Why These Frogs Make ‘the Grossest Blunder in Sexual Preference’

Interspecies mating has long been thought of as a mistake. But in the desert, it may sometimes be the best way to breed.

A male Mexican spadefoot toad and a female plains spadefoot toad
An odd couple for the ages: a male Mexican spadefoot toad (left) and a female plains spadefoot toad (right), members of different species that sometimes mate. (David Pfennig / University of North Carolina at Chapel Hill)

When rain falls and water is plentiful, the sex lives of plains spadefoot toads are pretty, well, plain. Females prowl ponds for the suitor with the most winsome call; they pair off to couple, churning out legions of eggs that will hatch into, as genetics might predict, more plains spadefoot toads.

But when the weather gets drier and deep ponds more scant, as they often do in the North American deserts where spadefoots live, this narrative acquires a twist. Female plains spadefoot toads start seeking out not the duckish quacks of their own species but the baritone trills of the Mexican spadefoot toad. Atop the parched landscape, these odd couples mingle, yielding mixed-breed offspring that turn out stunted in some quite serious ways: Males are sterile, and females produce far fewer eggs. It’s a fate that most animals would take great evolutionary pains to avoid. When Karin Pfennig, a behavioral ecologist at the University of North Carolina at Chapel Hill, first noticed the behavior some two decades ago, “I thought it had to be a mistake,” she told me.

It wasn’t. Female plains spadefoots are perfectly capable of telling males of different species apart, Pfennig and her colleagues have found. They’re just deliberately shunning their own kind to ensure their offspring’s success. During their tadpole stage, plains-Mexican hybrids develop much faster than purebred plains spadefoots do. That comes in handy when female plains spadefoots find themselves in especially shallow pools that, in the desert heat, may dry up and dehydrate the vulnerable tadpoles into mush before they mature into adults. The hybrids may not be the most fertile frogs around, but better to produce a brood of so-so babies than one uniformly doomed to death.

The spadefoots’ solution to their real-estate conundrum is one of the most nuclear biological options on hand. When push comes to shove, animals often weigh their options, says Karen Warkentin, an ecologist at Boston University who studies frogs. They might decide to put off breeding for a season, perhaps, or switch habitats before attempting to reproduce. But rarely, Warkentin told me, does the decision matrix point to Should I have hybrid babies? Hybridization—the interbreeding of species—is typically thought of as an error, a reproductive dead end; the British biologist Ronald Fisher once called the act “the grossest blunder in sexual preference” an animal might make. And in many cases, that’s a fair assessment. Mules and hinnies, born of unions between horse and donkey, are generally sterile; big-cat hybrids such as ligers and tigons—blends between lion and tiger that have been bred in captivity—often end up in very fragile health.

But in recent years, the view that hybridization has unilaterally catastrophic consequences has begun to shift. Looking outside one’s own species for sex can actually come with big benefits, especially as the planet becomes a less-hospitable place to live. With pollution on the rise, different species of killifish have traded genes and upped their resistance to toxic chemicals; in regions where winters are milder, snowshoe hares have melded with jackrabbits and donned a less-conspicuous brown coat. Even modern human genomes tout a mixed heritage of Homo sapiens, Denisovan, and Neanderthal. And hybrids may be much more common than researchers once thought: By some estimates, about 10 percent of animal species are known to regularly mix their genes with another’s. “That’s likely to be a pretty large underestimation,” says Stepfanie Aguillon, an evolutionary biologist at Stanford University who is studying hybridization in birds and fish.

A male Mexican spadefoot toad calls to females in an Arizona pond. (David Pfennig)

What’s especially unusual about the spadefoots, though, is that they showcase hybridization at both its best and worst—and female plains spadefoots seem capable of weighing the costs. In times of plenty, the calculus is simple. Female plains spadefoots can count on their ponds to be deep and homey, and will gladly shack up with males of their own ilk. It’s the obvious choice, given that hybridizing would produce less-fertile offspring.

