As expected, when given the choice between a combination of attractive audiovisual cues versus unattractive audiovisual cues (experiment 1), a significant majority of females preferred the ‘males’ with attractive calls. In contrast with this result, however, when attractive acoustic cues where associated with unattractive vocal sac coloration, and opposed to unattractive calls combined with attractive coloration, only half the females tested exhibited a preference for the males with attractive calls, the other half preferring the more brightly coloured males, despite their unattractive calls.
This study thus challenges the long accepted assumption that communication in nocturnal anurans is mainly acoustic. It clearly demonstrates that the outcome of female mate choice, as predicted on the basis of male calling quality, can be drastically different when additional communication modalities—in this case vision—are taken into account. This result could also help to explain the frequent failure of field studies to find a correlation between male calling behaviour and observed mating success (reviewed by Sullivan & Kwiatkowski 2007
). It now appears clear, that in such studies, once issues concerning the rigorous assessment of male acoustic signals have been addressed, a whole new modality of signals needs to be taken into account in order to accurately predict male quality.
In experiment 2, females were confronted with conflicting acoustic and visual cues. Since previous work (D. Gomez, C. Richardson, M. Théry, T. Lengagne, J.-P. Léna, S. Plénet & P. Joly 2007, unpublished data) has found acoustic and visual cues to be uncorrelated for this species in the study population, such a choice is likely one female is frequently faced with in nature. In this experiment, females equally adopted one of two strategies, preferring either attractive calls or intense vocal sac coloration. One interpretation of this result could be, according to the ‘sensory overload’ theory of Hebets & Papaj (2005)
, that by providing conflicting acoustic and visual information males are ‘jamming’ the females' reception and/or processing system, effectively leading them to choose their mate randomly. A second possibility is that females process individual cues to assess overall male quality (Candolin 2003
) and that in the present experiment both males, although they exhibited individual cues of differing attractiveness, were evaluated by females as being of equivalent overall quality, leading to females not preferring one male over the other.
Another explanation is that females may differ in the attention they pay to different male qualities and therefore present individual variation in preference rules for the various cues of multiple signals. Data relating to rules through which females prioritize several cues are rare. In her review of the role of multi-component sexual signals, Candolin (2003
) suggested that such signals containing ‘multiple message’ cues could enable females to make a more precise choice of mate, prioritizing cues according to their relative importance. In accord with this theory, in their study of Gryllus campestris
, Scheuber et al. (2004)
found that at the population level females preferred calls with attractive spectral characteristics, indicators of high-quality long-term development, over calls with attractive temporal components indicating good current condition. A more complex possibility is that females may vary individually in the rules through which they prioritize multiple cues, rules that could depend, for example, on female genetic make-up, life-history stage or the cost of mate choice (Candolin 2003
). To our knowledge, this is the first study that demonstrates that females may present individual variation in their rules of preference for multiple—and in this case multimodal—cues, with some females favouring attractive acoustic cues, potential indicators of energetic reserves (Prestwich 1994
), whereas others prefer bright carotenoid-based vocal sac coloration, a likely indicator of immune condition (e.g. Faivre et al. 2003
). In addition to the challenge of identifying the various—potentially multimodal—cues involved in sexual communication in nocturnal species, it therefore now appears crucial for our understanding of mate choice that students of sexual selection also take into consideration the possibility of individual variation in female rules of prioritization of these cues using repeatability tests.