By Rebecca Morelle

Science reporter, BBC News



See how the male toads' mating tactics compare

Male S. bombifrons Male S. multiplicata

US researcher Karin Pfennig found female spadefoot toads will flout the general evolutionary rule of not breeding with other species.

She discovered that the amphibians, under some conditions, will mate with other species to help boost the survival rates of their offspring.

The research is published in the journal Science.

Spadefoots breed in small ponds, which can often dry out, killing any developing tadpoles.

Dr Pfennig, from the University of North Carolina, has discovered that when a pond is very shallow, one species of female spadefoot, S. bombifrons, will often mate with another closely related species, S. multiplicata, rather than males of their own kind.

She believes the reason is down to tadpole development.

S. bombifrons tadpoles develop much more slowly than S. multiplicata, meaning they have to spend longer in the ponds.

But the hybrid offspring between the two species develop rapidly, meaning that they are more likely to survive if the pond dries out quickly.