'The European common frog, Rana temporaria, has long been thought to have a straightforward breeding strategy - male grabs female and fertilizes eggs once released into water. After centuries of study and thousands of published papers, a novel mating strategy been just discovered in the special population conditions (4-10 males : 1 female) of the ponds in the Pyrenees - post-mating clutch piracy, which has been observed for the first time in an externally fertilised vertebrate.
In these ponds, when a breeding female has allowed a male to clasp her, surrounding males often jostle the pair in an attempt to dislodge the male. After the female has laid the eggs and the parents have left, these pirate males grasp the eggs as they would a female, and release sperm in the floating clutches. In one pond, 84% of all clutches had been fertilized by more than one male.
At other times, gangs of males search the pond for newly laid clutches and fertilize them again within an hour or two of being laid. Often, parent males join a gang to engage in clutch piracy as well. Some clutches had as many as four fathers.'
"This is the first example of such a complex mating behavior in amphibians undergoing external fertilization," said David R. Vieites, now a postdoctoral fellow at the University of California, Berkeley's Museum of Vertebrate Zoology. "Because of a population excess of males, the males found a system to reproduce without the female."
See Science Daily and UC Berkeley news commenting on Vieities, DR, S Nieto-Roman, M Barluenga, A Palanca, M Vences & A. Meyer, 2004. Post-mating clutch piracy in an amphibian. Nature 431, 305 - 308 (16 September 2004); doi:10.1038/nature02879. Abstract. Video available in supplementray material (NUS access).