Initially the animals would have been rather like a seal in habit, using all four limbs to move around in the water and on land, but gradually, adapting to living in a cool, watery environment. The Mesonychids hind limbs were virtually useless in the water, and they were slowly lost. However, today's whales and dolphins still have the remnants of these hind-leg bones. The ancient whales have been named Archaeoceti. Fossil remains found in the Himalayan region of Pakistan, India, Egypt and Nigeria suggest that most Protocetidae were small-bodied animals less than 3m in length.
Water is an efficient conductor of heat, as it draws off heat twenty times faster than air.
Any warm-blooded creature which has to spend long periods of time immersed in it needs to find a way of conserving body heat. While some mammals, like otters, have evolved a coat of hair or fur, the cetaceans acquired a thick layer of blubber to help maintain body temperature. Water is also denser than air and therefore offers greater resistance to movement, which is what dictated not only the evolution of the cetaceans' overall shape, but also the development of flukes, flippers and the alignment and formation of muscles. While the sea presented some difficult challenges to the early cetaceans and their ancestors, it also offered immense opportunities. Food was most likely readily available in great abundance and, initially at least, there may have been little competition for this food source.