Male frogs in a mountain pond sneak their sperm directly onto egg masses of a female who’s already mated with another male, an extra effort at fatherhood that scientists haven’t reported before in amphibians, says an international team. In the study population, original matings fertilized on average only about three-quarters of the eggs.
A widespread European species called the common frog (Rana temporaria) engages in standard frog-mating behavior. Males firmly grasp a female and eject sperm while she releases a soft mass of eggs. As is typical for frogs, fertilization occurs in the water rather than inside the mother’s body.
Males in a pond in the Spanish Pyrenees, however, also search for egg masses already bobbing in water, says David R. Vieites of the University of California, Berkeley. He and his colleagues used both videotaping and genetic analysis to document that males treat these eggs as mating opportunities. About 80 percent of the egg masses in the study received sperm from at least one visitor, in a behavior the researchers call clutch piracy. The team reports its findings in the Sept. 16 Nature.
“It really is another example of a sneaky male,” notes Bryan Neff of the University of Western Ontario in London, Ontario (SN: 4/19/03, p. 246: Available to subscribers at Fishy Paternity Defense: Bluegill dads: Not mine? Why bother?). He finds similarities between the pirate-frog behavior and that of the fish he studies. Small bluegill males that don’t seek territories dart in and release sperm at the final moment of a territory-holding male’s courtship.
Variations on this sneaker-male strategy have shown up in at least 140 fish species, Neff says, plus some insects, crustaceans, lizards, and rodents.
Vieites and his colleagues watched their frog population for three breeding seasons. After a pair had spawned, a lurking male grasped the fresh egg mass and released sperm. The researchers found no size difference between the pirates and the original fertilizers. Some males both paired with females and fertilized other couples’ clutches.
The percentage of fertilized eggs rose measurably only when a pirate managed to tear open the jellylike egg mass and reach the interior. This insertion occurred about half the time. In a sampling of seven clutches, the additional sperm deliveries accounted for about one-quarter of the eggs fertilized.
Michael Tyler of the University of Adelaide in Australia greets the paper with “surprise and bewilderment.” His research on the same species has found very high fertilization rates from original couplings, so he speculates that Vieites might have examined an unusual population.
However, Ross Alford of James Cook University in Townsville, Australia, finds the piracy plausible. Given frogs’ “staggering diversity of reproductive modes and behavior,” he says, “it seems almost inevitable that some frogs should do it.”
Dale Roberts of the University of Western Australia in Crawley has found evidence of multiple paternity in a frog clutch arising from other behaviors. He notes that, concerning frog evolution, clutch piracy weakens the impact of any pattern of female choice. However, he also sees it as an “unsurprising outcome of male-male competition.”