It’s Saturday night down at the old mill pond, and gaggles of lonely anurans are looking for love. A question vexes researchers: Why the crowd? Each male would seem to have better odds of mating by setting off on his own. But males of Spea multiplicata and Spea bombifrons—spadefoot toad species that interbreed—sit in a flotilla that’s a veritable fraternity row of bachelor lily pads.
One easy answer is the same reason college boys can be found on Daytona Beach in April: It’s where the girls are. But there’s more to it than that, claim Karin S. Pfennig of the University of North Carolina at Chapel Hill and her colleagues in the June Behavioral Ecology and Sociobiology. Males are, at least in some cases, drawn to each other to win females.
To investigate the gregariousness, Pfennig’s team placed S. multiplicata males between two speakers at the edge of a wading pool. One speaker broadcast calls from males of the same species; the other, calls from S. bombifrons. Next, Pfennig played calls at either the average frequency for S. multiplicata, 31 calls per minute, or a less typical rate of 37 calls per minute.
Earlier studies had shown that females prefer average calls. Of 27 S. multiplicata males tested, 22 swam to the speaker broadcasting the calls of their own kind. In the contest between average and fast calling rates, smaller males chose the slower-frequency speaker, while larger males preferred the faster-frequency one, which more resembles the call of S. bombifrons.
Males evaluate each other’s calls, suggests Pfennig, because female S. multiplicata face a mating pitfall. If they breed with the S. bombifrons, the offspring hybrid males will be sterile and the females, only partially fecund. And spadefoot toads, observes Carl Gerhardt, a biologist at the University of Missouri in Columbia, “are notorious for trying to mate with any other frog of comparable size that moves near them.”
Males can help solve the hybridization problem by attempting to form single-species clusters, says Pfennig. S. multiplicata males who call at the species-typical rate rather than near the faster frequency offer additional reassurance to the would-be discriminating female.
Smaller S. multiplicata males are likely to be in poorer condition, says Pfennig, and may lack the energy reserves to do their own calling. Michael J. Ryan, a biologist at the University of Texas at Austin, notes that Pfennig’s study confirms his team’s unpublished observations that the silent, punier males are drawn to calls of average frequency. “It seems to me that if you were adapting a noncalling or nondisplaying strategy, then, sure, you would want to be around a male who was very attractive to females,” he says. “The way you’d get your copulations is by intercept.”
The reason larger males prefer the faster calling rate is less clear, but they may be attempting to exploit the effect of contrast. “It’s very difficult for a male, if he’s going to call at an average call rate which is attractive to the female, to out-average average,” notes Pfennig. A male who wants to stand out to a female, she speculates, might do better to find a male calling at the wrong rate and use his competitor’s amatory ineptitude to showcase his own savoir faire.
Pfennig concedes that her study doesn’t explore additional reasons males might aggregate. Each individual, for instance, may simply be choosing the best location for food. The hypotheses for aggregation aren’t mutually exclusive, however, and Ryan observes that Pfennig’s study is one of the few to establish that males evaluate potential competitors using the same rules that females do.
Simply finding that males seek out other males is striking, says Pfennig, who notes that biologists often separate male competition and female choice. Males in some species compete for mates in two stages, using one set of behavior to combat other males and another for wooing.
“What these results suggest,” says Pfennig, “is that those two categories may not be as separate as we sometimes tend to make them.”