Promiscuity leads to faster sperm: study
“[This study] provides some of the first strong comparative evidence based on lots of species that sperm swimming speeds have been shaped by female mating patterns,” said Sigal Balshine, the senior author of the study, as well as the associate professor in the Department of Psychology, Neuroscience and Behaviour at McMaster University in Hamilton, Ontario.
The research tested the effects of competition on sperm evolution. Where female species are more promiscuous and tend to mate with multiple partners in quick succession, competition exists between males for whoever could reach the egg first. In this race, the faster and more numerous sperm wins the egg.
Without this competition, there is no requirement for superior sperm, so there would be no need to expend energy on creating sperm of better quality or higher quantity.
The study, which began in 2004, made use of 29 closely related species in the cichlid fish family found in Lake Tanganyika, Africa.
Most fish participate in external fertilization, whereby sperm must swim through the water in order to reach and penetrate the eggs. This allowed the researchers to closely investigate the competition occurring between sperm in a biologically relevant fashion.
In particular, because the cichlid species were close relatives and were also gathered from the same lake, the differences in their mating behaviours could be identified as the most important factor influencing sperm capacity.
The species studied varied in their mating behaviours from strictly monogamous to highly promiscuous. This occurred in both in males and females.
After over 300 fish were collected, their sperm was carefully observed using a microscope, allowing the team to measure both the size and quantity of sperm. In addition, a digital video camera was able to capture the swimming speed in real time.
The intensive data analysis that followed — using computer simulation techniques — led to the eventual conclusion that the sperm improved in speed, size, quantity, and longevity in the context of promiscuous species. Alternatively, the monogamous species of cichlids produced sperm that were poorer in quality and quantity.
“Female fish mating behaviour influenced sperm traits,” said Balshine, emphasizing the importance of mating behaviour on the evolution of male traits.
Furthermore, sperm competition is evident across different species.
“The results of our study could be much more widely applicable,” said Balshine.
However, more in-depth research needs to now be carried out to understand precisely what machinery makes one sperm swim fast and another swim more slowly.