Reproduction is known to increase the basal metabolic rate and generate oxidative stress, a possible proximate cost of reproduction. Carotenoids have been shown to be in vitro antioxidant molecules and, in a number of instances, to contribute in vivo to the antioxidant protection of the organism against the deleterious effects of free radicals and oxidative stress. These compounds are also involved in the up- and downregulation of the immune system. Thus, carotenoids may improve a male's health status and condition during breeding and enhance his attractiveness through, for example, a higher investment into mating activities. The differential allocation hypothesis predicts that females should invest more in reproduction when mated to more attractive partners. Therefore, a supplementary dose of carotenoids during breeding should increase male attractiveness and translate into higher reproductive success via a higher reproductive effort by their mate. We tested this hypothesis in great tits by supplementing males with carotenoids during their female's fertile period. We subsequently transferred entire clutches into unmanipulated foster nests. Thus, any effect of our carotenoid supplementation to males on their reproductive success must be due to female differential reproductive investment. Offspring sired by carotenoid-supplemented males were found to grow bigger and heavier and to fledge more successfully. Carotenoid-supplemented males also lost less paternity. Our results illustrate the fitness benefits males can accrue from carotenoids and underline the selective pressure imposed on males to optimize carotenoid acquisitio
