Extrapair mating and the strength of sexual selection: insights from a polymorphic species

Abstract

Extrapair mating could drive sexual selection in socially monogamous species, but support for this hypothesis remains equivocal. We used lifetime fitness data and a unique model species, the dimorphic white-throated sparrow (Zonotrichia albicollis), to examine how extrapair mating affects the potential for sexual selection. In this species, the morphs employ distinct reproductive strategies, with white males pursuing extrapair mating at higher rates than tan counterparts. Social and extrapair mating is disassortative by morph, with paternity exchange occurring primarily between pairs composed of white males and tan females. Bateman gradients and Jones indexes indicated stronger sexual selection via mate numbers in white males than in females and tan males, and generally did not differ between females as compared with tan males. Extrapair mating contributed more to the Bateman gradient for white than tan males, and white males also had higher variance in annual reproductive success. However, variance in lifetime reproductive success did not differ between morphs or sexes. Moreover, extrapair mating did not increase variance in male reproductive success relative to apparent patterns, and within-pair success accounted for much more variance than extrapair success. Thus, extrapair mating by white males increases Bateman gradients and the potential for sexual selection via mate numbers. However, our latter results support previous research suggesting that extrapair mating may play a limited role in driving the overall potential for sexual selection.

Effects of haemosporidian co-infection and parasitemia on reproductive strategies in a polymorphic species

“…W´T pairs have higher extra-pair paternity and a lower ratio of genetic offspring belonging to the social father with lower body condition nestlings than T´W pairs. Our results are congruent with the pair-type reproductive strategies of the white-throated sparrow, where W´T pairs are more promiscuous, while T´W pairs have a more monogamous relationship involving mate guarding (Grunst et al 2019b).…”

“…Adult morph (white or tan) was confirmed by genetic analyses (Michopoulos et al 2007). To assess reproductive success, the number of genetic offspring (paternity) was determined and adjusted with the allocation of extra-pair paternity (EPP) by following the protocol of Grunst et al (2019b). In our statistical analysis, EPP was qualified present in a nest when the social fathers were not the genetic father of one or more of the nestlings.…”

“…However, the proportion of both morphs is relatively stable due to disassortative mating, where tan males mate with white females (T×W) and white males pair with tan females (W×T) (Tuttle et al 2016). The behavioral differences between morphs imply antagonistic reproductive strategies for each pair type, giving similar fitness to each morph and consequently keeping the equilibrium between the morphs in a population (Grunst et al 2019b).…”

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

“…1995; Grunst et al. 2019). The Seychelles warbler is a good candidate species to test this prediction, as it displays a socially monogamous and genetically promiscuous breeding system, as well as sexual dimorphism in body size (males being larger than females; Kingma et al.…”

“…2012; Grunst et al. 2019). Studies assessing whether genetic promiscuity increases the opportunity for selection among males in different social mating systems (including monogamy, polygyny and multimale groups with one or more females) also provided variable results (see Weatherhead and Boag 1997 [birds]; Jones et al.…”