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

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."

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.

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.