— 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 the amount of competition a male will face for fertilisation.
Rodents with relatively large testes also tended to have relatively long penises, Ramm found (The American Naturalist, vol 169, p 360). The advantage this confers on rodents is unknown, but a generously proportioned organ may deposit a male's sperm further up the female reproductive tract, giving them a head start in the race to the egg, Ramm speculates.
A similar, but weaker pattern occurs for carnivores. However, Ramm found no evidence that the correlation exists in either bats or primates.