Frogs are increasingly having more sex on dry land than in rivers and lakes. Previously thought to be an adaptation that prevents their eggs and tadpoles from being eaten by aquatic predators, new research says that the real reason might be for males to avoid competition from other males. In arriving at this theory, the researchers took a look at frog testes.

Because frog sex often involves external fertilization, frog eggs are particularly prone to being gobbled by hungry fish or other predators. The same holds true when the eggs turn into defenseless tadpoles. So it would make sense that frogs would hop out of aquatic environments teeming with egg-hungry creatures and attempt to get their groove on in more private locales. And that's exactly what they've been doing over time, according to researchers at Cornell University.

"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," said Rayna Camille Bell. Bell is a University of California at Berkeley postdoctoral fellow who took part in a study led by her doctoral thesis advisor, Kelly Zamudio, a Cornell University professor.

Frogs in tropical environments have been particularly prone to terrestrial sex and researchers think that's because the moist environment helps keep their eggs from drying. One favorite mating spot is inside bromeliads, large tropical plants that trap water inside.

But even on land, frog eggs can face predation and other dangers, such as newly hatched tadpoles falling off a leaf into a dangerous stream. So the researchers began to wonder if frogs were mating on dry land for another reason – for males to avoid competition from other males.

If the theory was correct, it would follow that the testes belonging to male frogs mating on land would be smaller than those possessed by water breeders. That's because in the relatively crowded aquatic environment, frogs would need to produce more prodigious amounts of sperm in the face of all the competition to try to ensure that their seeds were successful, which would lead to larger testes.

Sure enough, the team discovered this to be the case. Male frogs mating in private on land did indeed have smaller testes than their water-based cohorts, indicating that the move to land for reproduction purposes could have actually been sparked by a move away from competition rather than in an effort to protect eggs and young tadpoles.

Of course, another way to look at it is that frogs did indeed move onto dry land to keep their eggs safe and, as a result, their testes shrunk as the competition for females went down, although that's not the primary conclusion made by the researchers. In either case, it certainly seems that frogs that mate in land-based private locations do have smaller privates.

The study was published online today in the journal The American Naturalist.

"Hopefully our study will draw attention to how much we still have to learn about sexual selection and mating system dynamics in frogs," says Bell.

Source: UC Berkeley