The mechanisms involved in the production of red carotenoid‐based
ornaments of vertebrates are still poorly understood. These colorations
often depend on enzymatic transformations (ketolation) of dietary yellow
carotenoids, which could occur in the inner mitochondrial membrane
(IMM). Thus, carotenoid ketolation and cell respiration could share
biochemical pathways, favoring the evolution of ketocarotenoid‐based
ornaments as reliable indices of individual quality under sexual
selection. Captive male red crossbills (Loxia curvirostra
Linnaeus) were exposed to redox‐active compounds designed to penetrate
and act in the IMM: an ubiquinone (mitoQ) or a superoxide dismutase
mimetic (mitoTEMPO). MitoQ can act as an antioxidant but also distort
the IMM structure, increasing mitochondrial free radical production.
MitoQ decreased yellow carotenoids and tocopherol levels in blood,
perhaps by being consumed as antioxidants. Contrarily, mitoTEMPO‐treated
birds rose circulating levels of the second most abundant
ketocarotenoid in crossbills (i.e. canthaxanthin). It also increased
feather total red ketocarotenoid concentration and redness, but only
among those birds exhibiting a redder plumage at the start of the study,
that is, supposedly high‐quality individuals. The fact that mitoTEMPO
effects depended on original plumage color suggests that the
red‐ketocarotenoid‐based ornaments indicate individual quality as
mitochondrial function efficiency. The findings would thus support the
shared pathway hypothesis.