Female swamp sparrows do not show evidence of discriminating between the songs of peak-aged and senescent males

Sexual selection theory predicts that females face contrasting selection pressures when choosing the age of their mate. On the one hand, older males have demonstrated their ability to survive and they may be more experienced than younger males. At the same time, however, younger males are expected to have accumulated fewer deleterious mutations in their germline as compared to older males. These contrasting pressures on female preference may result in a preference for intermediate-aged males. A preference for males of a particular age can only be expressed, however, if females are able to identify males of different ages. We have previously shown that male swamp sparrows display age-related changes in vocal quality, such that males display sharp increases in vocal quality in early adulthood, followed by gradual senescent declines thereafter. We have also shown that territorial males discriminate these within-individual differences, giving stronger aggressive responses to songs of peak-aged males than to those of senescent males. Here, we use a copulation solicitation assay to test whether females also discriminate these within-signaler markers of senescence in song. Contrary to our prediction, females did not show any evidence of discriminating between songs recorded from peak-aged males as compared to songs from the same males following song senescence. We suggest that this difference in demonstrated discrimination between males and females may be the result of the two sexes attending to different song characteristics.

Automated summary

Sexual selection theory predicts that females may prefer intermediate-aged males when choosing mates, as older males have survival abilities while younger males may have fewer deleterious mutations. Research on swamp sparrows shows that males display age-related changes in vocal quality and territorial males can discriminate among different ages. However, females did not show discrimination between songs of peak-aged males and senescent males, which suggests that males and females may attend to different song characteristics.