Female toads seek a good man—even if he's another species

MARCH 20, 2020

by Brian Owens, Inside Science, American Institute of Physics

The one thing about species that most people probably remember from high school science class is that when it comes to sex, they generally stick to their own kind. Hybrids happen, but they are usually thought to be accidental, and the results typically have drawbacks—think of how a horse crossed with a donkey results in a sterile mule.

But the reality is more complicated than that. Karin Pfennig, an evolutionary biologist at the University of North Carolina at Chapel Hill, has for years been observing something strange in the southwestern U.S. Female plains spadefoot toads sometimes choose to ignore the males of their own species, and instead mate with males from a closely related species, the Mexican spadefoot toad.

The toads do this under very specific circumstances, said Catherine Chen, a behavioral ecologist who works in Pfennig's lab. Their tadpoles grow up in temporary ponds, and when the ponds are particularly shallow the female plains spadefoots prefer to mate with Mexican spadefoots. This seems to give their offspring a better chance of survival.

"Hybrid tadpoles develop more quickly, so they are more likely reach maturity before the shallow ponds dry up," said Chen.

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