Carotenoid ornaments and the spandrels of physiology: a critique of theory to explain condition dependency

Abstract

Even as numerous studies have documented that the red and yellow coloration resulting from the deposition of carotenoids serves as an honest signal of condition, the evolution of condition dependency is contentious. The resource trade‐off hypothesis proposes that condition‐dependent honest signalling relies on a trade‐off of resources between ornamental display and body maintenance. By this model, condition dependency can evolve through selection for a re‐allocation of resources to promote ornament expression. By contrast, the index hypothesis proposes that selection focuses mate choice on carotenoid coloration that is inherently condition dependent because production of such coloration is inexorably tied to vital cellular processes. These hypotheses for the origins of condition dependency make strongly contrasting and testable predictions about ornamental traits. To assess these two models, we review the mechanisms of production of carotenoids, patterns of condition dependency involving different classes of carotenoids, and patterns of behavioural responses to carotenoid coloration. We review evidence that traits can be condition dependent without the influence of sexual selection and that novel traits can show condition‐dependent expression as soon as they appear in a population, without the possibility of sexual selection. We conclude by highlighting new opportunities for studying condition‐dependent signalling made possible by genetic manipulation and expression of ornamental traits in synthetic biological systems.

A genomic basis of vocal rhythm in birds

“…Nevertheless, although we show there is no apparent correlation between IOI and either song frequency or body mass, there is a relationship between IOI and tinkerbird forecrown hue, for which a previous study associated a region on chromosome 8 with carotenoid conversion 32 . Indeed, besides in song, extoni and pusillus mostly differ from human observers in the coloration of their forecrown 32 , as their common names suggest, and such visual communication signals have been widely shown to influence mating preferences and thus could be an important pre-mating barrier to gene flow 71 .…”