Males of many species often unload their old sperm when no females are in sight, and now researchers may have figured out why. A new study shows that female crickets preferentially store young sperm in their reproductive tract, indicating that males who deliver fresh sperm during mating may have a reproductive advantage.
A sperm must compete with up to millions of its comrades to successfully fertilize the egg. Some studies have shown that swimming speed can play a role in this victory. Michael Siva-Jothy of the University of Sheffield in the United Kingdom and Klaus Reinhardt of Illinois State University in Normal wondered whether a sperm's age was also a factor.
To find out, the team injected crickets with a radioactive tracer that became incorporated into their sperm. The researchers varied the age of the labeled sperm by changing the number of days between injecting the tracer and mating the crickets. After the matings, the researchers found most sperm was less than 6 days old in females, but unmated males had plenty of 9- to15-day-old sperm in their reproductive tracts. This suggests males that deliver strapping young sperm get more access to the coveted eggs, the group reports online this week in The American Naturalist.
Whether it's the sperm or the females that are responsible for this age discrimination remains unknown. But once you look at sperm age as a driving factor in reproductive success, Siva-Jothy says, "almost every strange reproductive behavior you see can be reinterpreted in this light." For example, in addition to the sperm "shedding" seen in insects and primates, the results may also explain why the same pair of animals engages in multiple matings.
The discovery "sheds new light on the complexity of male-female interactions over fertilizing eggs," says Jonathan Waage, a behavioral ecologist at Brown University in Providence, Rhode Island. A number of old questions in sexual selection will have new hypotheses to explain them, he adds.