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).

Cynodonts evolved: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

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 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.

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 locomotionLarger brainsLanguage capabilitiesSymbolic thoughtThe manufacture and use of complex toolsShortened 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).

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.

Human Origins

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

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.

Bipedalism

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

Tool Use

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

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 2

and 1.4 million years.

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.

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.

Homo sapiens

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

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.