But that logic can turn on a dime. Out in the desert, ponds can evaporate in a matter of days, far fewer than the four or so weeks that purebred plains tadpoles usually need to complete their metamorphosis. “If that pond is totally dry before the tadpoles become little toadlets, all of them are toast,” Catherine Chen, a biologist who studied with Pfennig, told me. The little frogs desiccate into “this really gross, sort of crunchy stuff that we call tadpole brittle.” To avoid the massacre, female plains spadefoots in shallow pools will gravitate toward male Mexican spadefoots, whose genetic input can shave a couple days off of the typical plains developmental cycle. “Even having an afternoon head start can make the difference between dying and getting your legs and being able to hop off,” Pfennig said.

That the females can be this discriminating is its own froggy feat. They “will spend a couple of hours in their ponds, swimming around, presumably listening to males, and maybe evaluating the pond itself,” Pfennig said. During these recon missions, the females weigh both their physical surroundings—whether the pool they’re swimming in is shallow or deep—and their own health. Skinny, poorly nourished plains females, which have fewer resources to provision their eggs and tadpoles, seem more hot to trot with outsiders. Female plains spadefoots are even discerning enough to pick out the calls of male Mexican spadefoots that will be most likely to sire healthy, fast-growing hybrids—potential dads identifiable, apparently, by their particular slow-pulsing trills, according to Chen’s research. That amount of pickiness is one thing within a frog’s own species. Having an ear for another is even more impressive.

These early choices can cause all sorts of social ripple effects on the females’ fellow frogs. To minimize their chances of being overlooked, Chen and Pfennig have found, male plains spadefoot toads will actually hang around male Mexican spadefoots when their gals start to stray. “It may be a way to mooch off an attractive neighbor,” says Kim Hoke, a biologist and animal-behavior expert at Colorado State University. Pfennig thinks this stealthy strategy might be especially useful when the females’ choice between same species and other species isn’t obvious—when, say, females are in okay health, and ponds are of intermediate depth. An interloping plains male could easily rise above the Mexican spadefoot fray.

Mexican spadefoots don’t seem to get much out of this whole arrangement. Already graced with fast-maturing tadpoles, they stand only to lose out from extra-species mating. Things go especially awry when female Mexican spadefoots rendezvous with male plains spadefoots: “They’re extra messed up,” Pfennig said, ending up even less healthy and less fertile than other hybrids. Wary of stumbling into those pairings, female Mexican spadefoots will sometimes avoid fast-calling males of their own species, whose quick-clip croaks start to sound too plains-y.

A male plains spadefoot toad calls to females in a New Mexico pond. (David Pfennig)

As for the plains-Mexican hybrids in the middle? Because their males are sterile, they’re unlikely to be striking out as their own species anytime soon. But Pfennig is eager to better understand what happens when female hybrids mate back into purebred plains and Mexican spadefoot populations, and how all those genes will clash or collaborate in generations to come. The mixed-lineage frogs have, over the course of many years, become a conduit for DNA to shuttle between the two species—pathways that could become more important, Hoke told me, as the world continues to warm and deserts become even harsher, and deep ponds tougher to find. “Understanding hybridization really matters for the future of these frogs,” she said.

The plains-Mexican hybrids aren’t abominations or flukes. Where they sit, at the blurry line between species, turns out to be not such a bad place. Mixed heritage has imbued these frogs with resilience they would not have otherwise had. And it has challenged traditional notions of how creatures like them can be categorized. Species boundaries are really “human constructs,” Aguillon told me, and though crossing them has long been thought of as a biological taboo, they may not matter much to the rest of the world: “Does a frog really view another frog as different to themselves?” On the edge of definition, hybrids are a reminder that species aren’t always an either/or phenomenon—and that while blends between animals may sometimes be a dead end, they can also, on occasion, be a newly blazed trail.