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In the American Southwest, free love is vital to toad survival

Spadefoot toads are on the lookout for the fittest mates — species agnostic.

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The desert conditions of the Southwestern United States can make life tough for water-dwelling creatures. But some of the ecosystem's animals have learned to adapt — and get creative with their love lives.

Plains spadefoot toads, Spea bombifrons, are so concerned with finding the most suitable mate they're willing to set their sights outside the species.

In fact, they are most likely to get experimental with their love lives when living in a shallow pond, finds a new study, published Thursday in the journal Science. The finding suggests that interbreeding confers a survival advantage on the toads, helping them to safeguard the next generation.

The research also calls into question one of the biggest consensuses biologists have about interbreeding: namely, the idea that it’s always a disadvantage.

For spadefoot toads' sexual selection, it’s all about mate quality. That means creating offspring that will be well-suited to their environment. To manage that, Plains spadefoot toads appear to prefer mating with Mexican spadefoot toads, S. multiplicata, when they’re living in shallow ponds, but not when they live in deep ones.

The hybrid tadpoles those cross-breeders produce develop faster than non-hybrids, giving them a head start on reaching adulthood before the limited water in the pond dries up.

Mexican spadefoot toad calls out for mates. David Pfenning

What’s more, female spadefoot toads aren’t randomly choosing their partners. These ladies are picky.

They’re more likely to go for a male toad with an especially attractive mating call, found study authors Catherine Chen and Karin Pfenning from the University of North Carolina.

New theories about breeding across species

Interbreeding between species creates hybrids, and biologists tend to consider that a disadvantage. Hybrids have been thought to have some sort of downside, like a genetic issue. (Think of the mule, the outcome of a donkey-horse combo, which can’t reproduce.)

Traditional thinking has also been that these occurrences are usually random.

But in some cases, stepping outside the species is actually advantageous — and deliberate. Biologists are beginning to adjust their theory to accommodate the fact that hybridization may be preferable when the offspring has a chance of being better adapted to the environment.

The new study on the spadefoot toad provides an example, adding to other recent evidence: Butterfly species in the Heliconius genus are avid cross-breeders, according to a study published in October 2019.

The latest findings “contradict conventional wisdom about the disadvantages of hybridization and provide a connection between species diversification, sexual selection, and, ultimately, the context dependence of behavioral evolution,” writes biologist Marlene Zuk in an article accompanying the new study in Science.

Nate Edelman, a graduate student at Harvard University and author on the October 2019 butterfly study told Inverse at the time that hybridization has one obvious upside if you live in a tough environment — like an amphibian living in a desert region.

“One good thing about genetic diversity is if you have a really variable and diverse population, when environmental conditions change, you have a better chance of responding to them,” Edelman said.

Abstract: Hybridization—interbreeding between species—is generally thought to occur randomly between members of two species. Contrary to expectation, female plains spadefoot toads (Spea bombifrons) can increase their evolutionary fitness by preferentially mating with high-quality males of another species, the Mexican spadefoot toad (Spea multiplicata). Aspects of Mexican spadefoot males’ mating calls predict their hybrid offspring’s fitness, and plains spadefoot females prefer Mexican spadefoot males on the basis of these attributes, but only in populations and ecological conditions where hybridization is adaptive. By selecting fitness-enhancing mates of another species, females increase hybridization’s benefits and exert sexual selection across species. Nonrandom mating between species can thereby increase the potential for adaptive gene flow between species so that adaptive introgression is not simply happenstance.
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