Mating outside the species? For animals large and small, that’s generally a big no-no.
But UNC researcher Karin Pfennig has discovered that some spadefoot toads will jump the species barrier — if their offspring can benefit.
Pfennig observed that female spadefoot toads, S. bombifrons, will mate with S. multiplicata — a different but closely related species — if it seems to ensure better survival for their offspring.
The hybridization doesn’t create any sort of super-toad. But it does allow the trait of fast development that is present in S. multiplicata to transfer to S. bombifrons’ offspring. And in an environment where the ponds or puddles where toads lay their eggs can quickly dry up, that can mean the difference between life and death. Cross-species breeding helps S. bombifrons females ensure their offspring will develop out of the tadpole stage in time to survive.
This doesn’t mean that the female spadefoot toads always hybridize; only certain conditions encourage this behavior. Hybrid males are potentially sterile, and hybrid females produce fewer eggs than pure species spadefoots. If a pond lasts long enough, females mate with their own species rather than risk producing infertile offspring. But if a pond is drying so fast that pure S. bombifrons tadpoles can’t escape it, S. bombifrons females do better to hybridize. So while hybridization only happens in certain circumstances, Pfennig says, “it can dramatically affect the outcome of sexual selection and, ultimately, speciation.”
Danielle Del Sol was formerly a staff contributor for Endeavors.
Karin Pfennig performed her research at UNC and in Arizona over the course of seven years. She is an assistant professor in the Department of Biology, the Institute for the Environment, and the Curriculum in Ecology. Her findings were published in the November 9, 2007 issue of the journal Science.