Sex tricks: A cichlid Neolamprologus leleupi swimming in the southern shores of Africa's Lake Tanganyika.
Credit: Sigal Balshine
LONDON: Promiscuous behaviour in female cichlid fish may have caused males to evolve speedier, bigger sperm, according to a new study.
When females shop around for sexual partners, sometimes mating with several at once, males must compete, said study co-author Niclas Kolm, an evolutionary biologist from the University of Uppsala in Sweden.
In this case, male fish that produce faster sperm have a better chance of beating rivals in the race to fertilise the few eggs that females lay.
Super-sperm
Furthermore, Kolm and his co-workers found some of the fish produce sperm that have evolved to become larger, more numerous and more long-lived. These are a kind of "super-sperm", said the researchers, who detail their findings this week in the U.S. journal the Proceedings of the National Academy of Sciences.
Experts have known for some time that sperm competition plays a role in sexual selection, with larger and faster sperm expected to have the edge over rivals. However, the evidence has remained controversial, said Kolm.
To get a definitive answer the researchers compared sperm from 29 species of African cichlid fishes – a group known for their wide range of sexual behaviours and rapid evolution.
While the females of some species are strictly monogamous, others tend to hedge their bets. When mating, some female cichlids will lay their eggs and then collect them in their mouths, after which the male fertilises them by depositing sperm there.
Maternal mouthbrooding
While monogamous fish will then guard their spawn until they hatch, promiscuous females will shop around to get sperm from several different males. "The quickest and largest sperm were found in these species," said Kolm.
What's more, the team found evidence for the evolution of super-sperm. Previously, scientists believed that sperm production involved a trade-off, where sperm with certain positive traits would naturally lack others, but this latest research seems to suggest they can have it all.
But what about sperm in other animals that cheat? "We believe the results are general across the entire vertebrate kingdom," said Kolm. "Of course, we are very keen to see the effects in primates, especially because they are closer to humans."
