Condition dependence in animal colouration: tools for a comparative approach

The study of condition dependence in animal colour has undergone important changes in recent years. In particular, the focus on carotenoid-based colour traits is a thing of the past and a broader, comparative approach is becoming more common. Here, we refine a previously developed theoretical model describing pigment expression as a saturating function of resource intake and demonstrate its use as a tool to compare patterns of resource assimilation, resource allocation, and condition dependence between groups of organisms. Using a simple equation and non-linear regression analyses, we apply this saturating model to melanin-based colouration in a model insect species. We find that patterns of condition dependence differ between sexes, phenotypes, and populations, suggesting underlying differences in resource allocation priorities. Our results highlight both the usefulness of this method and the value of a comparative approach in identifying proximate mechanisms of condition dependence.