He Doesn’t Mind Being Shared, Unless His Mates Try to Eat Each Other’s Eggs

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From the standpoint of sex, male frogs tend to segregate into two camps: the monogamous bunch and the free wheeling philanderers.

This split seems to apply to all amphibians, which don’t really compromise between these two amorous extremes. Researchers have long found this odd; plenty of other animals practice group fidelity, wherein one male strikes up a long-term liaison with several females at once, but won’t engage with anyone else.

Now, a team of scientists has found a rule-breaking river frog in Brazil’s Atlantic rainforest called Thoropa taophora, whose sexual shenanigans involve everything from cannibalism to peacemaking hugs and some surprisingly well-muscled forearms.

Over the course of a single breeding season, some males will couple up with exactly two females, who alternately visit their mate like a sperm-dispensing timeshare.

“There is a bond between the male and the female, but there is more than one couple,” said Fábio de Sá, a biologist at the University of Campinas in Brazil and an author on a paper published Wednesday in Science Advances that describes the new behavior.

Much of Thoropa mating boils down to location. These frogs prize wet, rocky habitats called seeps, where their eggs can mature. Males will wage all-out war over seeps, emitting aggressive calls and jabbing at their rivals with the spine-studded thumbs at the ends of their bulging, battleworthy forelimbs. Once their real estate is won, the victors spend their nights on patrol, in the hopes that a female will drop by and leave behind a cache of fresh eggs.

Where seeps are abundant, both sexes of Thoropa taophora frogs take multiple partners, Dr. de Sá said. But when seeps become scarce enough to render some males homeless, the frogs are sometimes forced into a seismic sexual shift.

“For females, males become the limiting resource,” Dr. de Sá said. Under these circumstances, females queue up to mate with the seep-straddling males. In a series of video recordings, the researchers found that two, perhaps three, females might share the same beau, with one usually emerging as a “dominant” consort who monopolized most of the mating.

But the group courting the males “had consistent membership” over a long period of time, said Kelly Zamudio, a biologist at Cornell University and an author on the study. A genetic analysis of the eggs laid in each male’s seep also revealed that the tadpoles were all full- or half-siblings of various ages — a hint that they had come from two moms who repeatedly visited the same dad.

Mating systems like these are something “people have suspected,” said Lauren O’Connell, a biologist at Stanford University who wasn’t involved in the study. “But this was really a test of that idea.” She called it “a really heroic effort of explaining the mating system of a species in the wild.”

The females, however, were not always eager to share. Upon arriving at the seep for an evening tryst, some tried to eat the eggs already there, Dr. de Sá said. To curtail this filial cannibalism, the male sometimes attempted to create a diversion, clasping the female with his burly arms from behind — a typical mating position. But the encounter didn’t always end in sex; sometimes the male seemed to simply wrap the female in a platonic embrace, “like a distraction hug,” Dr. O’Connell said.

This whole situation might sound less than ideal, especially for non-dominant females, Dr. Zamudio said. But when breeding sites are this limited, “it’s better to be a secondary, or even the third, female in a group,” she said.” At least you have a chance of laying some eggs with the male, rather than going off to be on your own.”

The findings also help complete the puzzle of group fidelity, which has now been documented in all classes of four-legged animals, or tetrapods, Dr. de Sá said.

“In the big scheme of evolution, we can say we are filling in a piece of information,” he said. “That is pretty cool.”

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