Arboreal Calling Behaviour Across the Range of the Spring Peeper: Potential Consequences and Benefits
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Understanding the trade-offs that affect potentially adaptive traits is fundamental to our understanding of evolutionary diversification and speciation. Heterogeneous landscapes may lead to spatial variation in such traits among populations as they may experience different selective pressures. In this study, I investigate the spatial variation associated with arboreal calling behaviour in the spring peeper, Pseudacris crucifer. Finding marked variation in the median height of perching and the proportion of males perching within a population, I investigated the factors that might mediate this behaviour. Spring peeper males at the northern extent of their range almost exclusively call from ground-level, whereas individuals in populations towards the central and southern part of the species’ range are largely arboreal callers. Results of a playback transmission study using male advertisement calls at different perch heights suggest that there is a benefit to arboreal calling: less call degradation across longer distances. To understand some potential physiological costs associated with calling from elevated perches, where individuals would experience higher wind and evaporative water loss, I performed a desiccation experiment using plaster model frogs. Model frogs at high perches lose water at a markedly faster rate than those on the ground, suggesting an increased risk of desiccation for arboreal individuals. Arboreal calling behaviour may therefore be partially governed by a trade-off between reproductive attractiveness and physiological limitations. In addition to spatial variation in arboreality, I investigated morphological variation associated with this behaviour. I found longer limb length and larger body size in arboreal individuals, suggesting possible local morphological adaptation for populations with increased propensities for calling from elevated perches. Whether arboreal calling behaviour is a behaviourally- plastic, context-dependent trait or is fixed within populations remains to be studied, although the trade-offs mediating its prevalence in a population offers a unique opportunity to evaluate the genetic underpinnings of a potentially adaptive trait that is found in only some populations of a species with a dynamic evolutionary history.