Dolphins and porpoises are fast swimmers, and many species are outstanding deep divers. To land mammals like us, their routines are astonishing feats. Orcas, pantropical spotted dolphins and northern right whale dolphins are among the fastest species, yet each achieves speed with different body and fin shapes. Their bodies vary from the bulky orca, which weighs up to 5,000 kilograms, to the lithe 70-kilogram northern right whale dolphin. An orca dorsal fin can be 1.8 metres high, while the broad flippers are up to 2 metres long, almost one-quarter of the average orca's body length; the northern right whale dolphin has small flippers and no dorsal fin at all. At high speed, a dorsal fin may help provide stability, but it also adds drag. All three species use strong tail muscles for propulsion and, corn-
pared with humans, are streamlined.
Northern right whale dolphins have been clocked at 40 kilometres per hour and can sustain speeds of 25 kilometres per hour for up to 30 minutes. As they skim the water with their low-angle leaps, they can cover seven metres before they reenter the water and burst out again. Together with the absence of a dorsal fin, this technique allows them to avoid drag and to achieve greater speeds.
In one experiment, pantropical spotted dolphins trained to chase a lure accelerated to a top speed of 11 metres per second in just two seconds. For short speed bursts, the maximum power output was 50 percent greater per unit of body weight than that of human athletes. Orcas, estimated to travel at speed bursts of up to 48 kilometres per hour, may be the fastest of the dolphins.
How deep dolphins and porpoises dive depends on the availability of food. Because some fish live only at certain depths, scientists can sometimes tell where dolphins feed by examining the stomachs of dead dolphins and identifying fish remains.
Dolphins were trained in one experiment to trip switches on a submarine cable to indicate the depth of dive. One bottlenose dolphin repeatedly dived almost 300 metres and stayed down for 5 minutes. Pilot whales have reached 600 metres on 16-minute dives. The record dolphin dive belongs to an orca off Alaska that became entangled in telegraph cable at a depth of 1,030 metres.
Rauno Lauhakangas