SPERM that are forced into competing with sperm from other individuals evolve to be faster and fitter in just about every way.
It is known that sperm from promiscuous chimps move faster than those from relatively monogamous gorillas, but no one knew if this was common among animals. Studies in other species have produced conflicting results.
To settle the issue, Sigal Balshine of McMaster University in Hamilton, Ontario, Canada, and colleagues studied 29 species of cichlid fish living in Lake Tanganyika in central Africa. This is a major family of fish with species employing the full range of mating behaviours…
