Toads Know When To Look For Love Across Species Barriers

The love between these toads may seem destined to never be consumated, since they are a female S. bombiforns (left) and a male S. multiplicata, but in shallow water, they can make it work. David Pfennig

Hybridization is usually thought of as harmful for animal reproduction, but spadefoot toads (stars of viral drive-by text messaging) are making it work for them, specifically choosing to engage in interspecies mating when the conditions suit hybrid offspring.

Randy animals aren't always all that choosy, as numerous pictures of canoodling between wildly different species show (Google at own risk), but the preference is usually for their own kind. Related species will interbreed if they can’t find one of their own, but seldom do so as a preference.

Female plains spadefoot toads (Spea bombifrons) are the exception. The plains toads are known to sometimes hybridize with Mexican spadefoot toads (S. multiplicata) where their ranges overlap. Male offspring of such trysts are sterile, however, and while the females are not, they produce fewer eggs than their counterparts with two parents born of the same species.

It looks like a clear example of cross-breeding’s drawbacks, and something only pursued when no potential mate of the same species is around. That seems to be the way female S. multiplicata look at things. However, University of North Carolina PhD student Catherine Chen and supervisor Dr Karin Pfennig report female S. bombifrons will sometimes choose a S. multiplicata partner over one of their own.

That's not because male S. multiplicata are just too devilishly handsome to resist. Instead, hybrid tadpoles metamorphose into their adult forms faster than those with two S. bombifrons parents. For a female living in a rapidly drying waterhole, the choice could be between mating with a S. bombifrons male and probably having all her tadpoles die or mating with a S. multiplicata and laying eggs that produce young with reduced fertility who at least escape an early death.

It's not a great choice, but the latter at least offers some chance of grandchildren. When female S. bombifrons find themselves in waterholes that don't look set to last, they suddenly find S. multiplicata males more attractive.

You can see what female Spea bombifrons see in this handsome charmer, phwaor. David Pfennig

Nevertheless, all S. multiplicata males don’t look (or more importantly sound) alike. Chen and Pfennig bred 20 toads of each species together and studied the offspring. In Science, they report the best predictor of the young one’s fitness was the father’s pulse rate. Toads with slower pulses fathered larger young that developed more quickly, allowing them to escape the pond before it evaporated.

When playing batchalorette with different S. multiplicata males, bombifrons females preferred the one whose slow mating call matched his pulse, but only when in a shallow water pond. In deep water, S. bombifrons toads mostly ignored S. multiplicata mating calls, irrespective of pulse rate, a pattern that was not seen when exposed to calls of members of their own species. Intriguingly, S. multiplicata females are indifferent to pulse rate.

The interactions of toads may seem an amusing curiosity. However, hybridization with Neanderthals, Denisovans and other species of early humans shaped our own evolution, so the work may be more relevant to us than it appears.

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