Sexual rivalry may drive frog reproductive behaviors

Some frogs may have started laying eggs on land to get way from the mating frenzy in the water

Date: July 26, 2016

Source: University of California - Berkeley

It may be hard to imagine competing over who gets to kiss a frog, but when it comes to mating, a new study concludes that some frogs have moved out of the pond onto land to make it easier for the male in the pair to give sexual rivals the slip.

Biologists have long thought that some frog species evolved to mate on land -- sometimes in unusual places -- instead of in open water to better guard eggs and tadpoles from easily being eaten by fish and other predators. But the new research by a team of U.S. and Brazilian frog biologists suggests that mating on land in many species might in part be a strategy that male frogs use to ensure that their own DNA gets passed on, instead of the DNA of their rivals. Sexual selection may trump natural selection in the evolution of these reproductive behaviors, according to the new study, to be published online ahead of print on July 26 in The American Naturalist.

Frogs have a "dizzying array" of reproductive strategies, according to Rayna Camille Bell, a UC Berkeley postdoctoral fellow who contributed to data collection, analyzed and interpreted much of the data, and helped co-author the study, which was led by her doctoral thesis advisor, Kelly Zamudio, a professor at Cornell University.

Mating in frogs typically involves the male wrapping his arms around the female, the female depositing eggs and the male fertilizing the eggs, which will hatch into tadpoles and ultimately develop into froglets. The earliest frogs completed all of these steps in water, but among different frog species there are various strategies for accomplishing these reproductive tasks before a new generation hops or swims off on its own. Frog species vary in where they mate, where they lay eggs, where tadpoles develop and whether and how eggs and tadpoles are tended to by parent frogs. Some species even skip the egg stage, giving birth to live tadpoles or even froglets.

Moving toward mating on land

"Biologists noticed an apparent linear progression toward more terrestrial reproduction throughout frog evolution and proposed that frogs avoid putting their eggs and tadpoles in streams or ponds because they would be more vulnerable to aquatic predators," Bell said. The apparent trend toward increasingly terrestrial reproduction is most evident in tropical frogs, perhaps because more humid environments more easily permit reproduction on land without eggs or tadpoles drying up.

Continued …