Two years ago, Juan Díaz Ricaurte was hiking in the mountains of Brazil when a male yellow cururú toad latched onto his boot. Díaz Ricaurte gently detached the frog and placed it back on the ground, several meters away; Undeterred, he jumped back and wrapped his arms around the shoe again. “I was super focused on grabbing Juan’s boot,” says Filipe Serrano, Díaz Ricaurte’s biologist colleague, who witnessed the meet-cute. The frog seemed to have mistaken Diaz Ricaurte’s footwear “for a potential mate,” and kept re-tightening it again. Little Lotario, Serrano said, “would not let it go.”
Nor, in a certain sense, Serrano or Díaz Ricaurte. The date with the toad boot was finally over, but in the months that followed, the two researchers from the University of São Paulo couldn’t get the incident out of their minds. It wasn’t exactly the toad, or the boot, or even the doomed union between the two: frog mating, known as amplexus, usually involves a male bonding with a female, and it’s not uncommon for overeager suitors to initiate a wrong relationship. to hug. What stuck with the couple, they told me, was the possibility that these unfortunate events, which they had both heard of before, might be increasing in frequency, as the frogs try to navigate an increasingly fractured world. Scientists call this an evolutionary trap. “The environment changes and they make more mistakes,” says Ulrika Candolin, a biologist at the University of Helsinki. Temperatures are rising; habitats are wearing away; animals are forced to mix with new and unknown species. Sex, for some species, seems destined to take a hit.
Serrano and Díaz Ricaurte, along with their colleague Marcio Martins, began searching for earlier accounts of frog hugs gone wrong, formally called misdirected amplexus. They found nearly 400, a whole cabal of frogs clinging to things they would almost certainly never be able to fertilize: dead frogs, incompatible frog species, and frog embryos still inside eggs; coconuts, mangoes and apples; geckos, turtles, fish and slugs; plastic balls, rulers and cups; and even some cow and yak dung. The compendium makes it clear that randy frogs can sometimes be seriously misled.
However, what is not obvious is why. Misdirected amplexus is not always cause for alarm. A little chutzpah can actually befit a single frog, especially in species that mate only a few nights a year, or in populations where females are especially rare. “The males will go after anything they can get their hands on,” says Liz Lopez, a California-based wildlife biologist who has studied misdirected amplexus. Super-tight hugs are an ideal way to get males into the right place at the right time, when their mates release their eggs; they can also protect themselves from other suitors trying to break through. Serrano calls it a “squeeze first, ask questions later” strategy: It’s far better to be too aggressive and risk making a mistake than to give in to shyness and end up without sex altogether. There are even rare cases of sexual problems that turn out quite well: necrophilia that has actually resulted in the extrusion and fertilization of viable eggs; unions between species that, in some circumstances, give birth to offspring better adapted than staying with one’s own species.
Too many bugs, though, and the frogs can quickly find themselves in trouble. Each mistake marks a missed opportunity: time that could have been spent finding a more suitable partner and then relating to her. Some males cling to a mate for weeks, even months, without eating the whole time. “That’s very expensive,” and hardly worth the effort if no offspring arise, says Juan Carvajal Castro, a biologist at St. John’s University who has studied amplexus. (Males that accidentally grab other males sometimes elicit a protesting squawk that prompts them to let go; however, dead frogs and apples cannot give such warning calls.) A prolonged hold outdoors can also leave the wrong males more vulnerable to predators and disease, Diaz Ricaurte told me.
So there is a danger that the amplexus goes south too often. Which could very easily in today’s rapidly changing world. The precious terrain on which frogs live and mate is increasingly scarce; Carvajal Castro points out that some species that live in drier habitats, for example, may mate only after periods of rain fill local ponds, events that are becoming more dispersed as climate change accelerates. The frogs also have a hard time hearing and responding to the croaking and calls of their potential mates in the increasing urban din. Hungry for suitable mates, the frogs might start looking elsewhere. And as humans continue to encroach on wild spaces, frogs will undoubtedly come into more frequent contact with new species, or even unknown objects, that could distract them from better prospects. Candolin, from the University of Helsinki, has been observing that exact disaster with male European fireflies, which are sometimes lured away from luminous females by equally bright garden lights. “They fly around the lights, instead of mating and reproducing, and they die there,” he told me.
If something similar is happening to frogs now, Serrano and Díaz Ricaurte’s database of confused clingers will not be able to prove it. To really build the case for climate change or habitat alteration interfering with amplexus, researchers would have to slowly and systematically track frog mating over time while monitoring local environmental conditions; they would need to compare amphibian populations between locations and try to figure out which ones had the most sexual success. Most of the incidents in the database, on the other hand, were recorded by chance, by a select group of people who encountered a failed frog in flagrante delicto. “There’s a lot of potential for bias with that,” says Karin Pfennig, a frog biologist at the University of North Carolina at Chapel Hill.
Still, Serrano is intrigued by the glimpses of evidence the team saw during the search, during which researchers “tried to report as much as we could about each observation: air temperature, precipitation, altitude,” he told me. There have there have been many reports of misdirected amplexus in super degraded habitats, and many of them have happened in recent years. A recent study even suggested that climate change may be pushing certain European toads to live with the wrong species. And there’s something to be said for the presence of speckled human paraphernalia throughout the database, which the frogs wouldn’t have much to do to otherwise interact with. Just ask the American toad seen making out with a tennis ball in a Virginia park in 2007.
An increase in the misdirected complex is something many scientists could easily miss: The natural inclination, Lopez told me, is just to think oh that’s weird and move on. But the phenomenon is worth taking a closer look at, he said, especially considering “every other stressor for amphibians right now,” including deadly fungal outbreaks and dry ponds. Failed sexual encounters are the last thing the frogs need to add to their list of problems.
Sex, and the act of provoking it, is fundamental to the existence of any frog. “People often see these events as bugs and dismiss them as not significant,” Pfennig told me. “No one thinks of putting them in a larger context.” A database is a starting point: the core of something that could grow into something bigger and, perhaps more importantly, inspire bigger, bolder projects to determine the causes of a problem and develop solutions. Misdirected amplexus could become a kind of early indicator, demarcating endangered populations; could alert researchers to species teetering on the brink.
If misdirected amplexus among frogs really is on the rise at the hands of humans, it won’t be the first time people have changed the mating habits of other species. Birds stunned by traffic noises have had trouble hearing the songs of their mates. Fish mystified by chemical contaminants, which can interfere with social cues, have chosen suboptimal mates. Beetles attracted to the brown sheen of textured glass have tried to lay down on beer bottles in Western Australia. Spying on these misplaced acts of affection can seem strange, even voyeuristic. But sex is serious business, and perhaps people owe it to their neighbors to take a good look from time to time.