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Mr. Not Wrong: Not my species? Not a problem

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1:40pm, November 7, 2007
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Female toads that flirt with a male of another species may have their own best interests at heart.

The plains spadefoot toad spawns offspring that grow up faster if dad is a different species called the Mexican spadefoot, says Karin Pfennig of the University of North Carolina at Chapel Hill. The toads begin life as tadpoles in pools of water that can dry up quickly, so a little hop forward in speed of maturation can mean the difference between life and death. Pfennig has now found that in tough times, females tend to prefer Mexican males to plains males.

Spadefoot toads take their name from foot flaps that help the toads dig into mud. In winter, adult toads shovel their way underground and hibernate inside a mud cocoon. But tadpoles can't survive without a pond, so each mating season becomes a race between the next drought and the growing tadpoles.

In earlier research, Marie Simovich of the University of San Diego reported that the plains spadefoot toad (Spea bombifrons) hybridized with the Mexican spadefoot (Spea multiplicata) when they happened to share shallow pools. The hybrids were more likely to turn up in the shallowest pools that would dry out earliest.

Evidence suggested that the mixed offspring don't reproduce as readily as the purebred young of each species. Purebred Mexican tadpoles mature fast, however, and the hybrids beat by a day or two the 4 weeks that the plains tadpoles typically take to mature. "In a drying pond, that can be huge," says Pfennig.

She wondered whether females in desperate straits might actually be choosing the Mexican males. Simovich was skeptical, because the hybrids could just be an accident of numbers or crowding.

So Pfennig brought female toads into the lab and set them in tubs of water mimicking either shallow pools about to dry out or safely deep water. She then serenaded the females from speakers propped up in the tub. One broadcast a quacklike mating call of the plains males, and the other played the Mexican males' advertisements, which sound like someone running a finger over a comb. Females expressed a preference by hopping into the water and swimming toward one of the speakers.

Toads collected from ponds inhabited by both species strongly preferred their own males' calls—as long as the water was deep. In shallow water, the preference disappeared. Analyzing the data in more detail, Pfennig found that females with poor body condition were more likely to switch preferences to the Mexican toads. Eggs laid by skinny, stressed females are more likely to develop slowly, so the switch to hybrid young makes the most sense for such females, says Pfennig in the Nov. 9 Science.

The idea sounds plausible to Glenn-Peter Sætre of the University of Oslo. He has studied flycatcher nestlings that survive better at the end of the breeding season when dad's species differs from mom's.

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