Intraspecific variation in calls among environments has been explained through differences in the identity of emitters (see Ey and Fischer 2009
). However, we show the potential existence of a mechanism previously overlooked and operating at a finer scale. The causal structure identified through path analysis, combined with experimental evidence of males' ability to fine-tune their calls, suggests a role of phenotypic flexibility in the adjustment of calls to the environment.
Males of H. pulchellus
have the potential to adjust their calls in response to their local environment. The fact that call attributes were not significantly different at the start of the experiment between free field and enclosure records but differed significantly for the complete call indicates that some process of modulation is involved. Determinants of individuals' call decisions have been poorly considered. However, size-dependent energetic reserves, the effect of temperature on metabolic activity and their effects on the potential to allocate energy to reproductive effort are the main mechanisms involved (McLister 2001
; McNab 2002
; Kiss et al. 2009
). The existence of flexibility in call attributes draws attention to other underlying mechanisms. This kind of flexibility presumes the existence of a feedback response, involving a cognitive process between signal emission and detection of its potential attenuation and/or degradation (Kelley 2004
; Eliades and Wang 2008
). This phenomenon has been reported mainly in primates (Eliades and Wang 2008
). There exists only one report—for only one individual of one species—for amphibians (Lardner and bin Lakim 2002
). Although there are studies that address bird signal variation in response to ambient noise (see for examples Slabbekoorn and Peet 2003
; Patricelli and Blickley 2006
), this evidence is based on different individuals under different noise levels. In this sense, experimental and correlative approaches, as the ones here introduced, could improve the understanding of call modulation in other taxa, and its effects in ecological and evolutionary processes. Also, the present contribution suggests that individuals can use acoustic feedback of their own calls to match propagation properties of their environment in real-time. Yet, there is heterogeneity in the strength of the response and its direction (i.e., modulation upwards or downwards). Exploring the underlying causes of such heterogeneous response now appears as an interesting topic for future research.
Flexibility involves a more complex scenario for signal transmission than previously thought (Endler and Basolo 1998
; Foster 1999
; Boul et al. 2007
). Call flexibility in response to the acoustic properties of the environment affects signal transmission and female attraction (Endler 1992
). This implies a potential connection between flexible phenotypes and sexual selection and in how environmentally driven changes in mating behavior can set the context through which selection drives evolutionary changes (Price 2006
). In this sense, this article opens the door to the analysis of these phenomena within the framework of behavioral reaction norms (Dingemanse et al. 2010
). In addition, our results also support classic interpretations where different calls are attributed to the fact that individuals are different rather than individuals displaying flexible calls (Zimmerman 1983
; Bosch and De la Riva 2004
; reviewed in Ey and Fischer 2009
). Specifically, spectral attributes were related to body size of calling males (). As a consequence, both phenotypic plasticity and fixed attributes appear as important determinants of the observed call structure.
The process determining the association between calls and environment could change at different scales of observation, accounting for the mismatch between habitat type and call structure reported elsewhere (Feng and Schul 2007
). The association between habitat and call attributes has been focused mainly at the macrohabitat scale, with few exceptions (Bosch and De la Riva 2004
; Yao and Lin 2004
). This scale of analysis is congruent with the view of call variation as the result of local adaptations. Regarding amphibians, the limited information available shows little agreement on the potential of the AAH to account for observed call patterns (Bosch and De la Riva 2004
). This could be due to inconsistencies between the scale of observation and the scale at which mechanisms operate when call flexibility is involved (Zimmerman 1983
; Penna and Solís 1998
; Kime et al. 2000
; Bosch and De la Riva 2004
The match between call structure and environment has wide implications in organism performance and evolution. Although most natural variation in calling structure has been studied assuming it as a fixed attribute, our results suggest that call flexibility is also involved. The explicit consideration of a complex causal structure connecting environment, individual attributes, and call structure, allowed to jointly analyze the connection between components that have traditionally been separately considered. An additional implication of our results is that the temporal and spatial scale of changes in calls could be much smaller than previously considered. Thus, our study attempts to advance on the connection between call attributes and environment, highlighting the potential role of call flexibility and unraveling the association between environment, individual traits, and call structure.