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Why Don’t We Do It in the Tree? New research sheds light on frog lovin’

August 8, 2016
by Glen Martin

Generally speaking, frogs are a lusty and uninhibited bunch. When it comes to lovin’, most ranids prefer an orgiastic approach. They congregate in tepid ponds in huge numbers, the females laying masses of gelatinous eggs, which the males then inseminate. Typically, the males grasp the females with the forelimbs to stimulate egg extrusion. So many frogs can get into the act that they sometimes form huge “mating balls” of quivering amphibian flesh.

But some species, particularly tropical ones, take a different tack—more of a candlelight-and-champagne approach. They prefer to do their wooing on land, far from the madding and somewhat squishy aquatic crowd. They’ll mate in tree boles, bromeliad leaves, even meticulously constructed “mud volcanoes”—places, in short, that trap enough water to sustain eggs and provide a modicum of privacy.

For decades, researchers thought this strategy was a response to predation. The frogs were mating in terrestrial venues, it was believed, because there were fewer hungry critters on land than in the water, giving the eggs a better chance of making it to adulthood.

They found that the male frogs that went in for mating in little ter­res­trial pieds-à-terre have smaller tes­ti­cles than their pond-preferring ­breth­ren.

But new research from a team of herpetologists that included UC Berkeley postdoctoral fellow Rayna Camille Bell points to another likely motivation: rivalry.

“In ponds, the competition between males can be extremely high,” says Bell, who was recently appointed as the curator of amphibians and reptiles at the Smithsonian Institution’s National Museum of Natural History. “You get these huge mating aggregations, with multiple males hanging on, or trying to hang on, to the same female. That can really reduce any specific male’s chances for fertilizing eggs.”

Terrestrial mating can thus greatly increase a male’s procreative odds. It also offers some attractions for female frogs.

“They’re less likely to be stressed, because there isn’t going to be 10 or 20 males clamoring to get on them,” Bell says.

Makes sense. But how do you support the hypothesis? The researchers thought testicle size would be a good indication. In the frenzied milieu of the ponds, male frogs would have to produce a lot of sperm to stand a decent chance of fertilizing eggs. In that environment, large testes would be advantageous. Male frogs that mate on land, however, having little or no competition, shouldn’t need to waste tissue and energy growing bigger testicles; not all that much sperm is required to get the job done.

Bell and her colleagues tested their hunch on two frog families, one found only in the Neotropics and the other distributed widely in the Americas and Australia. And they indeed found that the male frogs that went in for mating in little terrestrial pieds-à-terre have smaller testicles than their pond-preferring brethren.

Bell emphasizes that the discovery “only came after decades of meticulous work from herpetologists in many countries who preceded us. And ultimately, we’re going to need many more years of field work to test our hypothesis.”

Bell hopes the growing research may bring frogs some long-overdue respect.

“We have so many words for birds—raptors, corvids, shorebirds, songbirds, and so on,” says Bell. “And the same is true for mammals. But the vocabulary for frogs is incredibly impoverished. Basically, it’s just ‘frogs’ and ‘tree frogs.’ There’s tremendous diversity among the frogs. They’re fascinating and beautiful animals, and they tell us a great deal about environmental conditions. They deserve more attention.”

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