Mammals Chapter 28
Class Mammalia • Mammals, class Mammalia, are represented by more than 5,000 species. • Kitti’s hognosed bat from Thailand weighs 1.5 g. • Blue whales exceed 130 metric tons.
Derived Characters of Mammals • Mammary glands, which produce milk are a distinctively mammalian character. • Hair is another mammalian characteristic. • Mammals generally have a larger brain than other vertebrates of equivalent size.
Early Evolution of Mammals • Mammals evolved from synapsids in the late Triassic period. • The earliest synapsids radiated into diverse herbivorous & carnivorous forms – the pelycosaurs.
Early Evolution of Mammals • One group of carnivorous pelycosaurs led to the therapsids. • The only synapsid group to survive into the Mesozoic. • Limbs positioned under the body.
Early Evolution of Mammals • Cynodonts were a group of therapsid reptiles that radiated in the Mesozoic. • One branch of cynodonts eventually led to the early therians (marsupial and placental mammals).
Early Evolution of Mammals • Cynodontsevolved: • A high metabolic rate that supported a more active life. • Enhanced jaw musculature • A secondary bony palate permits breathing while holding prey or chewing food. • Important later to mammal evolution by allowing young to breathe while suckling
Early Evolution of Mammals • Turbinate bones in the nasal cavity aided in the retention of heat.
Early Evolution of Mammals • The jaw was remodeled during the evolution of mammals from nonmammalian synapsids. • Two of the bones that formerly made up the jaw joint were incorporated into the mammalian middle ear.
Early Evolution of Mammals • Living lineages of mammals originated in the Jurassic, but did not undergo a significant adaptive radiation until after the Cretaceous.
Skin & Hair • Skin is composed of the thinner epidermis and the underlying, thicker dermis. • Hair is one of the defining characteristics of mammals. • Composed of dead, keratin-packed epidermal cells.
Hair • Two kinds of hair form the coat of most mammals: • Dense and soft underhair for insulation. • Traps a layer of insulating air. • Extremely dense in aquatic mammals. • Coarse, longer guard hair for protection from wear and coloration.
Hair • Mammals shed or molt once or usually twice each year. • Summer coat usually thinner. • May be a different color.
Hair • Hair can be specialized into bristles, spines, and whiskers. • Whiskers, vibrissae, are sensory hairs that provide tactile sense to many mammals. • Especially long in nocturnal and burrowing mammals.
Horns & Antlers • Horns, like those of sheep & cattle, are hollow sheaths of keratinized epidermis with a core of bone. • Found in both sexes, grow continuously, are not shed.
Horns & Antlers • Antlers, like those of deer, are branched and composed of solid bone. • During growth, they have a highly vascular soft skin called velvet covering them. • Usually only males have antlers. • Both sexes of Caribou have antlers.
Glands • Mammals have the greatest variety of integumentary glands of any vertebrate. • Sweat glands – involved in heat regulation. • Scent glands – for marking territories, warning, defense. • Sebaceous glands – associated with hair follicles. • Mammary glands – provides milk for offspring, gives mammals their name.
Food and Feeding • Mammals eat a wide variety of food. • Dentition often reflects the food they eat. • Heterodont dentition
Food and Feeding • Mammals also have adaptations that help them digest the different types of foods they eat. • Herbivores can’t digest cellulose – they rely on microorganisms to metabolize cellulose.
Body Weight and Food Consumption • The smaller the animal, the higher its metabolic rate. • More food must be consumed.
Migration • Some terrestrial mammals, like caribou and bison, migrate twice annually between summer and winter ranges.
Migration • Migration is more common among oceanic whales & seals.
Flight and Echolocation • Many mammals can glide from tree to tree. • Not powered flight. • Gliding & flying evolved independently.
Flight and Echolocation • Bats have evolved true flight. • Mostly nocturnal or crepuscular. • Echolocation is used to navigate and locate food.
Territoriality • Many mammals defend a territory that includes resources such as food, shelter, or mates that is defended from others usually of the same species (conspecifics) – those that would utilize the same resources.
Territoriality • A mammal usually has a larger, undefended home range that may overlap with the home range of conspecifics.
Monotremes • Monotremes are a small group of egg-laying mammals consisting of echidnas and the platypus.
Monotremes • Monotremes are oviparous mammals. • Eggs have a thin, leathery shell. • Babies hatch in a relatively undeveloped state. • Echidnas have an abdominal pouch where they keep the young. • Young feed on milk secreted by the mother’s mammary glands.
Marsupials • Marsupials include opossums, kangaroos, and koalas.
Marsupials • Before it is a born, an embryonic marsupial receives nourishment by absorbing nutrient secretions in the uterus.
Marsupials • A marsupial is born very early in its development and completes its embryonic development while nursing within a maternal pouch called a marsupium.
Convergent Evolution • In Australia, convergent evolution has resulted in a diversity of marsupials that resemble eutherians (placental mammals) in other parts of the world.
Eutherians – Placental Mammals • Compared to marsupials, eutherians have a longer period of pregnancy. • Usually larger placental mammals have longer gestations.
Eutherians – Placental Mammals • Young eutherians complete their embryonic development within a uterus, joined to the mother by the placenta.
Primates • The mammalian order Primates include: • Lemurs, tarsiers, monkeys, and apes • Humans are members of the ape group.
Primates • Most primates have hands and feet adapted for grasping. • Primates also have: • A large brain and short jaws. • Forward-looking eyes close together on the face, providing depth perception. • Well-developed parental care and complex social behavior. • A fully opposable thumb.
Living Primates • There are three main groups of living primates: • The lemurs of Madagascar and the lorises and pottos of tropical Africa and southern Asia form one group.
Living Primates • The tarsiers of Southeast Asia.
Living Primates • The anthropoids, which include monkeys and hominids worldwide.
Primate Evolution • The oldest known anthropoid fossils are about 45 million years old. • Indicate that tarsiers are more closely related to anthropoids.
Primate Evolution • The fossil record indicates that monkeys first appeared in the New World (South America) during the Oligocene (~36 mya). • The first monkeys evolved in the Old World (Africa and Asia) about 40 mya.
Primate Evolution • New World and Old World monkeys underwent separate adaptive radiations during their many millions of years of separation.
Primate Evolution • The other group of anthropoids, the hominoids, consists of primates informally called apes. • Hominoids diverged from Old World monkeys about 20–25 million years ago.
Primate Evolution • African apes – chimpanzees & gorillas have been moved into the familyHominidae to more accurately reflect relatedness. • So, now, humans, chimps and gorillas are hominids. • Many researchers advocate placing them in the genus Homo as well.
Humans – Bipedal Hominoids • Homo sapiens is about 200,000 years old which is very young considering that life has existed on Earth for at least 3.5 billion years.
Derived Characters of Hominids • A number of characters distinguish humans from other hominoids: • Upright posture and bipedal locomotion • Larger brains • Language capabilities • Symbolic thought • The manufacture and use of complex tools • Shortened jaw
Human Origins • The study of human origins is known as paleoanthropology. • Paleoanthropologists have discovered fossils of approximately 20 species of extinct hominoids that are more closely related to humans than to chimpanzees.
Human Origins October 2, 2009 issue of Science is full of articles on Ardipithicus ramidus. Available for free! http://www.sciencemag.org/ardipithecus/
Human Origins • Hominins (modern humans and their extinct ancestors are placed in the subfamily Homininae) originated in Africa approximately 6–7 million years ago. • Early hominins had a small brain, but probably walked upright, exhibiting mosaic evolution (different traits evolving at different rates).