Men are typically about fifteen percent bigger than women. It’s also true that males are bigger than females in some familiar animals, like gorillas and lions. Evolutionary biologists thought it was a general rule among mammals that males are bigger. Mammals are the animal group that humans belong to. Like us, they all have a backbone, generate their own body heat, and secrete milk to feed their young.
Many have hair on their bodies. Biologists even had a theory to explain why males were always bigger. The idea was that female mammals devote more time to caring for their young and nursing them with milk. This means that, at any given time, more males than females are available for mating. With females in short supply, males have to compete for mating opportunities. Bigger and stronger males are more likely to win, and pass on the genes for this advantage. New research now shows that this simple story is wrong, or at least not right as often as biologists thought.
The research comes from a team of American biologists who studied data on the typical body weights of 429 different species of mammals. The researchers were surprised to find that, in more than half of the species, males weren’t bigger than females. Specifically, males were bigger in only about forty five percent of species, females were bigger in about sixteen percent, and in the rest—about thirty nine percent—males and females were the same size. The researchers expect the new findings will lead to a more complicated and varied story of male and female roles across the mammals.
