Characteristics Of Mammals

Mammals are air breathing vertebrate animals; a class with highly evolved internal and external characteristics. ‘Most intelligent animals’, like the primates including human beings, belong to this class. There are a set of characteristics like presence of mammary glands and sweat glands, specialized dentition etc. that make mammals superior to other animals. Except for five species of monotremes which lay eggs, all mammals give birth to young ones. The name ‘mammals’ is derived from the specialized mammary glands with which they feed their young ones. Mammals are distributed in aquatic, terrestrial, arboreal and aerial ecosystems. Aquatic mammals include whales, dolphins etc. whereas bats lead arboreal and aerial life. Monkeys and primates live on trees whereas human beings live on land. Mammals feed on a range of resources; they can be insectivorous, carnivorous, omnivorous and herbivorous. The digestive tracts of mammals are inhabited with several types of beneficial bacteria that establish a symbiotic relationship with mammals. They help mammals in digesting complex components in their diet and help themselves by deriving their nutritional requirements. Read the following section to learn more about mammals and their characteristics.

General Characteristics Of Mammals

Morphological Characteristics

• Morphology of mammals greatly varies according to species. However, there are certain characteristics which are general to the entire class. 
• Mammals are the only class of animals which have hair. Other animals have structures like cilia, setae, whiskers, feathers etc., which appear like hair but are structurally, as well as, functionally different. Therefore, hairs are definitive characteristics of mammals.
• Marsupials, members of the infraclass marsupialia, have a specialized pouch-like structure called marsupiam in which they carry their offspring. The young one is born relatively underdeveloped. It is then kept in the pouch, which has multiple nipples for nourishment, until it gains proper development. Kangaroos, koalas, wombats etc. are some examples for marsupials. This feature, being unique to this infraclass, is a definitive character for members of this group.
• Some primates and marsupials have coloured skin with shades of green, violet or blue. You may have seen polar bears with green fur but that occurs due to algal growth; it is not their skin color.
  Mammals can be identified by the presence of mammary glands with nipples. However, monotremes do not have nipples but, the females do have mammary glands with a mammary patch on their bellies. Young ones lick milk from this mammary patch.
  Different species of mammals have different types of locomotory organs depending on their habitat.
• Terrestrial mammals have limbs like legs and hoofs, an adaptation to walk. Arboreal animals have limbs with claws which enable them to climb trees. Aquatic mammals, like whales and dolphins, use their tail flukes to propel their massive bodies through water. Mammals like bats live aerial lives and hence, have wings.

Anatomical Characteristics

• Mammals generally have seven neck bones (cervical vertebrae). However, there are exceptions for this. Manatee and the two-toed sloth have only six cervical vertebrae whereas the three-toed sloth has nine cervical vertebrae.
• Mammals have spongy lungs, which are their chief respiratory organs. Large muscular diaphragm aids in respiration by adjusting the volume of thoracic and abdominal cavities.
• Mammalian lungs are ‘bellows lungs’ which means that they expand and contract, depending on the pressure gradient.
• Mammalian diaphragm is a dome like muscular structure which is convex towards the thoracic cavity.
• Small air sacs called alveoli form the functional units of lungs. Gas exchange takes place in these alveoli.
• Mammalian skin is made of three layers of cells – epidermis, dermis and hypodermis. The lowermost layer, hypodermis, is made of adipose tissues which store fats and also provide cushioning and insulation.
• ‘Intelligent mammals’, like the primates, have larger cerebrum compared to the rest of the brain. The higher level of intelligence is indicated through ability of fast learning and behavioral flexibility.

Physiological Characteristics

• All mammals inhale oxygen and exhale carbon dioxide.
• Mammalian respiration depends upon pressure gradients of the respiratory gases. Inhalation occurs when the pressure gradient of oxygen is greater outside the body and lesser inside. Exhalation takes place when the pressure gradient of carbon dioxide is greater inside the body and lesser outside.
• Mammals are warm-blooded animals which means, they have relatively higher temperatures than the atmosphere. The body temperature is maintained by high metabolic activities. Birds, though not mammals are also endotherm and hence, this feature is not unique to mammals.
• The amount of heat lost from mammals is greater compared to the heat-generating volume. Hence, mammals have high metabolic rates to keep a healthy balance between the two.
• Some mammals can be extremely specific about their diet. Other than the regular meat eating habits, some specialized feeding habits of the mammals can be; granivory – seed eating; frugivory – fruit eating; folivory – leaf eating (giraffes); gumivory (or gummivory) – gum eating; nectivory – nectar eating and mycophagy – fungus eating.

Mammals are a highly evolved group of organisms with very specialized structures that vary from one species to another. The above article has some of the general characteristic of mammals, which may vary according to the species.