Spadefoot toads break the rules in dry weather

A metamorph (with head encased in mud) crawls out of a drying pond and emerges onto land for the first time. Most of the remaining tadpoles (the muddy, pebble-shaped objects) will probably die from dehydration.

Desert-dwelling animals have all kinds of clever tricks for surviving in their dry environments. This includes the spadefoot toad, which is named for the hard, pointy “spade” on its hind feet, which is used for digging.

Spadefoot toads breed on a single night following a rainstorm. The male spadefoot toad (left) will call throughout the night to attract a mate. The female (right) can use the male's calls to tell what species he is and decide whether he would make a good mate.

Spadefoot toads have quite a problem in dry weather. The females lay their eggs in small pools that form during rainy seasons, but sometimes the pools dry up before the tadpoles develop into little toads that can survive on dry land.

A new study shows that spadefoot parents get around this problem in a very unusual way.

Most animal species don’t breed with each other, and spadefoots generally don’t either -- but they will during dry seasons if it helps their young survive, Karin Pfennig of the University of North Carolina reports in the 9 November 2007 issue of the journal Science.

Two spadefoot species, Spea bombifrons and Spea multiplicata, both live in the southwestern United States, where they often occupy the same territory. They can mate with each other, though there are pros and cons to doing so.

On the “pro” side for S. bombifrons, the hybrid tadpoles develop faster than tadpoles with two S. bombifrons parents. The hybrid tadpoles are therefore more likely to survive if they are born in a shallow pool that dries up quickly. On the “con” side, the hybrids tend to have problems reproducing once they become adults.

Dr. Pfennig has discovered that when S. bombifrons females are breeding in particularly shallow pools, they seem to decide that the pros of breeding with the other species outweigh the cons. They will break the usual rule and mate with S. multiplicata males in order to produce fast-developing tadpoles that will hop out of those shallow pools before they dry up.

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