Abstract
Carotenoids are a class of pigments which are widely used by animals for the expression of yellow-to-red colour signals, such as bill or plumage colour. Since they also have been shown to promote immunocompetence and to function as antioxidants, many studies have investigated a potential allocation trade-off with respect to carotenoid-based signals within the context of sexual selection. Although an effect of carotenoids on non-visual (e.g. acoustic) signals involved in sexual selection has been hypothesized, this has to date not been investigated. First, we examined a potential effect of dietary carotenoid supplementation on overall song rate during the non-breeding season in captive male European starlings (Sturnus vulgaris). After only 3–7 days, we found a significant (body-mass independent) positive effect of carotenoid availability on overall song rate. Secondly, as a number of studies suggest that carotenoids could affect the modulation of sexual signals by plasma levels of the steroid hormone testosterone (T), we used the same birds to subsequently investigate whether carotenoid availability affects the increase in (nestbox-oriented) song rate induced by experimentally elevated plasma T levels. Our results suggest that carotenoids may enhance the positive effect of elevated plasma T levels on nestbox-oriented song rate. Moreover, while non-supplemented starlings responded to T-implantation with an increase in both overall song rate and nestbox-oriented song, carotenoid-supplemented starlings instead shifted song production towards (reproductively relevant) nestbox-oriented song, without increasing overall song rate. Given that song rate is an acoustic signal rather than a visual signal, our findings therefore indicate that the role of carotenoids in (sexual) signalling need not be dependent on their function as pigments.
Highlights
Carotenoids are a class of pigments which are widely used by animals for the expression of yellow-to-red colour signals, such as bill or plumage colour, often within the context of sexual selection [1,2]
A potential effect of antioxidants on acoustic signals involved in sexual selection has recently been suggested [12], the authors focussed on potential effect of carotenoids on developmental processes rather than on direct effects on song production during adulthood
Carotenoid supplementation and overall song rate Our results indicate that, during the non-breeding season, song rate in adult male European starlings was significantly elevated less than one week after starting Caro-supplementation, compared to non-supplemented starlings
Summary
Carotenoids are a class of pigments which are widely used by animals for the expression of yellow-to-red colour signals, such as bill or plumage colour, often within the context of sexual selection [1,2]. Two recent studies have reported a positive effect, respectively, of dietary carotenoid supplementation on paternal fanning behaviour in male three-spined sticklebacks (Gasterosteus aculeatus) [10] and of dietary supplementation of an antioxidant (vitamin E) on begging behaviour in yellow-legged gull chicks (Larus michahellis) [11]. It could, be hypothesized that the availability of carotenoids, for instance through their effect on immunocompetence or their antioxidative capacity, could affect noncarotenoid-based signals within a sexual selection context
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