Characteristics, Classification and Examples of Mammals

Antarctic Fur Seal with Pup / Mint Images - Art Wolfe / Mint Images RF / Getty Images
White Antarctic fur seal adult female, with a white seal pup at her side on South Georgia Island on the Falklands islands. Mint Images - Art Wolfe / Mint Images RF / Getty Images

Many of the animals you're most familiar with are probably mammals. The group of mammals includes such well-known animals as humans, dogs, cats, bears and whales.

Characteristics of Mammals

All mammals have:

  • Hair
  • Mammary glands that produce milk
  • Three bones in their middle ear

In addition, most mammals give birth to live young, are endothermic and have a highly-developed brain. 

Keeping Warm

Marine mammals, especially those that live in colder regions, have special considerations when it comes to keeping their bodies warm.

 It's easier to lose heat to water than to air. Marine mammals have blubber and sometimes fur to insulate their bodies and keep heat in. They can also maintain core body temperatures by using a countercurrent heat exchange system, which involves veins absorbing heat from blood in arteries in flippers, fins and flukes and carrying it back to the core of the body. It is only with these adaptations that marine mammals can survive in nutrient-rich cold waters and take advantage of the rich sources of prey there.

Mammal Classification

In addition to being in the animal kingdom, all mammals have a skeleton made of bone, placing them in the phylum Chordata. All mammals are chordates, but not all chordates are mammals. 

Mammals may be further classified into three major groups, based on how they bear young:

  • Theria - mammals that give birth to live young, without use of a shelled egg. All marine mammals are in this group. 
  • Prototheria or monotremes, which are egg-laying mammals
  • Metatheria or marsupials, whose young develop in a pouch

Marine mammals in the subclass Theria are further classified into three orders:

  • Carnivora - pinnipeds, sea otters, polar bears
  • Cetacea - whales, dolphins and porpoises
  • Sirenia - manatees and dugongs

    What Do Mammals Eat?

    Mammals are a very diverse group of animals, and as such, have a variety of diets. They may be herbivores (plant eaters), carnivores (meat eaters) or omnivores (eat both plants and animals).  

    Most marine mammals, including whales, polar bears, sea otters, pinnipeds are carnivores. The diets of these animals often include fish, squid, crustaceans and zooplankton.  Sirenians (manatees and dugongs) are herbivores that get their nutrition from aquatic plants.

    Where Do Mammals Live?

    Mammals live in a variety of habitats around the world.  Marine mammals can be found in warm and cold water (even in very cold, polar regions which provide habitats for marine mammals such as whales, pinnipeds and polar bears).

    Mammal Reproduction

    Mammals reproduce sexually, with internal fertilization. In marine mammals, all young (fetuses) develop within their mother's uterus and are born live. 

    Whales, sea otters and sirenians give birth in the ocean, but pinnipeds and polar bears give birth on land. While cetaceans can swim immediately upon birth, pinniped pups and polar bear cubs require some time on land before they learn to swim.

    Female marine mammals invest a lot of energy in raising young.

    The gestation period for most marine mammals is several months, and young may nurse for several months, or even years, before they are weaned. Marine mammals typically have very rich milk that ranges from 25-60% fat.

    Largest and Smallest Marine Mammals

    The smallest marine mammal by weight is the sea otter. By length, the smallest marine mammals are the sea otter, Commerson's dolphin and the vaquita, which all grow to a maximum length of 4-5 feet.

    The largest marine mammal is the blue whale, which is also the largest species in the world. Blue whales can grow to a maximum length of about 100 feet and a maximum weight of about 150 tons.

    References and Further Information:

    • Morrissey, J.F. and J.L. Sumich. 2012. Introduction to the Biology of Marine Life (10th Edition). Jones & Bartlett Learning. 467pp.
    • Wund, M. and P. Myers. 2005. Mammalia. Animal Diversity Web. Accessed September 30, 2015.