Odontoceti: Toothed Whales
In contrast, the faster-moving, smaller-bodied toothed whales—including dolphins and porpoises—pursue squid, fishes of many sizes, and in the case of killer whales (Orcinus orca), sea birds and mammals, including other cetaceans. Many toothed whales travel in groups of from five to many dozens of animals, whose purpose seems to be in part to hunt cooperatively. Killer whales have been observed to gang up on and kill larger whales such as gray whales. They also collectively hunt seals resting on ice floes; once a seal is spotted, the whales will dive together, causing a great wave that upsets the ice, dumping the unfortunate seal into their midst. (Interestingly, there is no record of a killer whale having killed a human.) Cooperative behavior has also been suggested for several dolphin species; bottlenose dolphins (Tursiops truncatus) have been observed to circle a school of fish, causing them to group more tightly together, and then take turns lunging through the school, grabbing mouthfuls of fish as they pass. Perhaps in these species, individuals hunting together can catch more food than each would hunting alone. This is probably not true of the plankton-eating baleen whales.
There are many solitary species of odontocetes, however, each with innovative "solo" feeding strategies. Odontocete teeth are generally conical in shape, except in porpoises; these animals have spade-shaped teeth, and they lack the dolphin's protruding rostrum, or beak. However, the bouto (Inia geoffrensis) of the Amazon and Orinoco rivers in South America, has rear teeth that resemble molars, used for crushing the armored catfish which is among its favorite foods. The susu river dolphins (Platanista minor and P. gangetica) of the Indian subcontinent, have long, pincer-like jaws for grabbing prey out in front of them. The largest odontocete, the sperm whale (Physeter catodon), is also the deepest diver. These animals have been observed diving to 4,000 ft (1,220 m), and have been captured with squid inhabiting depths of 10,000 ft (3,050 m) within their stomachs; however, their average dives are probably around 1,100 ft (335 m).
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