Linking amphibian call structure to the environment: the interplay between phenotypic flexibility and individual attributes

Abstract

The structure of the environment surrounding signal emission produces different patterns of degradation and attenuation. The expected adjustment of calls to ensure signal transmission in an environment was formalized in the acoustic adaptation hypothesis. Within this framework, most studies considered anuran calls as fixed attributes determined by local adaptations. However, variability in vocalizations as a product of phenotypic expression has also been reported. Empirical evidence supporting the association between environment and call structure has been inconsistent, particularly in anurans. Here, we identify a plausible causal structure connecting environment, individual attributes, and temporal and spectral adjustments as direct or indirect determinants of the observed variation in call attributes of the frog Hypsiboas pulchellus. For that purpose, we recorded the calls of 40 males in the field, together with vegetation density and other environmental descriptors of the calling site. Path analysis revealed a strong effect of habitat structure on the temporal parameters of the call, and an effect of site temperature conditioning the size of organisms calling at each site and thus indirectly affecting the dominant frequency of the call. Experimental habitat modification with a styrofoam enclosure yielded results consistent with field observations, highlighting the potential role of call flexibility on detected call patterns. Both, experimental and correlative results indicate the need to incorporate the so far poorly considered role of phenotypic plasticity in the complex connection between environmental structure and individual call attributes.

Figure 1

Path analysis representing the causal model that best explained the relationship among abiotic and call variables. Path coefficients are indicated for each path (link) between variables. local veget. = local vegetation, corresponding to the first axis of a PCA for vegetation density; %H₂O = percentage of open water in the calling site; temp = temperature; and call temporal and call spectral = temporal and spectral call parameters, corresponding to the first and second PCA axes, respectively. Epsilons (ϵ) represent variances unexplained by the model. RMS = root mean square error.

Figure 2

Effect of the styrofoam enclosure on call structure. Boxplots show the result of Student's t-test for paired comparisons. FFM, male free-field recording; ECM, males in the enclosure; FFPB, free-field playback; and ECPB, playback in the enclosure. Individual graphs represent call parameters: DF1, dominant frequency of note 1; DF2, dominant frequency of note 2; DN1, duration of note 1; DN2, duration of note 2; CD, call duration; and INI, internote interval. * for p < 0.05, ** for p < 0.01.

Figure 3

Scatterplot showing the trajectory of temporal variables for males tested with the styrofoam enclosure. Each starting dot corresponds to a male calling in natural conditions, and the arrowheads show the final position, with that male calling from inside the enclosure. Factor 1 and Factor 2 correspond to the first and second axes, respectively, obtained from the PCA performed for temporal variables. Factor 1 represents all temporal variables with the exception of note 1 duration, which is represented by the second factor. Black circles correspond to males calling in a pond on the night of 26 October 2008; gray circles correspond to males calling in a pond on the night of 5 January 2009; and white circles correspond to males calling in a section of forested stream on the night of 27 October 2008.