Oxidative stress constrains avian mate choice: Experimental evidence for song- and testosterone-mediated pathways in Zebra Finches (Taeniopygia castanotis)

Oxidative stress is proposed as a key physiological mediator of reproduction, yet the specific pathways through which it influences avian mate choice remain poorly understood. Here, we experimentally manipulated oxidative stress levels in male Zebra Finches (Taeniopygia castanotis) through BSO-induced glutathione depletion (high oxidative stress) or dietary supplementation with CoQ10 (low oxidative stress). Female mate preference was then assessed in a three-choice arena, and male phenotypes—including coloration (cheek, chest, beak, flank), song characteristics (syllable duration, frequency parameters), body condition, circulating testosterone and gonadotropin-releasing hormone levels—were systematically quantified. Results showed that oxidative stress significantly reduced male attractiveness, as females spent less time with BSO-treated males (P < 0.001) and exhibited a stronger preference for CoQ10-supplemented ones (P = 0.01). These effects were not attributable to coloration or body conditions: although multiple coloration parameters (cheek brightness, chest brightness, beak chroma, cheek hue) were altered (all P < 0.05) and body condition (BSO: 0.31 ± 0.17, Control: −0.21 ± 0.18, P < 0.05) were improved, these changes did not directly mediate mating decisions. Piecewise structural equation modeling revealed two primary indirect pathways: a song-mediated pathway, in which oxidative stress impaired song structure (t = −2.685, P = 0.009), thereby reducing attractiveness; and a testosterone-mediated pathway, in which stress induced testicular damage and lowered testosterone levels (t = −2.311, P = 0.024), further diminishing male appeal. This multifaceted assault, from internal physiological state to external secondary sexual traits, collectively diminishes overall male attractiveness. Our study provides a comprehensive mechanistic account of how oxidative stress acts as a potent physiological constraint on sexual selection, advancing insight into the physiological constraints affecting sexual selection.