Female promiscuity has led to the development of bigger, faster sperm in fish, according to a researcher at The University of Western Australia who is the main author of a paper published in the US journal Proceedings of the National Academy of Science.
Postdoctoral Fellow Dr John Fitzpatrick, at UWA's Centre for Evolutionary Biology, and his international team studied 29 species of cichlid fish in Lake Tanganyika in Africa's Rift Valley, one of the largest, deepest lakes in the world.
They found that among monogamous cichlid species which share parental care and practise oral sex - fertilisation in the female fish's mouth - low sperm competition means the males have relatively small, slow sperm. But in polygamous species in which only the females care for offspring and which practise external fertilisation, where the sperm and ova mingle in the lake-water, sperm is bigger and faster.
Dr Fitzpatrick's research focuses on sexual selection, with particular emphasis on sperm competition, and the evolution of ejaculate traits in animals. His current research aims to uncover how inbreeding influences ejaculate traits and fertilisation success in guppies and Australian rainbowfish. Previously he examined how social status and sperm competition influences ejaculate traits in highly social species and species with alternative male reproductive tactics.
Cichlids belong to one of the largest vertebrate families and are a favourite subject for evolutionary biologists as they rapidly evolved into many closely related but diverse species.
"Our study shows how selection in the form of female mating behaviour may have acted in evolutionary time to shape various sperm traits and to produce superior ejaculates," Dr Fitzpatrick said. "Further use of this methodology in other animals can highlight the common selective pressures that have driven the evolution of ejaculate traits in animals in general."
The paper's co-authors are: Dr Robert Montgomerie from Queen's University, Canada; Dr Julie Desjardins from Stanford University in the US; Kelly Stiver of Yale University in the US; Dr Niclas Kolm of Uppsala University, Sweden; and Dr Sigal Balshine of McMaster University, Canada.
For a summary of the article, "Female promiscuity promotes the evolution of faster sperm in cichlid fishes" visit http://www.newscientist.com/article/dn16442-competition-drives-evolution-of-super-sperm.html