Edited sentence: Hairlessness is only an advantage for aquatic mammals such as whales and dolphins that have spent millions of years adapting to aquatic lifestyles involving diving, fast swimming and migration over long distances; such animals show considerable skeletal and cardiovascular adaptations to an aquatic environment.However, humans vary strongly in the amount and distribution of body hair and comparably sized mammals adapted to semi-aquatic lifestyles actually have dense, insulating fur or large, barrel-shaped bodies that retain heat well in water.Langdon, in his 1997 critique of the hypothesis, stated that the streamlining features attributed to hair follicle distribution and direction would be more reasonably achieved through changes in the shape of the skeleton and soft tissues.Morgan claimed the relatively hairless skin of humans was due to comparable adaptations in aquatic mammals and land-dwelling mammals that have aquatic ancestors as well as those that currently spend much of their time in wet conditions, and what body hair humans do have follows the flow of water over the body.Though a variety of explanations have been proposed for human hairlessness, the best-supported hypothesis involves improved cooling through perspiration; while fur helps cool inactive animals, hairless skin that sweats vigorously is much better at cooling humans who generate body heat through activity