Puddles: the hottest pick-up joint for toads. Credit: Science

When the spring rains soak the western United States, two types of spadefoot toads surface from their underground burrows and converge on ponds and puddles to find a suitable mate. Male toads fill the night air with a lusty cacophony of croaks and females use the songs to zero in on a good partner. With only one mating season a year, the girls need to be picky.

But surprisingly, researchers have found that under some conditions the females will chose a partner of another species from the puddle of possible mates. Karen Pfennig, a biologist at the University of North Carolina at Chapel Hill, has discovered that they take this extreme mating measure to ensure that their offspring have the best chance of survival ? even if it means that those offspring will themselves have a lower chance of reproducing successfully.

The two spadefoot toad species involved, Spea bombifrons and S. multiplicata, look much the same to the untrained eye. But S. bombifrons has more of a pug nose and a bump between its eyes, whereas S. multiplicata lacks the bump and has a nose "more like Kermit the frog" says Pfennig.

Generally, when cross-species mating occurs, fertile offspring are not produced. When these two toads get together, they yield males that are sometimes sterile, and female offspring that produce about half the number of eggs as a purebred. That's enough for scientists to label them as different species, and for the two to usually avoid interbreeding.

However, Pfennig had previously noticed that there are some circumstances in which S. bombifrons females will take a chance and seek out a male partner from the other species.

Fancy that

To investigate this further, Pfennig played recordings of songs from the males of each species: the baritone croak of S. multiplicata (video) from one speaker and the tenor quack of S. bombifrons from another (video). She then recorded which speaker the female S. bombifrons approached.

Female S. bombifrons were more likely to approach the call of an S. multiplicata male when in shallow waters, she found.

This suggests that S. bombifrons might benefit in some way from cross-species hybridization when waters are shallow. One possibility is that the toads need their offspring to develop more quickly when water is in short supply, so that the tadpoles turn to toads before the shallow puddles run dry. S. bombifrons are slower to develop; S. multiplicata and hybrids are both faster.

Field tests confirmed that hybrid tadpoles were more likely to survive through metamorphosis in rapidly drying pools. The results are published this week in Science.

Driven to another species

Other animals have been shown to vary their mate preferences according to their surroundings, but this is the first time that harsh environmental conditions have been shown to drive an animal across the species barrier.

?It?s quite spectacular,? comments Maurice Sabelis of the University of Amsterdam in the Netherlands. ?You have an example of one species relying on another to produce viable offspring.?

This type of dependence on another species could have implications for researchers trying to preserve toads, notes Sabelis. Researchers should take heed that things may be more complicated than they have assumed, he says. ?It?s a warning. Such complexities might play a crucial role for species conservation.?

Pfennig says that the results emphasize that mate selection isn?t just a matter of sizing up male ornamentation. ?Females are probably assessing a lot more out there than just how long the male?s tail is,? she says. ?They are probably more sensitive to their own condition and environment when choosing a male.?