Calling fast and slow: Hybridization risk affects female choice in Spadefoot Toads

The presence of another species changes female preferences.

From an evolutionary perspective, individuals “want” to get their genes into the next generation, and beyond. Finding a suitable partner is thus of utmost importance. Mating with a different species is a tricky strategy that can be successful, as illustrated by cases of adaptive introgression (see for example this blog post). In most cases, however, hybridization will be detrimental. The resulting hybrid offspring can be unviable or sterile, sending an individuals genes into an evolutionary dead-end.

The risk of maladaptive hybridization could act as a selective force, potentially shaping the evolution of certain behaviors. Take, for example, Mexican Spadefoot Toads (Spea multiplicata). Females of this species tend to prefer males with fast calling rates, because a rapid fire of calls signals a good condition. However, a closely related species – the Plains Spadefoot Toad (Spea bombifrons) – also produces fast calls. Females of the Mexican species might mistake males of the Plains Spadefoot Toad for potential partners, which might lead to maladaptive hybridization. Selection against hybridization would favor Mexican females that prefer slower calling males of their own species.

Choice Experiment

Gina Calabrese and Karin Pfennig put this idea to the test. They collected 53 females of the Mexican Spadefoot Toad and subjected them to a choice experiment. The toads were exposed to two speakers: one with a fast calling rate (37 calls per minute) and one with a slow calling rate (26 calls per minute). Meanwhile, the researchers played a background chorus of calling toads. This chorus consisted of either only Mexican Spadefoot Toads or a mixture of Mexican and Plains Spadefoot Toads. This clever experimental set-up allowed the researchers to see whether female toads will adjust their preference – by hopping to one of the speakers – when hybridization with another species is a possibility.

At the beginning of the results section, we find a good summary of the findings: “Female preferences depended on the presence of heterospecifics in the chorus background. In the mixed-chorus background, females preferred the slower call rate. In the pure-conspecific chorus treatment, however, females as a group did not express a preference.” So, indeed, female Mexican Spadefoot Toads change their preference for calling males if they run the risk of hybridizing.

The choice experiment revealed that females show no preference when only one species is calling in the background (left section). When a mixed chorus is playing, however, the females mainly select slow-calling males (right section). From: Calabrese & Pfennig (2022).

Sexual Selection

The outcomes of this experiment suggest that female choice – and thus sexual selection – can change when the risk of maladaptive hybridization is high. Although this study focuses on amphibians, I cannot help but extend this insight to birds. It is easy to imagine that similar situations can occur in our feathery friends. And indeed: in a 1997 Nature paper, Glenn-Peter Saetre (who I actually met a few months ago, see this blog post) and his colleagues reported that female Pied Flycatchers (Ficedula hypoleuca) prefer dull males when Collared Flycatchers (F. albicollis) are around. Normally, females of this species select high-quality black-and-white males. But the risk of maladaptive hybridization – hybrids are sterile – forces females to change their preferences. Mate choice is not an easy process…


Calabrese, G. M., & Pfennig, K. S. (2022). Females alter their mate preferences depending on hybridization risk. Biology Letters18(11), 20220310.

Saetre, G. P., Moum, T., Bureš, S., Král, M., Adamjan, M., & Moreno, J. (1997). A sexually selected character displacement in flycatchers reinforces premating isolation. Nature, 387(6633), 589-592.

Featured image: Mexican Spadefoot Toad (Spea multiplicata) © William L. Farr | Wikimedia Commons