or two cusps for shearing and slicing. Molars have larger bodies and variable cusp arrangements for crushing and grinding. Teeth A primitive mammal tooth formula is three incisors, one canine, four premolars and three molars. Mammals do not continually replace teeth; they have one deciduous set and a permanent set. Generally, the incisors, canines and premolars are deciduous; molars are a single permanent set. Specializations for Feeding Insectivores Shrews, moles, anteaters and most bats are insectivores. They eat little fibrous vegetable matter so their digestive tract is short. Many other mammals occasionally feed on insects, making this distinction blurred. Specializations for Feeding Herbivores Browsers and grazers include horses, deer, antelope, cattle, sheep and goats. Herbivores have reduced or absent canines, but molars are broad and highcrowned. Rodents have chisel-shaped incisors that grow throughout life. Cellulose is a chain of glucose molecules, but the chemical bonds are difficult to break. Herbivores use anaerobic fermentation chambers so microorganisms can metabolize cellulose. A side pocket or cecum may also serve as a fermentation chamber and absorptive area. Hares, rabbits, and some rodents eat fecal pellets in order to provide additional fermentation. Specializations for Feeding Herbivores Ruminants have a huge four-chambered stomach. Food is regurgitated, rechewed, and passed to the rumen, reticulum, omasum and abomasum. Herbivores generally have long digestive tracts for the prolonged time needed to digest fiber. 4

5 Specializations for Feeding Carnivores Most carnivores feed on herbivores. This requires specialization for killing the prey. A high protein diet is easily digestible and therefore the digestive tract is shorter. Carnivores do not have to continuously graze and they have more leisure time. Capturing prey also requires more intelligence, stealth, and cunning. Other Feeding Specializations Predation has driven herbivores to have keen senses and escape behaviors. Some herbivores use size (i.e. rhinos, elephants) or defensive group behaviors. Omnivores Omnivores feed on both plant and animal tissues. Many carnivores will switch to fruits, berries, etc. when normal food is scarce. Some mammals cache food stores during times of plenty, a common behavior of rodents. Body Weight and Food Consumption Fig The smaller the animal, the greater is its metabolic rate and the more it must eat per unit size. The amount of food varies in proportion to the body surface area rather than the body weight. Surface area is proportional to about 0.7 power of body weight. Amount of food a mammal or bird eats is also about 0.7 power of body weight. A 3 gram mouse will consumer 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. Small mammals must spend much more time hunting and eating food than do large mammals. 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. In contrast, a mountain lion may kill an average of one deer a week. Reproductive Cycles Most mammals have mating seasons timed to coincide with most favorable time to rear young. Female mammals usually restrict mating to a fertile period during the periodic estrous cycle. This time of female receptivity is known as heat or estrous. 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. Animals with only one breeding season a year are monestrous; recurrent breeding is polyestrous. Reproductive Patterns Egg-Laying Monotremes Monotremes, such as the duck-billed platypus, lay eggs with one breeding season per year. Eggs are fertilized in the oviduct before albumin and a thin, leathery shell are added. She lays eggs in a burrow nest where they are incubated for 12 days. Similar to reptiles and birds, there is no gestation and the egg provides all nutrients. However, after hatching, young suck milk from the mother s fur near her mammary glands. 5

6 Reproductive Patterns Pouched Marsupials Marsupials are pouched, viviparous mammals. Although only eutherians are placental mammals, marsupials do have a primitive choriovitelline placenta. The embryo is first encapsulated by shell membranes and floats free for several days. After hatching from shell membranes, the embryo erodes a shallow depression in the uterine wall and absorbs nutrient secretions by a vascularized yolk sac. Gestation is brief and marsupials give birth to tiny young that are still embryos. Early birth is followed by a prolonged interval of lactation and parental care. In red kangaroos, the first pregnancy is followed by a 33-day gestation and then birth. The mother immediately becomes pregnant, but the presence of a suckling young arrests development of the new embryo at the 100-cell stage. Such a period of arrest is called embryonic diapause. It is possible to stairstep three young with one external, one suckling, and one embryonic. Placental Mammals Eutherians are viviparous placental mammals. They have an investment in a prolonged gestation in contrast to marsupials with an investment in prolonged lactation. The embryo in the uterus is nourished through a chorioallantoic placenta. Gestation is longer than in marsupials and is much longer for large mammals. Gestation and body size are loosely correlated because there is variation in maturity at birth. Humans are slower developing than any other mammal; this contributes to our uniqueness. Reproductive Patterns Fig Reproduction Patterns The ultimate number of young produced per year also depends on mortality rate. Small rodents that are prey for carnivores usually produce more than one litter each season. Meadow mice can produce up to 17 litters of four to nine young each year! At the other extreme, an elephant produces on average four calves during her 50-year life. The End. 6