Allometric deviations of plasma carotenoids in raptors

Because large species ingest proportionally less food than small ones, it may be predicted that they should incorporate relatively fewer carotenoids to a proportionally equal volume of blood. However, some species may increase their levels of circulating carotenoids by ingesting unusual food. We tested whether the plasma concentration of carotenoids scales to the three‐quarter power of mass in nine predatory and scavenger raptor species. No significant allometric relationships were found due to the unusually high concentrations of carotenoids in the Egyptian Vulture Neophron percnopterus and the Andean condor Vultur gryphus. To assess whether these two species deviate from the allometric rule through the exploitation of unusual sources of carotenoids, or due to a physiological adaptation to improve the uptake of carotenoids, we determined allometric patterns in individuals of these two species kept in captivity with an exclusive diet of flesh. Our results provided support for the allometric rule because the slope of the allometric equation did not differ from a three‐quarter exponent when carotenoid levels of the two outliers were replaced by those of captive birds. This adjustment to the allometric rule suggests a lack of any physiological adaptation to improve the uptake of the low concentrations of carotenoids contained in flesh. Differences between species in carotenoid incorporation into the bloodstream may be ultimately due to contrasting evolutionary history, physiology and associated colour‐signalling strategies, but proximately due to the acquisition of these micronutrients from both usual and unusual dietary sources.