Active choice, passive attraction and the cognitive machinery of acoustic preferences

Female preferences for loud calls are widespread among animals that communicate acoustically, but their ‘strategic’ role is still controversial. According to the ‘passive attraction’ hypothesis, females are more likely to respond to loud calls simply because these calls are more effective at stimulating their acoustic receptors and primary neurons. The ‘active choice’ hypothesis, however, assumes that females use sound intensity as a cue to assess the power and location of sound sources and to respond accordingly. To test which hypothesis could best explain the preference for loud calls of female green toads, Bufo viridis, we carried out multitrial discrimination experiments, in which females were given a choice between two stimuli differing in either the power or the distance of their sources or both. In the laboratory, females could discern differences in intensity caused by differences in power or location of the acoustic sources. This ability increased in field experiments, when the speakers and receivers were further apart than in the laboratory experiments. Phonotactic behaviour did not vary significantly between females, but did within females: in their second trial, females were more likely to approach the closer speaker than in the first trial. These findings suggest that sound intensity plays a more complex role in female mate choice than that recognized by the passive attraction hypothesis. To understand female preferences and, ultimately, mate choice, attention should be focused directly on the computational processes of the femaleʹs central nervous system.