Infidelity Produces Faster Sperm, Swedish Fish Study Finds

Until now, it has been difficult to prove that fast-swimming sperm have an advantage when it comes to fertilizing an egg. But now a research team at Uppsala University can demonstrate that unfaithful females of the cichlid fish species influence the males’ sperm. Increased competition leads to both faster and larger sperm, and the research findings now being published in the scientific journal PNAS, Proceedings of the National Academy of Sciences, thus show that the much mythologized size factor does indeed count.

“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."

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