The Freeloading Boyfriends That Just Won’t Let Go
For male Santa Marta harlequin toads, sex is an exercise in patience.
The ping-pong-ball-size frogs, which are native to a mountainous strip in northern Colombia, spend most of their days milling about the region’s burbling brooks, hoping to chance upon a mate. They don’t often get lucky: Only rarely, for a few days a year around the start of the rainy season, will the species’ much-larger females venture down from the trees to flit through these loose froggy frats. That means the window of amorous opportunity is painfully tight, and “the probability of meeting is very low,” says Luis Alberto Rueda Solano, a biologist studying the toads at the University of Magdalena, in Colombia. The first unattached female a male sees that year might be the only unattached female he sees that year—his one shot at shattering his celibacy until the rains come again.
So when a male harlequin gets a hold of a potential mate, he cannot afford to let her go. He leaps atop her and presses his belly to her back. He snakes his arms beneath her armpits and around her chest. He clings to her, fierce and unflagging, until the female releases her eggs for him to fertilize and the act of mating is officially complete—a milestone, Rueda Solano told me, that the pair might not hit until four or five months down the line.
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While locked in their embrace, males do not eat. They may lose up to 30 percent of their body weight. By the end, some of the frogs—which don’t have ribs or a diaphragm—may end up so emaciated that “you can see the spine through the stomach,” Rueda Solano said. In rare cases, a latched-on male will starve and fall off dead before fathering anything at all. The toads’ perseverance is a lesson in fortitude, a demonstration of serious forearm strength. And it’s all meant to be worth it in the end, if these stubborn suitors get to pass their genes on.
Among frogs, this ultra-clingy act, also known as amplexus (Latin for “embrace”), isn’t that strange. Most species practice amplexus in some form, and there are many unique ways of carrying it out. In some, the male grips the female’s chest; in others, it’s the chin or the hips. A few bulbous-bodied bachelors with arms too short to encircle their mate will secrete a sticky substance and glue themselves to their beau’s bum. No matter how it’s done, amplexus generally allows a pair to stay in close proximity while the female finds a good spot to extrude her eggs, which the male then fertilizes with an external squirt of sperm.
But the harlequin toad’s riff on the method represents “a very extreme case,” because of its duration, says Johana Goyes Vallejos, a biologist at the University of Missouri. Other amplexus-style trysts may last only minutes or hours. Male harlequins, meanwhile, regularly staple themselves to a female weeks before she is ready to drop her reproductive load.
Somehow, the haul has to be worth it. Rueda Solano thinks sheer scarcity may be to blame: Encounters between males and females are vanishingly rare, not just because the sexes spend so much time apart but also because males outnumber females roughly three to one in Santa Marta. Under those conditions, males that call dibs on their mate as soon as possible will win out—even if that’s way, way, way in advance of when she’s ready to actually do the deed. “It’s better to just secure her now,” Goyes Vallejos told me, rather than wait for another available female to show up, which may never happen at all.
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And the early frog doesn’t just get the girl; he also has a better chance of keeping her too. Males that don’t end up partnered at first will hunt down amplexed duos and spend hours, even days, trying to pry them apart, yanking at the first male’s limbs, or trying to wedge his body between theirs. But they don’t often succeed. When Rueda Solano lets males duke it out over females in his lab, he finds that the first suitor to amplex holds his ground some 85 percent of the time. Even smaller, wimpier males don’t get ousted in most cases, says Andrew Crawford, a biologist at the University of the Andes, in Colombia, who studied the frogs with Rueda Solano. That could give scrawny frogs an even heftier incentive to amplex early in the season—it may be the only way they win out over bigger, beefier males.
Even those petite frogs’ grips are ridiculously strong. A few years ago, Rueda Solano and his colleagues collected amplexed pairs from streams and their lab, slipped a little vest made of duct tape onto each male’s torso, and used a pulley to slowly, gently wrest the frogs apart. Separating the couples took forces up to 80 times a male’s body weight—like using an African elephant to lever a 150-pound person out of a hug. (Rueda Solano reassured me that after each experiment, the displaced male climbed back atop the female, unfazed.)
The frogs’ feats of strength don’t shock Karin Pfennig, a biologist at the University of North Carolina at Chapel Hill who frequently lets male frogs amplex her fingers. “It can kind of hurt,” she told me. That same pinchiness makes her wonder what’s in it for the female, who may have to lug her spermy cargo all over the place for a third or more of a calendar year. Unlike the males, females can eat just fine throughout the duration of the tryst. Still, it “can’t be comfortable,” Crawford told me.
The researchers can’t yet say for sure, but maybe the female is hedging her bets too. In the same way that females are a precious commodity for males, “males may somehow be a limiting resource” for females, Sinlan Poo, an amphibian ecologist at the Memphis Zoo, told me. Good real estate for family planning isn’t always easy to come by. Maybe, when females find the right place to lay their eggs, it’s just “nice to have my male with me,” Pfennig said, like a convenient rucksack full of quick-draw sperm. That notion offers a reminder of just how much females can dictate the terms of their relationships—that they’re not just “passive actors” on the receiving end of an annoying hug, Pfennig told me. They can snub males with subpar croaks. They can kick unworthy bachelors away. They can decide which suitors end up tragically unsexed and which get a monthslong free ride.