Published: 2 December, 2004

Not all mammals live on land

Why do dolphins live in the water when they are mammals?

There are several thousand species of mammals living today, and all share a set of common characteristics, or are descendant from a previously living group that already had many of these characteristics. Most mammals live on land and are terrestrial. Some live on terrestrial structures, but feed in the sky such as bats, which fly. Some mammals live in water most of their lives, but emerge onto land to give birth to young. These include seals and sea lions. Whales and dolphins are entirely aquatic, having modified their appendages in favor of swimming at the expense of not being able to walk on land. Within the whales and dolphins there are two major groups, the toothed whales and the baleen whales. Toothed whales include the dolphins and others like the sperm whale. The baleen whales are large oceanic whales and include the blue whale, the largest living mammal. The baleen whales are named because of the large plates of baleen they have in their mouths that strain food out of the water column and pass it into their digestive system.

What about arms and legs? The arms or forelimbs have been modified into flippers for swimming. The hindlimbs have become vestigial, or have been lost. Look at the right whale skeleton at the Paleontological Research Institute to see the tiny remnants of the pelvis and femur (thighbone). We think that whales evolved from ancestors of artiodactyls, the group that includes modern camels, deer and cows. The earliest whales and dolphins may have been partially aquatic, moving in and out of the water, but not fully committed to life at sea. The fluke that powers whale swimming is a modification of the tail region of these mammals and may be critical to their eventual life as obligate but agile swimmers. Paleontologists have fossils of intermediate forms of whales, with teeth, forelimbs and hindlimbs designed for walking on land, including hooves on the toes of the hindlimbs.

Finally, there is good molecular evidence linking dolphins and whales with modern artiodactyls, like cows and deer. Mammals have been adept at solving the problems associated with living in the sky, on earth's surface, or in the oceans.

Here's some extra arguments about why dolphins are mammals. The presence of modified skin glands, or mammary glands, that are used to provide nourishment to newborn animals is a hallmark of the mammals. Whales or dolphins have these glands and are able to provide milk to their babies. Unlike reptiles and birds, mammals are born alive, and do not hatch from a typical egg. A complex set of teeth comprise yet another characteristic that sets mammals apart from most other vertebrate animals. Another common characteristic of most mammals is a coat of hair.