Female salmon fussy over males

Friday 15 August 2008

Cryptic choice allows female salmon to be fussy over males

When salmon spawn, the sperm of competing males are in an all-or-nothing race to be the first to reach and fertilise the eggs. New findings by a University of Otago researcher now show that, surprisingly, female salmon can influence the race results from afar.

Professor Neil Gemmell, Director of the University’s Centre for Reproduction and Genomics, says the research reveals that the ovarian fluid that females release with their eggs can help or hinder sperm swiftness, depending on the male it comes from.

“This appears to be the first example of ‘cryptic female choice’ found in an externally fertilising species,” Professor Gemmell says.

Cryptic female choice involves female attempts to control who fertilises their eggs after mating. It is known to occur in many species but, until now, only in ones which fertilise internally. An example is found in ducks, where females can favour one male over another through ejecting the sperm of less favoured males, he says.

“We were startled to find that female salmon are also able to exert this kind of influence. Previously, it had been thought that once their eggs had been released, females had very little say over which of the males around her would fertilise them.”

The findings have just appeared in the international journal Behavioral Ecology.

Professor Gemmell’s PhD student, Patrice Rosengrave, together with colleagues from Canterbury, Lincoln and Queen’s University in Canada compared the swimming speeds of 11 salmon’s sperm in the diluted ovarian fluids of seven females.

“We found that the speed of a male’s sperm varied significantly according to which female’s ovarian fluid it was swimming in,” he says.

For example, a particular male’s sperm was among the fastest in one set of fluid, but in another set, it trailed the field. In this latter set, a different male’s sperm was able to swim twice as fast as it had in another, he says.

The reason why female salmon try to swing the odds for some sperm over others might relate to avoiding inbreeding or ensuring that their mates had complementary immune system genes, he says.

“From the female’s perspective, it makes sense that when you only have one opportunity to reproduce in your life, you want to make sure you get it right.

“For the males, you could say that the upshot is that there may be someone for everyone in the salmon world – the race does not always go to the naturally fastest runner, as the conditions on the racecourse differ.”

Professor Gemmell says the findings also may have wider implications for the understanding of animal, or even human, reproduction.

“For instance, whatever it is in this ovarian fluid that is enhancing or retarding sperm function may have a similar effect on sperm in other species,” he says.

Professor Gemmell now plans to investigate the genetics involved in the phenomenon, and also hopes to establish a research collaboration to identify what mechanism in the fluid is responsible.

The research was supported by a Marsden Grant.

ENDS

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