Among the tiny insects known as water striders, males who aggressively attempt to mate with females don't wind up with as many offspring as their more gentlemanly counterparts. How can aggressive mating ever be a losing strategy?
A group of researchers in the United States decided to do an experiment with water striders, in which they observed the mating success of prudent, "nice" males versus aggressive, "psychopathic" males. The latter group tried often to mate with the females very aggressively, and in previous experiments they had the most reproductive success. But these scientists discovered that the success of the psychopaths depended on very specific laboratory conditions
It turned out that other studies of sex among water striders had kept the population contained in a limited area, where females had access to very few males. When the researchers opened up the insects' habitat, allowing the females to roam freely, they discovered that the less aggressive males attracted the highest number of mates.
According to a release about the research, published yesterday afternoon in Science:
"The presence of psychopaths dramatically reduced the productivity of the population," [biologist David Sloan] Wilson said. "When all the males were gentlemen, the females laid about three times more eggs than they did when all the males were psychopaths. And yet within each group the psychopaths were doing better than the gentlemen. How do the gentlemen persist if they're disadvantaged within the group?"
Once the females could move between groups, the researchers had their answer. [Researcher Omar Tonsi] Eldakar and Michael J. Dlugos, then also a Binghamton graduate student, devised a wading pool equipped with special doors that could restrict movement between groups or allow the insects to move freely.
"When they opened the doors, the females would leave whenever a psychopath came around," Wilson said. "The whole thing resulted in a heterogeneity in which the females were clustered with the gentlemen. It's the movement of individuals that creates these differences between groups that favor nonaggressive males."
Who knows how much research into sexual selection has been flawed because researchers forgot the crucial ingredient of female freedom?
Ultimately, what's interesting about this study is that it shows why isolated populations might engage in a different mode of sexual selection than a free-ranging population that has a lot of contact with outside groups.