When bottlenose dolphins swim at a cruising speed around six miles an hour or a sprinting speed about twice that fastthey are constantly fighting against the waters drag,which only gets worse as they swim harder.
Shawn R. Noren,a marine biologist at the University of California,Santa Cruz,set out to find how much extra drag would be experienced by a specific,aerodynamically disadvantaged subset of the dolphin population: adult females nearing the end of pregnancy.
Dolphinswhich,like cows,descended from ancient ungulates have their calves one at a time,after a 12-month gestation period. Over the course of pregnancy,a females belly will grow until her girth increases about 50 percent.
The result is a drastic increase in drag: When a dolphin is within weeks of giving birth,she will encounter the same amount of water resistance at a speed of about four miles an hour as she normally would at a speed of eight miles an hour,according to Dr Norens analysis.
Pregnancy affects a dolphins swimming ability in other ways as well. Pregnant animals are fatter, Dr Noren said,but the blubber has a higher proportion of lipidwhich makes the dolphins more buoyant and less able to dive for prey.
Pregnant dolphins also swim differently,making shorter but more frequent strokes. And the growing fetus stretches out the mothers muscles,probably making it more difficult for her to thrust her tail up and down.
Norens research appears in the Journal of Experimental Biology under the title Pregnancy Is a Drag. RITCHIE S. KING