Males of One Frog Species Go to Great Leaps to Ensure Lineage

The European brown frog is one of the most thoroughly studied in the world, but researchers have found that it has a long-unsuspected reproduction strategy, one driven by the large excess of males in the population.

In the higher altitudes of the Pyrenees where the study took place, on the border between France and Spain, male frogs outnumber females four- to tenfold.

In an effort to preserve their lineage, male frogs that are unable to mate with females “kidnap” clutches of eggs laid by the female and deposit sperm in hopes of inseminating any eggs not inseminated by the female’s mate. This process can occur three to four times with a given batch of eggs.

Researchers used to believe that the frog, Rana temporaria, had a simpler mating process: a male would grasp a pregnant female and hold on to her -- a process called amplexus -- for as long as two days until she released her eggs. The male would then fertilize them and the game would be over.

But zoologist David R. Vieites of UC Berkeley and his colleagues from Spain, Germany and the Netherlands found that as soon as the male finished his business, a second male would grasp the clutch and release his sperm, hoping to get the eggs missed by the first male. As many as 30% of the eggs are often missed by the first male.

Sometimes, a third and a fourth male would do the same thing. In one pond, the team found that 84% of all clutches had been fertilized by more than one male.