Age-dependent trajectories differ between within-pair and extra-pair paternity success

Reproductive success is associated with age in many taxa, increasing in early life followed by reproductive senescence. In socially monogamous, but genetically polygamous species, this generates the interesting possibility of differential trajectories of within-pair and extra-pair siring success with age in males. We investigate these relationships simultaneously using within-individual analyses with 13 years of data from an insular house sparrow (Passer domesticus) population. As expected, we found that both within- and extra-pair paternity success increased with age, followed by a senescence-like decline. However, the age trajectories of within- and extra-pair paternity successes differed significantly, with the extra-pair paternity success increasing faster, albeit non-significantly so, in early life, and showing a delayed decline by 1.5 years on average later in life compared to within-pair paternity success. These different trajectories indicate that the two alternative mating tact...