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Chapter 3: Introduction to small mammals.


After completing this chapter, the student should be able to

* explain what a eutherian is.

* explain what a metatherian is.

* explain what a monotreme is.

* describe what a herbivore eats.

* describe what a carnivore eats.

* describe what an omnivore eats.

Key Terms




















Mammals are a very diverse group within the animal kingdom. They represent a great variety of species, ranging from the very smallest of the known mammals, Kitli's Hog-nosed Bat, which weighs considerably less than ounce, to the greatest of all living creatures, the Blue Whale, with an estimated weight of 150 tons. Although the range of mammalian species is vast, they all share distinguishing characteristics.

All mammals are vertebrates, possessing a spinal column that is part of the endoskeleton, a framework of bones which provides structure and support to the body. Mammals have a highly developed central nervous system and brain. They are all endotherms (warm-blooded), have hair, and produce milk to feed their young.

Mammals are separated into three distinctive groups based primarily on the manner in which their young develop prior to birth. Most familiar are the eutherians, those mammals with a placenta that nourishes the developing young, connecting them to the uterus until the time of birth. The young of some species are precocial. They are fully developed at birth, with their eyes open and a full coat of hair and they are able to function with a degree of independence. A few of these species include the degu, chinchilla, and guinea pig. More commonly the young are delivered altricial. They are born with their eyes and ears closed and have no visible hair growth. They are entirely dependent on maternal care for survival. Examples of these species are mice and rats, rabbits, and ferrets.

The second group is metatheria, or marsupials. The young of all marsupials are born in an embryonic state. They must make their way to either a pouch or teat attachment to complete their development. Two examples of marsupials commonly kept as pets are sugar gliders and Brazilian short-tailed opossums.

The third group, monotremes, are mammals which lay eggs. There are only two living species of monotremes, the duck-billed platypus and the echidna, a type of spiny ant-eater. While they have many features characteristic to other mammals, adult monotremes do not possess teeth. The echidna has a specialized snout for probing into anthills and a very long tongue, while the platypus has a bill, not dissimilar to that of some ducks, which is used to probe underwater in search of food items.

With the exception of the monotremes, all mammals have two sets of teeth. The deciduous or baby teeth are shed and replaced by permanent, adult teeth. If adult teeth are lost they are not replaced.

Other features unique to mammals include the structure of the lower jaw, or mandible. The mandible consists of one bone in mammals, and many smaller bones in nonmammals. Fossil evidence in the evolution of mammals has determined that these smaller bones of the lower jaw were once present but have evolved to form the bones of the middle ear in mammals. They have become the malleus (the hammer), the incus (the anvil), and the stapes (the stirrup).

Additionally, mammals possess a diaphragm, a strong muscle which assists in respiration. It is absent in birds and reptiles. Mammals also possess a soft and a hard palate which separate the esophagus and the trachea.

Mammals consume a variety of diets. They may be generally classified as herbivores, animals which eat only plant material, carnivores, those which eat only meat, and omnivores, those mammals which eat both plants and other animals. Some mammals may be referred to as insectivores and these feed primarily on insects.

Review Questions

(1) What is one reason mammals are divided into groups?

(2) Give definitions for the following groups of animals:

a. eutherians

b. metatherians

c. monotremes

(3) What is the function of the placenta?

(4) How does the structure of the mammalian mandible differ from other classes of animals?

(5) What are the names of the bones of the middle ear?

(6) What are the deciduous teeth?

(7) A diaphragm is unique to mammals. What is the function?

(8) Define the following terms:

a. herbivore

b. carnivore

c. omnivore

d. insectivore

(9) The young of mammals are described as being either precocial or altricial. What is the difference between these two?

Further Reading

Earth-Life Web productions,, April 06 (accessed December 4, 2006). (accessed December 4, 2006).
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Title Annotation:UNIT II
Publication:Exotic Animal Care and Management
Geographic Code:1USA
Date:Jan 1, 2008
Previous Article:Chapter 2: Zoonotic diseases.
Next Article:Chapter 4: Ferrets.

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