Female salmon are able to choose the sperm that will eventually be used to fertilize their eggs by secretions released together with their eggs, according to a study by Patrice Rosengrave and co-workers to be published in the journal Behavioral Ecology.
The authors compared how ovarian fluid from each of 7 female chinook salmon (Oncorhynchus tschawytscha) affected the sperm behaviour of 11 different males by using computer-assisted sperm analysis to measure sperm velocity, duration of sperm motility, swimming path trajectory, and progressive motility from the ejaculates of each male activated in the ovarian fluid from each female.
The authors found that the mean sperm swimming speed, path trajectory, and longevity differed significantly for the same male in response to different females ovarian fluids (the sperm of one male swam up to four times faster in the ovarian fluid of one female compared to another).
Because the speed at which the sperm swims is the most important variable influencing fertilization success in this species, this suggests that the females are favouring sperm from one male over another.
The exact reasons for this cryptic female choice is unclear, but the authors think that it is highly likely that the females are selecting for genetic compatibility to ensure healthier offspring.
For more information, see the paper: Rosengrave, P, MJ Gemmell, V Metcalf, K McBride and R Montgomerie (2008) A mechanism for cryptic female choice in chinook salmon. Behavioral Ecology, doi:10.1093/beheco/arn089