Quirks and Quarks

A desert toad chooses to mate with another species when times get tough

Under certain environmental conditions, the female plains spadefoot toad chooses the best male of another species to give its offspring the best chance of surviving.

The female plains spadefoot toad will mate with another species for the good of her offspring

Under specific conditions, a female plains spadefoot, left, will mate with another species, the male Mexican spadefoot, right. (David Pfennig)

In order to ensure the best quality offspring, the female plains spadefoot toad chooses the best mate around — even when that male belongs to another species.

The plains spadefoot toad lives in the southwestern United States. For most of the time, it lives underground, emerging to mate with the arrival of monsoon rains which form temporary desert ponds.

Tadpoles grow in the ponds, but if the rains are inadequate, the ponds will dry up before the tadpoles are fully developed, killing them.   

So the female plains spadefoot toad engages in what's known as "adaptive hybridization."

Research suggests the female plains spadefoot toad has evolved heterospecific mate preferences, but only when it is to her advantage to do so. (David Pfennig)

The female plains spadefoot toad normally prefers to mate with males of its own species, but when the ponds are not sufficient to sustain life, she chooses another option that produces very surprising results, according to a study by biologist Catherine Chen and her colleagues

In drier years the female plains spadefoot toad mates with another species —  the Mexican spadefoot toad —  because the offspring of that species develop much faster than other tadpoles. This gives the female's offspring a better chance of survival.

Mexican spadefoot male calling to attract a mate. (David Pfennig )

Chen and her colleagues were intrigued to discover that this alternative mating strategy is not random. The female plains spadefoot toad chooses the highest quality Mexican male, based on his vocalization.  The fact that the female was able to discriminate based on the quality of a separate species call was quite surprising.

There is a cost to this hybrid mating.  While the offspring can develop fast enough to escape the ponds before they dry up, the male hybrids are sterile.  However the evolutionary logic, for the toads, is that in a bad season some sterile offspring are better than no offspring at all.