Frogs started to breed out of water to mitigate sexual competition

Research shows sexual selection may also have contributed to an evolutionary shift in frogs' reproductive modes, with some species preferring to breed on land.

By Elton Alisson  |  Agência FAPESP – Among terrestrial vertebrates, frogs display the greatest diversity of reproductive strategies on land.

Until recently, the main hypothesis raised by biologists to explain what led these animals to take this evolutionary step and begin reproducing in such varied ways on land was the need to flee fish and other aquatic predators that feed on frogspawn and tadpoles.

Now, however, a study performed by researchers at São Paulo State University (UNESP) in Brazil in collaboration with colleagues at Cornell University and the University of California Berkeley in the United States has shown that not only natural selection via reduced predation but also sexual selection favors terrestrial breeding by frogs.

Sexual selection enables some animal species to develop morphological and behavioral characteristics that give them reproductive advantages rather than just increased chances of survival.

The findings from the study, conducted as part of a Thematic Project supported by FAPESP, have been published in The American Naturalist.

“We discovered that the males of some frog species build subterranean chambers, attracting females to lay eggs in these less exposed sites and fertilizing the eggs there,” said Cynthia Peralta de Almeida Prado, a professor at UNESP’s Jaboticabal campus and one of the authors of the study.

“This reduces competition with other males that opportunistically try to fertilize the female’s eggs, a frequent occurrence in aquatic environments,” Prado told Agência FAPESP.

In a previous study, Prado and her collaborators had already observed that the testicles of male tree frogs of species belonging to the Hylidae family are smaller and vary less in size than those of species that breed in water. The Hylidae family comprises 952 land-breeding species found in several regions of the world.

Smaller testes mean less production of sperm by males that breed on land, a less competitive environment as far as the fertilization of females’ eggs is concerned.

“This finding raised the hypothesis that in addition to natural selection, leading them to keep as far away as possible from predators, sexual selection via competition among males could have led to the evolution of terrestrial reproduction in these species, which may have begun laying eggs in hidden places protected from opportunistic males,” Prado said.

During a recent research project, also supported by FAPESP under its Young Investigators Grants, Prado collected data on the behavior and testicle size of males from several species of frogs that breed on land or in water.

In this study, Prado observed that males from some water-breeding species competed more among themselves to fertilize females’ eggs than males from land-breeding species.

The reason, she surmised, is that more males seek to mate with females in ponds and lakes, for example, and eggs deposited by females in aquatic environments are more exposed to opportunistic males. Males in aquatic environments typically crowd around females that have just laid their eggs, competing for fertilization.

“This behavior, which we call polyandry, when a female mates with several males during a reproductive episode, had already been observed in other groups of animals,” Prado said.

Morphological variations

To evaluate whether competition to fertilize eggs influences males’ morphological characteristics, Prado worked with colleagues at UNESP, Cornell and UC Berkeley on a study of reproductive modes and testis size variation in other species of tree frogs in the Hylidae family.

According to their findings, testis size in tree frog species that prefer amplexus (mating) in water also exceeds testis size in species that prefer amplexus on land.

One of the hypotheses to explain this morphological variation is that in water males have to compete with each other much more to fertilize females’ eggs and hence produce more sperm to increase their chances of reproductive success.

The researchers also observed that some frog species mate in water that accumulates in bromeliads, for example, whereas others build clay nests shaped like volcanoes.

“Frogs’ reproductive strategies are exceedingly varied, and many have yet to be discovered,” Prado said. “Furthermore, Brazil has the greatest diversity of frogs and toads in the world, and it’s extremely important to study the biology of these species, some of which are endangered, if we want to extend our knowledge of evolution.”

The article “Polyandry, predation, and the evolution of frog reproductive modes” (doi: 10.5061/dryad.v67g3) by Kelly R. Zamudio, Rayna C. Bell, Renato C. Nali, Célio F. B. Haddad and Cynthia P. A. Prado can be retrieved by subscribers to The American Naturalist from journals.uchicago.edu/doi/abs/10.1086/687547.