Colour Morphs as Alternative Solutions to the Trade-Off Predicted by the Immuno-Competence Handicap Hypothesis

Colour morphs in polymorphic species are associated with a suite of heritable traits governed by distinct genetic loci, each corresponding to alternative fitness peaks. Hormonal pleiotropy has been proposed as a mechanism maintaining these morphs, though experimental evidence is lacking. In this study, we tested whether white and yellow morphs of the common wall lizard (Podarcis muralis) adopt alternative strategies shaped by the immuno-competence handicap hypotheses (ICHH). Specifically, we experimentally elevated plasma testosterone levels via transdermal administration and measured changes in (i) immune response using phytohaemagglutinin (PHA) swelling and (ii) the aggressive behaviour in response to a mirror test simulating territorial intrusion. We found that testosterone-induced immune suppression was stronger in white males, while aggression decreased in both morphs. Elevated testosterone eliminated the baseline differences between morphs in both immunity and aggression. These findings provide the first experimental support for morph-specific life-history strategies in common wall lizards, consistent with the ICHH: Yellow males prioritize aggression over immunity, while white males invest more in survival at the cost of competitiveness.