Biology 201Dr. Edwin DeMont

St. Francis Xavier University

Chapter 22Mammals: Specialized Teeth, Hair,

Endothermy and Viviparity

Part 1

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Mammals

Mammals are distributed on all continents except

Antarctica, and they live in all oceans.

Some of the most distinctive features of mammals involve adaptations of the epidermis and the skeletal system.

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Epidermis

Notable features of mammalian skin are:

- Hair (with arrector pili muscle)- Large variety of epidermal glands- Highly stratified epidermis.

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Epidermis: Hair

Hair is a unique mammalian structure. The purpose of hairs includes conservation of body heat, protection (porcupine quills), warning coloration (rump patches on antelopes),

camouflage (baby deer), and sensation (whiskers or vibrissae on carnivores).

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Epidermis: Hair

Hair is greatly reduced in most marine mammals (except sea otters), as it would increase drag. Marine mammals

have thick layers of fat for insulation.

The sea otter has the greatest number of hairs per area of skin, important in insulating this marine mammal. Otters

have 100,000 hairs/cm2; compare to a dog with 9,000

hairs/cm2.

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Epidermis: Glands

Sebaceous glands are associated with hair follicles

and produce oil.

(2) Large apocrine glands, which can produce products with a smell.

(1) Small eccrine glands produce sweat, which

functions in evaporative cooling in a few mammals

Two types of sudoriferous glands :

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Mammary Glands

Mammary glands are derived from apocrine glands, and function to provide nourishment for the young.

In humans, many ducts lead from the glands to a nipple and parts of the duct system are enlarged to store milk.

Cattle have teats that form by the extension of a collar of skin around the opening of

the mammary ducts.

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Milk

Milk contains water, butterfat, lactose, albumin and salts. The composition of

milk varies with the species.

In human milk, albumin is low, as well as fat (roughly 4%), resulting in slow

growth. In guinea pigs, high albumin in the milk leads to a doubling of weight of

the offspring every few days.

The fat content of milk varies, as high as 25% in sea otters, and

50% in whales

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Teeth

One of the hallmarks of mammalian evolution has been the development of

highly specialized dentition.

Species vary in the number of each type of tooth and the degree of specialization.

Most mammals do have highly adaptive teeth that are used in tearing

grass, grasping prey or crushing bones.

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TeethSome mammals (i.e. humans) are omnivorous; feeding on a variety

of plant and animal material. They have anterior teeth with sharp

ripping and piercing surfaces and posterior teeth with flattened

grinding surfaces for rupturing plant cell walls.

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Teeth

Mammals that eat plant material often have flat, grinding posterior

teeth and have chisel shaped incisors for nipping or gnawing

plant material.

Deer (cow skull looks similar)

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Teeth

Mammals that eat plant material often have flat, grinding posterior

teeth and have chisel shaped incisors for nipping or gnawing

plant material.

A rodent (beaver)

In rodents the incisors grow throughout life.

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Teeth

Canines are typically pointed, and are particularly important to carnivores. They

are usually long, conspicuous, and have one point. They are pointed for capturing and

killing prey.

A carnivore, the coyote

Canines are very enlarged in walruses.

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Digestive System

The digestive system of mammals is similar to that of other

vertebrates, but has specializations that reflects their diets. Ruminants (i.e. cows) have a four-chambered stomach to allow fermentation of

cellulose by microorganisms.

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Ruminants

Ruminants (animals that ‘chew their cud’) have the most

unusual modifications of their stomach. These animals eat grasses and other vegetation

that has cellulose-based walls. Cellulose contains a large

amount of energy but animals generally can’t digest it.

Gut microorganisms can produce cellulase – an enzyme

that can digest cellulose.

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Ruminants

Upper portion expands to form a large pouch – the

rumen and a smaller reticulum.

Lower portion contains a small antechamber - the

omasum and the true stomach (abomasum).

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Ruminants

Food first enters the rumen where it encounters the microorganisms where it is partially digested (heat and churning help). Pulpy mass moved into

reticulum.

Moves into the omasum and abomasum where digestive

enzymes encountered and digestion continues.

Reswallowed food goes back to the rumen where it becomes more liquid.

Mouthful regurgitated (cud) and chewed.

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Temperature Regulation

Mammals are widely distributed and some face harsh environmental conditions. Most face temperatures that require them to dissipate heat at some

times and to conserve and generate heat at other times.

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Temperature Regulation

Mammals may produce heat by (1) shivering thermogenesis and (2)

non-shivering thermogenesis. The second process involves the

metabolism of special fat deposits called brown fat.

Heat production is effective in thermoregulation because mammals can conserve heat several different ways: (1) insulated by hair and/or

fat deposits and (2) with the use of heat exchangers.

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Heat conservation

Countercurrent heat exchangers conserve heat in animals adapted

to cold environments.

In the winter lower part of a reindeer’s leg may

be 10oC while body temperature is 40oC.