Well-hung males may enjoy an evolutionary advantage over their less well-endowed competitors – in certain rodents, anyway. The finding may help answer the vexing question of why penis size is so variable among mammals.

Steven Ramm, an evolutionary biologist at the University of Liverpool, UK, gathered published measurements of the length of the penis bone in four orders of mammals: rodents, primates, bats and carnivores, then corrected for the fact that related species tend to have similar-sized penises. He compared these adjusted lengths with body weight and testis size, which is a good indicator of a species’ promiscuity and so of …