The contribution of extra-pair paternity to the variation in lifetime and age-specific male reproductive success in a socially monogamous species

Abstract

In socially monogamous species, extra-pair paternity (EPP) is predicted to increase variance in male reproductive success (RS) beyond that resulting from genetic monogamy, thus, increasing the "opportunity for selection" (maximum strength of selection that can act on traits). This prediction is challenging to investigate in wild populations because lifetime reproduction data are often incomplete. Moreover, age-specific variances in reproduction have been rarely quantified. We analyzed 21 years of near-complete social and genetic reproduction data from an insular population of Seychelles warblers (Acrocephalus sechellensis). We quantified EPP's contribution to lifetime and age-specific opportunities for selection in males. We compared the variance in male genetic RS vs social ("apparent") RS (RS_ap ) to assess if EPP increased the opportunity for selection over that resulting from genetic monogamy. Despite not causing a statistically significant excess (19%) of the former over the latter, EPP contributed substantially (27%) to the variance in lifetime RS, similarly to within-pair paternity (WPP, 39%) and to the positive WPP-EPP covariance (34%). Partitioning the opportunity for selection into age-specific (co)variance components, showed that EPP also provided a substantial contribution at most ages, varying with age. Therefore, despite possibly not playing the main role in shaping sexual selection in Seychelles warblers, EPP provided a substantial contribution to the lifetime and age-specific opportunity for selection, which can influence evolutionary processes in age-structured populations.

Keywords: Age-specific reproduction; Seychelles warbler; extra-pair paternity; lifetime reproductive success; opportunity for selection; sexual selection.

Figure 1

Distribution of lifetime extra‐group paternity (LEGP, top left), within‐group paternity (LWGP, top right), reproductive success (LRS, bottom left), and apparent reproductive success (LRS_ap, bottom right) in male Seychelles warblers (n = 237). Genetic paternity measures—LEGP, LWGP, and LRS—consist of the number of extra‐group, within‐group, and total offspring sired by males throughout life. The social (“apparent”) reproduction measure LRS_ap corresponds to the number of young produced by a male's social female(s), and not necessarily sired by that male, throughout the male's life

Figure 2

The standardized variance in lifetime reproductive success and its lifetime (co)variance components for male Seychelles warblers (n = 237). The variance in lifetime reproductive success is partitioned into the variance in extra‐group paternity (EGP) and within‐group paternity (WGP), plus twice their covariance, 2Cov(EGP,WGP)