Observing body sizes of ancient and modern aquatic mammals and their terrestrial colleagues reveals that life in water restricts mammals to a narrow range of body sizes: big enough to stay warm, but not so big that they can't find enough food
These findings by researchers at the Stanford School of Earth, Energy & Environmental Sciences, is in divergence to preceding theories proposing that pressure on body size should be more relaxed in water, perhaps because of the enormous surroundings and ability for animals to float rather than have to support their body weight on legs.
As a substitute, the group found that aquatic mammal size is restricted at the minor end by the need to retain heat and at the bulky end by complications getting enough food to survive. While mammals that live in water share a similarly oblong body shape, they are not narrowly linked. Rather, seals and sea lions are closely related to dogs, manatees stake ancestry with elephants, and whales and dolphins are related to hippos and other hoofed mammals.