Carotenoid-dependent signals: Indicators of foraging efficiency, immunocompetence or detoxification ability?

Carotenoids are a large group of more than 600 different biochemicals synth esized by bacteria, fungi, algae and plants. Higher animals obtain these bi ologically active pigments in the diet, and they are subsequently used in d iverse bodily functions. They are disproportionately common components of t he color of animal signals such as those used in sexual communication, sign aling between offspring and their parents and in warning colors. Carotenoid s also play important roles in various aspects of immune function and detox ification, and a trade-off between signaling and these physiological functi ons has been hypothesized. They may either signal foraging efficiency, immu nocompetence or anti-oxidative potential of signalers. Carotenoids are usua lly limiting the maximum possible expression of a signal since supplementat ion experiments have commonly produced more exaggerated signals. There is c onsiderable evidence supporting the prediction that parasites reduce the ex pression of carotenoid-based signals, and that such signals reliably reflec t the ability of the signaler to produce a strong immune response. There is also some evidence consistent with the hypothesis that signals may reflect the antioxidant potential of signalers. If carotenoids commonly have impor tant physiological functions among captive and free-living organisms, we hy pothesize several ways in which this should affect the design of animal bre eding programs and conservation programs involving captive breeding.