Three yellow patches differently correlate with escape behaviour, morphological traits, leukocytes, parasites, and hormones in a lizard species
Description
Multiple within individual and non-redundant signals can convey complementary information about individual quality in lizards. This visual information is commonly provided by colour patches of different hues (red, yellow, blue). However, whether different within-individual colour patches of a single hue can contain non-redundant information remains understudied. To test this idea, we investigated the spectrophotometric reflectance of three colour patches in adult males of Acanthodactylus erythrurus, a lizard of the family Lacertidae that has colour patches that look similarly yellow to the human eye. We modelled the spectral properties of these patches using model averaging and cross-validation as a function of head volume and body length (proxies of resource allocation to somatic growth), escape behaviour (proxy of quality to cope with stress), body condition (proxy of nutritional state), leukocytic profiles (proxy of immune state), and faecal testosterone metabolites (proxy of reproductive state and aggression). The different relationships of the three “yellow” patches with the independent predictors analysed suggested that they can provide complementary information about the males’ quality in the context of the sexual selection theory.
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We ran all statistical models in R version 3.4.3 (R Core Team, 2017). We performed a within-effects F test that compared the spectral variables of the three “yellow” patches investigated within a given individual. In addition, we performed twelve models, one per spectral variable (luminance, chromaVIS, chromaUV, spectral hue) and colour patch (cheek, first eyespot, outer ventral scale). All the models included as independent variables z-standardized ordinal date of capture, body condition, head volume, body length, log10-transformed counts of both mites and blood parasites, [FTM], and the two WBC principal factors. We modelled the PC of escape behaviour of the lizards with the four spectral properties of the three colour patches as predictors and included [FTM] as covariate (Moore and Marler, 1987). [FTM] was later removed from this model because its effect was estimated not significant, which allowed us to include in the analysis those lizards without faecal sample (N = 41). To reduce over parametrization, we used the Akaike’s criterion to compare four separate models, each containing or luminance or chromaVIS or chromaUV or spectral hue of the three colour patches. We checked the residual distribution and the autocorrelation of the models based on variance inflation factor (VIFs, which were < 2 for all predictors in the four models) with the vif() function of the ‘car’ package (Fox and Weisberg, 2019). We used an information criterion corrected for small sample sizes (Bedrick and Tsai, 1994) and we applied a multimodel inference approach (package ‘MuMIn’; Barton, 2018), which is a method of variable reduction recommended in ecology (Hegyi and Garamszegi, 2011). We considered sufficiently informative all models with ΔAICc ≤ 4 in relation to the best model (i.e., the one with the lowest AICc) (Burnham and Anderson, 2004). We also summed the AICc weights of all the models where the predictor appeared (i.e., conditional average) to calculate the relative importance of each variable in the averaged model. Using this procedure, we calculated the significance (α < 0.05) of the effects and their z-standardised ß coefficient ± adjusted standard error (SE). The latter z-standardization enables comparing the magnitude of the effects. The resulting final models were cross-validated using a k-fold split of 3 using the R-package ‘DAAG’ (Maindonald et al., 2015).