In a species of crickets, males are able to take advantage of investments made by other males to increase the quality of their own sperm.
New research from Leigh W. Simmons and Maxine Beveridge at the University of Western Australia shows that high quality seminal fluid from one male can rescue sperm from a “rival” male that had lower quality seminal fluid. This research suggests that males of this species might invest less in their own ejaculates if they think the female has already mated. That way, they can rely on the other male to increase the quality of their sperm, while saving the good stuff for a later mating with less competition.
This kind of strategy is important for sperm competition. Sperm competition is what happens when females mate with more than one male. The sperm of the two (or more) males fight it out inside the female to win the ultimate prize: the fertilization of her eggs.
But how can males increase their odds of winning?
In a 2007 paper, Melissa L. Thomas and Leigh W. Simmons showed that male crickets of this same species (Teleogryllus oceanicus) can adjust the proportion of live and dead sperm in their ejaculates. The more live sperm in the ejaculate, the more expensive that mating is for the male.
What they saw was that males invested the most in trying to win sperm competition when the female had already mated once. When she was a virgin, they had more dead sperm in the ejaculate, presumably because they didn’t have to worry about competing with any other male.
Relationship between female mating status and male ejaculate investment.
But if a male mated with a female who had already mated twice before, he also invested less (more dead sperm). In this case, it doesn’t do him much good to give her his best quality ejaculate, because most of her eggs are going to be fertilized by other males anyway–it’s just not a fair fight no matter what he does.
Or maybe, as the new study suggests, he might simply be able to rely on those other males’ seminal fluid to bump up the quality of his own sperm.
It’s hard to compare the two studies, because the newer one is done completely in vitro, by mixing sperm and seminal fluid from different males. The first study was done using actual matings. But it suggests that males use many different strategies to make the best use of their resources when mating under different conditions.
Interestingly, more and more studies have come out showing that males of many species are able to tailor their ejaculates–both the quality of sperm and the quality of seminal fluid–to suit the sperm competitive environment in which they find themselves.
Even men can strategically change their ejaculates depending on the context. In one study (paper here), men who viewed sexually explicit images depicting sperm competition scenarios (i.e.: two men and one woman) had more motile sperm in their ejaculate than when they viewed non-sperm competition images (three women). Other factors also played a role, such as age, time of day, and whether the man was raised in an urban vs. rural environment. But, the biggest factor was the images themselves.
The big question still remains: how do males tailor their ejaculates? What physiological processes are working to make sure that the right amount of proteins and high quality sperm get used for an individual mating? These “decisions” have to be made very quickly to adjust the ejaculate quality for a specific mating. Hopefully, future research will give us some clues as to how this all works.
Finally, just for fun, a video of crickets mating. Enjoy