Chapter 28

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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.

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.

Early Mammals of the Triassic Period

• 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.

Fig. 28.3

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.

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.

Source

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.

Source

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 high crowned.

• 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, re chewed, and passed to the rumen, reticulum, omasum and abomasum.

• Herbivores generally have long digestive tracts for the prolonged time needed to digest fiber.

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

• 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.

Fig. 28.14

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.

Source

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. Source Source

• 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

Source

Patterns

Fig. 28.21

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.