Swimming and Diving
Many claims have been made about the swimming speeds and diving abilities of dolphins and whales but there are surprisingly few valid measurements of either.
Swimming:
The many Pelagic dolphin species that feed on fast-moving fish and muscular squid can attain speeds of 24km per hour(15mph); coastal species, which tend to feed on slower moving prey, rarely exceed 16km per hour (10mph). Few species manage to exceed 35km per hour (22mph). Exceptions to this are the bottlenose dolphins, that can reach speeds of up to 27km per hour (17mph), and spotted dolphins which have been recorded as having reached 40km per hour (25mph).Dolphins achieve such speeds by leaping from the water in a series of curves and spending as little time as possible under the water. This is known as 'running'. They can attain much greater speeds by riding the bow wave of a fast-moving vessel.
Diving:
Although they cannot compete with the diving abilities of the sperm whale, the deepest diver of all cetacean species, dolphins and porpoises can nevertheless reach impressive depths. Bottlenose dolphins can repeatedly reach depths of 300m (1,000ft) and can hold their breath for as long as six or seven minutes. Fraser's dolphins and Dall's porpoises have been known to dive as deep as 500m (1,640ft), in search of food. Belugas are capable of making prolonged dives to depths of 650m (2,130ft), while a killer whale has been recorded at a depth of 1km (3,300ft).
In order to obtain and retain enough oxygen to reach these depths and survive, the dolphin has two useful adaptations. firstly, it can exchange up to 80 per cent of the gases in its lungs at each breath (by comparison, the human exchange rate is 30 per cent) and it dives with full lungs. Secondly, its circulatory system and muscles have been modified to increase the dolphin's capacity to store oxygen.
In adapting to cope with the intense pressure experienced during such deep dives, dolphins have evolved a collapsible rib cage and, as the animal dives, air in the lungs is forced into the passages leading to its blowhole. At the same time, the lining of the lungs thickens, which may have the effect of reducing the absorption of nitrogen into the blood. This is the main cause of 'the bends', which can maim or even kill human divers returning from such depths.
The dolphins heartbeat also responds to the changing availability of oxygen by slowing down while it is diving and speeding up rapidly when it takes a breath so that oxygen can be quickly supplied to the bloodstream and waste product rapidly removed.
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