Divorce and extra-pair paternity in the Lundy house sparrows

The question of why socially monogamous females engage in extra-pair
behaviour is long-standing in evolutionary biology. Due to a lack of
empirical support among passerine birds, recent work has moved away from
the indirect-benefits hypothesis to explain extra-pair mating behaviour by
females, instead favouring the hypothesis that this is the result of a
pleiotropic effect. That is, a trait under strong positive selection in
either or both sexes are genetically linked with another, potentially
unrelated, trait. For example, genes beneficial to female fecundity (that
promote within-pair solicitation of mating from a male partner) might also
lead to extra-pair behaviour (by also promoting solicited copulations from
extra-pair males).  Here, we test two predictions from this
hypothesis: We test the prediction that female divorce, measured as the
number of social mates within a given year, is linked with (1) the number
of extra-pair males engaged by the female and (2) the proportion of the
female’s offspring that are extra-pair. Our results show that females who
divorce their social partner are more likely to produce extra-pair
offspring than those who maintain social monogamy, supporting the
pleiotropy hypothesis. However, those females did not also have a higher
proportion of extra-pair offspring. The number of broods initiated was
also positively correlated with the number of extra-pair males that sired
a female’s offspring, probably through increased opportunity for
extra-pair males to sire offspring over a longer breeding season. Our
results support the intrasexual pleiotropy hypothesis as a driver of
female extra-pair behaviour.