Sex Difference in Condition Dependence of Carotenoid Gapes in the Eurasian Roller (Coracias garrulus).

In altricial birds, sex differentiation can start early in the ontogeny in the form of color, physiology, and/or growth and may potentially result in sex-specific condition dependence of traits mediating parent-offspring communication. Carotenoids have long been hypothesized to modulate the expression of gape coloration, but their sex-specific role enforcing honesty of gape coloration remains poorly studied. In a within-nest design, we provided carotenoid supplementation to nestlings of the Eurasian roller (Coracias garrulus) and measured the response in circulating carotenoids, coloration of the gape, cutaneous immune responsiveness to phytohemagglutinin, and growth while accounting for the sex of nestlings. Male nestlings supplemented with carotenoids displayed enhanced pigmentation of their gapes and grew faster than control nestlings, but there was no within-individual correlation between gape color and growth in either carotenoid-supplemented or control males. Female nestlings, however, diverted most supplemented carotenoids into growing fast at the expense of reducing their level of circulating carotenoids and displaying less-pigmented gapes. Nestling cutaneous immune response was not affected by carotenoid supplementation in either sex. Our results provide only weak support for the hypothesis that carotenoids enforce the honesty of gape color signals in nestling rollers and demonstrate sex specificity in how nestlings divert a surplus of carotenoids into different physiological functions.