The diet of whales and the sounds of dolphins
One group of whales has specialised in feeding on tiny shrimp like crustaceans, krill, which swim in vast quantities in the sea. Just as teeth are of no value to mammals feeding on ants, so they are of no use to those animals eating krill. These whales have lost their teeth and instead have baleen, sheets of horn, feathered at the edges, that hang down like stiff parallel curtains from the roof of the mouth. The whale takes a large mouthful of water in the middle of the shoal of krill, half shuts its jaws and then expels the water by pressing its tongue forward so that the krill remains and can be swallowed. Sometimes it gathers the krill by slowly cruising where it is thickest. It also can concentrate a dispersed shoal by diving beneath it and then spiralling up, expelling bubbles as it goes, so that the krill is driven towards the centre of the spiral. Then the whale with its jaws pointing upwards rises vertically in the centre of the spiral it has created and gathers them in one gulp. On such a diet, the baleen whales have grown to an immense size. The blue whale (Balaenoptera musculus) the biggest of any animal to inhabit our planet, grows to over 30 metres long and weighs up to 130 tonnes. There is a positive advantage to a whale being so large. Maintaining body temperature is easier the bigger you are and the lower the ratio between your volume and surface area. This phenomenon had affected the dinosaurs but their dimensions were limited by the mechanical strength of bone. Above a certain weight, limbs would simply break. The whales are less hampered. The function of their bones is largely to give rigidity. Support for their bodies comes from the water. Nor does a life spent gently cruising after krill demand great agility.
The toothed whales fed on different prey. The largest of them, the squid eating sperm whale (Physeter macrocephalus), only attains half the size of the blue whale. The smaller ones, dolphins, porpoises and killer whales, hunt both fish and squid and have become extremely fast swimmers, some reputedly being able to reach speeds of over 40 kph. Moving at such speeds, navigation becomes critically important. Fish are helped by their lateral line system, but mammals lost that far back in their ancestry and the toothed whales have instead a system based on the sounds used by shrews and elaborated by bats, sonar. Dolphins such as Bottle-nosed (Tursiops truncatus) produce the ultra sound with larynx and maybe an organ in the font of the head, the melon. The frequencies they use are around 200 000 vibrations a second, which is comparable to those used by bats. With this aid, they can not only sense obstacles in their path, but identify from the quality of the echo, the nature of these objects ahead. This can be demonstrated easily enough, for dolphins flourish in oceanaria and eagerly cooperate in training. Blindfolded dolphins demonstrate that they can, without difficulty, pick out particular shapes of floating rings and will swiftly swim through the water, with blindfolds on their eyes.