Female canaries invest more in response to an exaggerated male trait

Females choose their mate on the basis of secondary sexual characters such as physical ornaments, courtship behaviours or vocalizations. In male birds, vocalizations are known to increase their attractiveness and consequently their probability of copulation. Moreover, male songs have been shown to shape maternal investment, particularly with regard to egg quality. Artificial manipulation of male attractiveness has been found to affect female sexual preferences and maternal investment. In this regard, supranormal stimuli are thought to increase such responses. The present study was designed to assess whether supranormal acoustic stimuli, known to induce high levels of sexual responsiveness in female canaries, Serinus canaria, could increase maternal investment beyond levels observed in birds exposed to natural songs. Exposure to supranormal songs (G phrases) increased maternal investment: compared to females exposed to moderately attractive (Control) or naturally attractive (A) phrases. Females exposed to supranormal songs laid bigger eggs (egg volume and mass, albumen mass); moreover, females in the G group laid eggs with heavier yolks than females in the Control group. In contrast, no differences were found between groups with regard to testosterone deposition in the eggs. Thus, the adjustment of femalesʹ investment in their brood seems to take various forms that appear to be related to different features of malesʹ sexually selected traits, which raises the questions of how and why these different mechanisms have been selected.