Mammals

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.

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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.

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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

• 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.

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Early Evolution of Mammals

• Living lineages of mammals originated in the Jurassic, but did not undergo a significant adaptive radiation until after the Cretaceous.

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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.

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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.

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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.

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Monotremes

• Monotremes are a small group of egg-laying mammals consisting of echidnas and the platypus.

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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.

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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.

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Convergent Evolution

• In Australia, convergent evolution has resulted in a diversity of marsupials that resemble eutherians (placental mammals) in other parts of the world.

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Eutherians – Placental Mammals

• Compared to marsupials, eutherians have a longer period of pregnancy.
• Usually larger placental mammals have longer gestations.

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Eutherians – Placental Mammals

• Young eutherians complete their embryonic development within a uterus, joined to the mother by the placenta.

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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.

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Living Primates

• The tarsiers of Southeast Asia.
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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.

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Primate Evolution

• New World and Old World monkeys underwent separate adaptive radiations during their many millions of years of separation.

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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.

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Primate Evolution

• African apes – chimpanzees & gorillas have been moved into the family Hominidae to more accurately reflect relatedness.
• So, now, humans, chimps and gorillas are hominids.
• Many researchers advocate placing them in the genus Homo as well.
• Check out this article!

Humans – Bipedal Hominoids

• Homo sapiens is about 160,000 years old which is very young considering that life has existed on Earth for at least 3.5 billion years.

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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.
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Human Origins

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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).

Human Origins

• Two common misconceptions of early hominids include:
• Thinking of them as chimpanzees.
• Imagining human evolution as a ladder leading directly to Homo sapiens.

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Human Origins

• Australopiths are a paraphyletic assemblage of hominids that lived between 4 and 2 million years ago.

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Human Origins

• Some species walked fully erect and had human-like hands and teeth.
• Australopith brains were smaller than modern humans, but larger than chimps.

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Bipedalism

• Hominins began to walk long distances on two legs about 1.9 million years ago.

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Tool Use

• The oldest evidence of tool use – cut marks on animal bones – is 2.5 million years old.

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Early Homo

• The earliest fossils that paleoanthropologists place in our genus, Homo, are those of the species Homo habilis, ranging in age from about 2.4 to 1.6 million years.
• Stone tools have been found with H. habilis, giving this species its name, which means “handy man”.

Early Homo

• Homo ergaster was the first fully bipedal, large-brained hominin.
• Existed between 1.9 and 1.6 million years.

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Early Homo

• Homo erectus originated in Africa approximately 1.8 million years ago.
• The first hominin to leave Africa.
• Social – living in tribes of 20-50.

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Neanderthals

• Neanderthals, Homo neanderthalensis lived in Europe and the Near East from 200,000 to 30,000 years ago.
• Large, thick-browed hominins.
• Became extinct a few thousand years after the arrival of Homo sapiens in Europe.

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Homo sapiens

• Homo sapiens appeared in Africa at least 200,000 years ago.

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Homo sapiens

• The oldest fossils of Homo sapiens outside Africa date back about 50,000 years ago.

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Homo sapiens

• The rapid expansion of our species may have been preceded by changes to the brain that made symbolic thought and other cognitive innovations possible.
• Binocular vision, visuotactile discrimination and manipulative skills that resulted from our arboreal ancestry likely played a role as well.