Hairlessness: 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.[32][33] However, humans vary strongly in the amount and distribution of body hair[34] and comparably sized mammals adapted to semi-aquatic lifestyles actually have dense, insulating fur[30][35] or large, barrel-shaped bodies that retain heat well in water.[30] 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.[4][30]