Mammals

The Tell-Tale Hair
If Fuzzy Wuzzy, the bear that had no hair (according to the children’s rhyme), was truly hairless, he could not have been a mammal or a bear. For hair is as much an unmistakable characteristic of mammals as feathers are of birds. If an animal has hair it is a mammal; if it lacks hair it must be something else. It is true that many aquatic mammals are nearly hairless (whales, for example) but hair can usually be found (with a bit of searching) at least in vestigial form somewhere on the body of the adult. Unlike feathers, which evolved from converted reptilian scales, mammalian hair is a completely new epidermal structure. Mammals use their hair for protection from the elements, for protective coloration and concealment, for waterproofing and buoyancy, and for behavioral signaling; they have turned hairs into sensitive vibrissae on their snouts and into prickly quills. Perhaps most important of all, mammals use their hair for thermal insulation, which allows them to enjoy the great advantages of homeothermy. Warmblooded animals in most climates and at sunless times benefit from this natural and controllable protective insulation.

Hair, of course, is only one of several features that together characterize a mammal and help us to understand the mammalian evolutionary achievement. Most mammals have a highly developed placenta for feeding the embryo; mammary glands for nourishing the newborn; and a surpassingly advanced nervous system that far exceeds in performance that of any other animal group. It is doubtful, however, that even with this winning combination of adaptations, the mammals could have triumphed as they have without their hair.

Mammals, with their highly developed nervous system and numerous ingenious adaptations, occupy almost every environment on earth that supports life. Although not a large group (about 4600 species as compared with more than 9000 species of birds, approximately 24,600 species of fishes, and 800,000 species of insects), the class Mammalia (mam-may´lee-a) (L. mamma, breast) is overall the most biologically differentiated group in the animal kingdom. Many potentialities that dwell more or less latently in other vertebrates are highly developed in mammals. Mammals are exceedingly diverse in size, shape, form, and function. They range in size from the recently discovered Kitti’s hognosed bat in Thailand, weighing only 1.5 g, to blue whales, exceeding 130 metric tons.

Yet, despite their adaptability and in some instances because of it, mammals have been influenced by the presence of humans more than any other group of animals. We have domesticated numerous mammals for food and clothing, as beasts of burden, and as pets. We use millions of mammals each year in biomedical research. We have introduced alien mammals into new habitats, occasionally with benign results but more frequently with unexpected disaster. Although history provides us with numerous warnings, we continue to overcrop valuable wild stocks of mammals. The whale industry has threatened itself with total collapse by exterminating its own resource—a classic example of self-destruction in the modern world, in which competing segments of an industry are intent only on reaping all they can today as though tomorrow’s supply were of no concern whatever. In some cases destruction of a valuable mammalian resource has been deliberate, such as the officially sanctioned (and tragically successful) policy during the Indian wars of exterminating the bison to drive the Plains Indians into starvation. Although commercial hunting has declined, the ever-increasing human population with the accompanying destruction of wild habitats has harassed and disfigured the mammalian fauna. Approximately 300 species and subspecies of mammals are considered endangered by the International Union for the Conservation of Nature and Natural Resources (IUCN), including most cetaceans, cats (except domestic cats), otters, and primates (except humans).

We are becoming increasingly aware that our presence on this planet as the most powerful product of organic evolution makes us responsible for the character of our natural environment. Since our welfare has been and continues to be closely related to that of the other mammals, it is clearly in our interest to preserve the natural environment of which all mammals, ourselves included, are a part. We need to remember that nature can do without humans but humans cannot exist without nature.

Position in the Animal Kingdom
Modern mammals are descendants of the synapsid lineage of amniotes that appeared in the Permian period. The synapsid lineage is characterized in the primitive condition by having a skull with a single temporal opening (Figure 30-1). Modern mammals are endothermic and homeothermic, have bodies partially or wholly covered with hair, and have mammary glands that secrete milk for the nourishment of the young. These derived characteristics, together with several distinctive skeletal characteristics, a highly developed nervous system, and complex individual and social behavior, distinguish the mammals from all other amniotes. Their genetic plasticity and numerous derived adaptations have enabled mammals to invade almost every environment on earth that supports life.

Biological Contributions

1. Mammals share with birds both endothermy and homeothermy which permit a high level of activity at night, and year-round penetration into low temperature habitats denied to ectothermic vertebrates.
2. The placenta in placental mammals allows developing young to obtain nourishment and grow in a protected environment during the most vulnerable period of their lives. After birth the young continue to feed by suckling from mammary glands. A long period of parental care and education allows the young to acquire skills necessary for survival.
3. Specialization of mammalian teeth for different functions permitted the evolution of many different feeding specializations in mammals. The secondary palate, which separates the air passageway from the food passageway, enables mammals to hold and partially break down food in their mouths without interrupting breathing.
4. The highly evolved brain, especially the large neocortex, has bequeathed mammals with a well developed memory and the capacity to learn rapidly and to respond appropriately to problems not previously encountered. Highly elaborated sense organs and special senses, particularly those of hearing, smell, and touch, contribute an inflow of environmental information that, together with their processing brain centers, provide mammals with a level of environmental awareness and responsiveness unequaled in the animal kingdom.