What doesn't kill you makes you stronger: Detoxification ability as a mechanism of honesty in a sexually selected signal

Abstract

Abstract Sexual selection leads to the expression and maintenance of colourful signals. The metabolic pathways to produce such signals often involve toxic byproducts that can reduce survival. However, rather than discarding these otherwise harmful byproducts, animals may use them by integrating them into sexually selected traits. We tested this using the damselfly Hetaerina americana, where males bear a red wing spot (RWS) that has evolved by intrasexual competition. By using confocal microscopy and several biochemical techniques, we determined that the RWS are generated by ommochrome pigments derived from tryptophan metabolism. Second, we injected a group of males with the toxic precursor of these ommochromes, 3‐hydroxy‐kynurenine (3‐Hk), confirming the toxicity of this compound in adult males. Finally, we showed that adult males injected with a median lethal concentration of 3‐Hk had more ommochromes in their RWS than controls but similar survival, suggesting that the deposition of ommochrome pigment in the wing counteracts the 3‐HK toxicity. Thus, we report that sexually selected pigmented signals may involve the co‐option of excreted compounds that could otherwise have lethal effects, a hypothesis we call ‘detoxifying ability signalling’. Our results provide new insights about the evolution of sexual signals, elucidating a mechanism for the evolution of honest indicators of quality that could have arisen due to natural selection.