Dietary canthaxanthin reduces xanthophyll uptake and red coloration in adult red-legged partridges

Abstract

Carotenoids give color to conspicuous animal signals that are often the product of sexual selection. To know the mechanisms involved in carotenoid-based signaling is critical to understand how these traits evolve. However, these mechanisms remain partially understood. Carotenoids are usually viewed as scarce dietary antioxidants whose allocation to ornaments may trade against health. This trade-off would assure its reliability as individual quality signals. In the case of red (keto)carotenoids, the literature suggests that some species may show constraints in their uptake. Canthaxanthin is one of the most common ketocarotenoids in red ornaments of animals. It is often commercially used as a dietary supplement to obtain redder birds (e.g. poultry). We increased the dietary canthaxanthin levels in captive red-legged partridges (Alectoris rufa). This species shows red bare parts mostly pigmented by another common ketocarotenoid: astaxanthin. We studied the impact on the uptake of carotenoids, vitamins and, finally, on coloration. We also tested the potential protecting effect of canthaxanthin when exposing birds to a free radical generator (diquat). Canthaxanthin did not apparently protect birds from oxidative stress, but interfered with the absorption of yellow carotenoids (lutein and zeaxanthin). Zeaxanthin is a precursor of astaxanthin in enzymatic pathways, and their values in tissues and eggs were lower in canthaxanthin-supplied birds. This led to lower astaxanthin levels in ornaments and paler colorations. As far as we know, this is the first report of a carotenoid supplementation decreasing animal coloration. The results have implications for understanding carotenoid-based signaling evolution, but also for improving husbandry/experimental procedures.