Thermal dependence of signalling: do polymorphic wall lizards compensate for morph-specific differences in conspicuousness?
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Selection for signal efficacy (detectability) is an important factor driving the evolution of chromatic signals. Communication theory predicts that colour signals should evolve to show those properties that maximize their conspicuousness to receivers in their own visual environment. In the ventrally polymorphic lizard Podarcis muralis, visual modelling has shown morph-specific differences in chromatic conspicuousness (orange > yellow > white). Although this suggests that morphs may incur different detectability costs, the differences in conspicuousness could be compensated behaviourally if individuals of the more conspicuous morphs adopted postures that made their colour patches less visible. We quantified the degree of exposure of the lizard ventral and ventrolateral coloration in the field to investigate the relationship between body posture and colour morph. We used a classification based on four lizard postures, from ventral surface completely hidden to full exposure of the ventral and ventrolateral colour patches (when lizards extend their forelegs and raise the head). As these postures may have consequences for thermoregulation, we also recorded substrate and lizard body temperatures using a thermographic camera. Results did not reveal differences among morphs in the frequency with which they adopt postures that expose their colour patches. In contrast, we found a strong relationship between body temperature and lizard posture. Overall, our results support the view that, regardless of colour morph, perching wall lizards adopt the elevated postures that maximize exposure of their ventral colour patches only when their body temperature is high enough to allow for an efficient predator avoidance response.
Selection for effective communication favours flashy, highly conspicuous coloration, but conspicuousness also makes colour signals more detectable to unintended receivers (e.g. predators). Some animals offset the costs associated with conspicuous coloration by evolving compensatory traits. In the lizard P. muralis, ventral colour morphs differ in conspicuousness and we hypothesized that the most conspicuous morphs might compensate for their increased conspicuousness by adopting postures that minimize exposure of their conspicuous colour patches. Results show that the morph-specific differences in conspicuousness are not compensated behaviourally. In contrast, we found a strong relationship between body temperature and lizard posture, suggesting that lizards, regardless of colour morph, adopt postures exposing their conspicuous colour patches only when their body temperature allows for an efficient predator avoidance response.
KeywordsDetectability Colour polymorphism Signal efficacy Thermal ecology Thermography Visual ecology
The study was supported by FEDER through the COMPETE program (ref. 008929) and Portuguese national funds through the FCT (Fundação para a Ciência e a Tecnologia, Portugal) project PTDC/BIA-BEC/101256/2008 and a grant (CGL2011-23751) from the Spanish Ministerio de Ciencia e Innovación. GPL was supported by post-doctoral grant (SFRH/BPD/94582/2013) from FCT under the Programa Operacional Potencial Humano–Quadro de Referência Estratégico Nacional funds from the European Social Fund and Portuguese Ministério da Educação e Ciência. We thank the editor and two anonymous reviewers for their valuable comments.
Compliance with ethical standards
An attempt was made to minimize lizard disturbance during observations. At worst, lizards avoided the observer by moving away. No lizards were captured or manipulated during the observations. The habitat (i.e. rocks, vegetation) was not altered in any way. Field work was performed under research permit number 2013095-0001 issued by the Préfecture des Pyrénées-Orientales (France). This research complied with the ASAB/ABS Guidelines for the Use of Animals in Research.
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