Abstract
Mate choice is expected to be important for the fitness of both sexes for species in which successful reproductionrelies strongly on shared and substantial parental investment by males and females. Reciprocal selection maythen favour the evolution of morphological signals providing mutual information on the condition/quality oftentative partners. However, because males and females often have differing physiological constraints, it isunclear which proximate physiological pathways guarantee the honesty of male and female signals in similarlyornamented species. We used the monomorphic king penguin (Aptenodytes patagonicus) as a model to investigatethe physiological qualities signalled by colour and morphological ornaments known to be under sexual selection(coloration of the beak spots and size of auricular feather patches). In both sexes of this slow-breeding seabird, weinvestigated the links between ornaments and multiple indices of individual quality; including body condition,immunity, stress and energy status. In both sexes, individual innate immunity, resting metabolic rate, and theability to mount a stress response in answer to an acute disturbance (capture) were similarly signalled by variousaspects of beak coloration or auricular patch size. However, we also reveal interesting and contrastingrelationships between males and females in how ornaments may signal individual quality. Body condition andoxidative stress status were signalled by beak coloration, although in opposite directions for the sexes. Over anexhaustive set of physiological variables, several suggestive patterns indicated the conveyance of honestinformation about mate quality in this monomorphic species. However, sex-specific patterns suggested thatmonomorphic ornaments may signal different information concerning body mass and oxidative balance of malesand females, at least in king penguins.