<P> The differences between families of odontocetes include size, feeding adaptations and distribution . Monodontids consist of two species: the beluga and the narwhal . They both reside in the frigid arctic and both have large amounts of blubber . Belugas, being white, hunt in large pods near the surface and around pack ice, their coloration acting as camouflage . Narwhals, being black, hunt in large pods in the aphotic zone, but their underbelly still remains white to remain camouflaged when something is looking directly up or down at them . They have no dorsal fin to prevent collision with pack ice . Physeterids and Kogiids consist of sperm whales . Sperm whales consist the largest and smallest odontocetes, and spend a large portion of their life hunting squid . P. macrocephalus spends most of its life in search of squid in the depths; these animals do not require any degree of light at all, in fact, blind sperm whales have been caught in perfect health . The behaviour of Kogiids remains largely unknown, but, due to their small lungs, they are thought to hunt in the photic zone . Ziphiids consist of 22 species of beaked whale . These vary from size, to coloration, to distribution, but they all share a similar hunting style . They use a suction technique, aided by a pair of grooves on the underside of their head, not unlike the throat pleats on the rorquals, to feed . </P> <P> Whales are descendants of land - dwelling mammals of the artiodactyl order (even - toed ungulates). They are related to the Indohyus, an extinct chevrotain - like ungulate, from which they split approximately 48 million years ago . Primitive cetaceans, or archaeocetes, first took to the sea approximately 49 million years ago and became fully aquatic 5--10 million years later . What defines an archaeocete is the presence of anatomical features exclusive to cetaceans, alongside other primitive features not found in modern cetaceans, such as visible legs or asymmetrical teeth . Their features became adapted for living in the marine environment . Major anatomical changes included their hearing set - up that channeled vibrations from the jaw to the earbone (Ambulocetus 49 mya), a streamlined body and the growth of flukes on the tail (Protocetus 43 mya), the migration of the nostrils toward the top of the cranium (blowholes), and the modification of the forelimbs into flippers (Basilosaurus 35 mya), and the shrinking and eventual disappearance of the hind limbs (the first odontocetes and mysticetes 34 mya). </P> <P> Whale morphology shows a number of examples of convergent evolution, the most obvious being the streamlined fish - like body shape . Other examples include the use of echolocation for hunting in low light conditions--which is the same hearing adaptation used by bats--and, in the rorqual whales, jaw adaptations, similar to those found in pelicans, that enable engulfment feeding . </P> <P> Today, the closest living relatives of cetaceans are the hippopotamuses; these share a semi-aquatic ancestor that branched off from other artiodactyls some 60 mya . Around 40 mya, a common ancestor between the two branched off into cetacea and anthracotheres; nearly all anthracotheres became extinct at the end of the Pleistocene 2.5 mya, eventually leaving only one surviving lineage--the hippopotamus . </P>

What part of the ocean do whales live in