Infidelity Produces Faster Sperm, Swedish Fish Study Finds

“The competition among sperms to fertilize a female’s eggs is an extremely powerful evolutionary force that influences various characteristics of sperms, such as size and speed,” says Niclas Kolm, a researcher at Uppsala University, who, in collaboration with scientists from several other universities, has studied the mating system of 29 species of Tanganyika cichlids. “For the first time, we can show a strong link between the degree of sperm competition and the size and speed of the sperms. Males with promiscuous females develop faster and larger sperms than the monogamous species,” says Niclas.

"In promiscuous species we found that males produced larger and faster sperm than in closely related species that were monogamous," says Sigal Balshine, associate professor in the department of Psychology, Neuroscience & Behaviour at McMaster University, and senior author on the study. "This research offers some of the first evidence that sperm has evolved to become more competitive in response to females mating with multiple males."

Female promiscuity is a major problem for males because the sperm from rival suitors will compete in the race to procreate, explains Balshine. While the idea that sperm would evolve to become more competitive when males compete for fertilization seems obvious, to date there has been little evidence to support this theory.

“[One] unique aspect of the study is that we based our study on an unusually large base, with many fish from many different species. The fish were caught in lakes in Africa, and a special characteristic of this group of fishes is that there are incredible numbers of species,” says Niclas. “There’s an unbelievable variety of species and different kinds of mating behaviors. There’s the whole spectrum of mating systems, from monogamous males to females that mate with many many males.”

The findings also show that the speed and the size of sperm are closely related: larger sperms are faster. These sperm swim faster thanks to the greater power of a larger flagellum, but faster sperm also need to have a larger store of energy, which in turn results in larger sperm.

Thanks to new analytical methods, they have also managed to demonstrate the order of this development. The sperm first become faster, then larger, following increased female promiscuity in a species.

“No one has previously been able to show what causes what. Here we can clearly see that female promiscuity determines the character of sperms,” says Niclas.