Colour morphs as alternative solutions to the trade-off predicted by the Immuno-Competence Handicap Hypothesis
Colour morphs in polymorphic species represent a suite of heritable traits controlled by separate sets of loci, which correspond to alternative peaks in the fitness surfaces. Pleiotropic effects of hormones have been hypothesized to be involved in the maintenance of morphs, but no experiments have been done to test it so far. In this study, we experimentally increased testosterone plasma levels in colour morph of the common wall lizards to test if white and yellow morphs follow two alternative strategies imposed by trade-off depicted in the Immuno-Competence Handicap Hypotheses. Yellow males invest in a more aggressive behaviour at the expense of a reduced immune function, whereas white males invest in greater survival at the expense of a reduced competitiveness. Plasma T levels were increased through transdermal administration. Before and after treatment, we measured i) the immune-response of lizards through phytohaemoagglutinin (PHA), and ii) the aggressive response by the introduction of a small mirror into the plastic enclosure to mimic the intrusion of a stranger male in its own territory. The hormone administration caused a higher immune-suppression in white than yellow males, and a switch to less aggressive behaviour against the mirrored image. Overall, increased plasma testosterone levels resulted in the disappearance of differences between morphs of both immune-response and aggressive behaviour. These results supplied experimental evidence of the existence of morph-specific strategies in common wall lizards depending on the investment in territorial aggression or in a longer survival, as alternative solutions to the trade-off predicted by the Immuno- Competence Handicap Hypothesis.