Evolution of cetaceans "Caperea marginata") all have derived characteristics presently unknown in any cetothere and vice versa (such as a sagittal crest). The adaptation of echolocation occurred when toothed whales (Odontoceti) split apart from baleen whales, and distinguishes modern toothed whales from fully aquatic archaeocetes. This happened around 34 million years ago in a second cetacean radiation. Modern toothed whales do not rely on their sense of sight, but rather on their sonar to hunt prey. Echolocation also allowed toothed whales to dive deeper in search of food, with light no longer necessary for navigation, which opened up new food sources. Toothed whales
Michael Bigg key discoveries of Dr. Bigg's team was that there were two types of killer whales living near the British Columbia coastline: "residents" which ate almost exclusively fish, and "transients" which hunted marine mammals and other warm-blooded prey. Resident and transient killer whale societies are separate, and the two types tend to avoid each other. The resident killer whale population of British Columbia and Washington is further divided into two "communities", one northern and one southern, that do not interbreed. Another discovery was that resident killer whales have one of the most stable social structures of any animal species. Pods of