The European common frog, Rana temporaria, has long been thought to have a straightforward breeding strategy -- one lucky male grabs the female and fertilizes her eggs as soon as she releases them into the water. End of story.
But that's not the end of the story, according to Spanish researcher David R. Vieites, now a postdoctoral fellow at the University of California, Berkeley. In high-altitude ponds in the Pyrenees, on the border between Spain and France, so many males are vying for fatherhood that they pirate the egg clutches after they're laid. Grasping them as they would a female, they release sperm in the floating clutches, often successfully fertilizing the eggs left unfertilized after the initial encounter. In one pond studied, 84 percent of all clutches had been fertilized by more than one male.
In addition, numerous male frogs typically gang around the egg clutch for their turn at fertilization. According to Vieites, genetic analysis of the eggs in a single clutch shows fertilization by as many as four separate males, including the male that actually had embraced the female.
"This is the first example of such a complex mating behavior in amphibians undergoing external fertilization," Vieites said. "Because of a population excess of males, the males found a system to reproduce without the female."
He and colleagues in Spain, Germany and The Netherlands report their findings in the Sept. 16 issue of Nature.
Vieites also noticed a possible female response to this so-called "clutch piracy." Many females delayed release of their eggs until no other males were around, often floating around for several days with a male onboard before releasing eggs. Vieites noticed, too, that some females were releasing thei
Contact: Robert Sanders
University of California - Berkeley