Promiscuous female fish speed up Sperm

 

Underwater, some females succeed at changing males.

The sperm of males of some species of cichlid fish competes to fertilize the eggs of promiscuous females, according to a new study by McMaster University researchers.

"What is new and exciting about our results is this is the first evidence that female behaviours influence sperm swimming speeds," said Sigal Balshine, a professor of psychology, neuroscience and behaviour at McMaster who co-authored the report.

The four-year study on sperm competition looked at 29 varieties of cichlid fish from Lake Tanganyika in central Africa. Some cichlid species are monogamous, while others have multiple sexual partners.

"We had a digital video camera stuck onto a microscope to measure sperm in real time," Balshine said. "We found that the species in which the female was promiscuous, the (males') sperm swam faster ... was also longer and swam for longer periods."

Computer technology was used to track the changes in sperm over time in response to female behaviour.

"Just like a mechanic could make a car drive faster by installing a better engine, evolution appears to act first on the engine that drives sperm movement," report co-author John Fitzpatrick, a McMaster graduate student now at the University of Western Australia, said in a news release.

"Slow, short sperm was ancestral and evolved to become fast, long sperm," Balshine said.

Balshine plans on examining the biochemical changes that make cichlid sperm swim faster and longer.

The study's findings have broad application. Sperm competition also exists in mammals, insects and birds, Balshine said.

And sperm competition exists in humans, as well. When a woman mates with multiple partners in a short period of time, there is competition between different men's sperm in her body to fertilize her eggs. Balshine said the speed of human sperm has also been shaped by this behaviour.