Is it one call or two? Perception of inter-pulse and inter-call intervals in the grey treefrog, Hyla versicolor

Female grey treefrogs, Hyla versicolor, treat a call with an anomalously large gap between call pulses as relatively unattractive. However, whether such a ‘call’ is treated (or perhaps perceived) as a single call or two separate calls, an issue relevant to the problem of perceptual segregation of calls of different males, was unclear. We tested whether there could be gaps shorter than a typical inter-call interval that would be sufficient to elicit the latter percept by offering females a choice between a 5-pulse call and 13-pulse call separated by a 1575-ms gap and these pulse strings separated by shorter gaps. Preliminary results suggested that perception of inter-call gaps was categorical, and the distributions of such gaps between males in choruses were compatible with this finding. However, when we also offered females call alternatives designed to specifically test the hypothesis of categorical perception, predictions were only sometimes met.

Adaptive plasticity in calling site selection in grey treefrogs (Hyla versicolor)

Calling behaviour is an essential component of gaining access to mates, and calling site selection may be an important component of effective communication. Environmental factors like microclimate, or the presence of competitors and predators often show seasonal or spatial variation, and behavioural plasticity that allows the caller to adjust to this variation may be adaptive. Prompted by the observation of across-season variation in elevated calling site use in our grey treefrog population, we formulated three hypotheses about their calling site choice and tested them using field observations and behavioural trials in the lab. We found that calling site selection is largely determined by local temperature regimes, and suggest that this temperature-based plasticity in calling site selection is adaptive because it allows males to increase their metabolic efficiency and mate attraction effectiveness. The mere presence of heterospecific competitors and predators did not affect calling site selection at the pond, but close proximity to a predator during behavioural trials did reduce calling activity. This suggests that grey treefrog males attend to the presence of predators, that they can assess the degree of risk associated with predator proximity, and that they can adjust calling behaviour adaptively to reduce the chances of being detected by a predator.