Abstract
The question of why socially monogamous females engage in extra-pair
behaviour is long-standing in evolutionary biology. Recent theoretical
work has moved away from the indirect-benefits hypothesis to explain
female extra-pair behaviours, instead favouring suggestions that they
are the result of pleiotropic effects. That is, a trait under strong
positive selection in either or both sexes are genetically linked to
another, often unrelated, trait. For example, where genes beneficial to
female fecundity (contributing to within-pair solicitation of her social
partner) are linked with extra-pair behaviour (soliciting copulations
from extra-pair males). Here, we test two predictions from this
hypothesis: We test the prediction that female divorce, measured from
the number of social mates within a given year, is linked with (1) the
number of extra-pair males and (2) the proportion of her offspring that
are extra-pair. Our results suggest that females who frequently divorce
social partners are more likely to produce extra-pair offspring than
those who maintain social monogamy. However, by contrast, those females
do not also have a higher proportion of extra-pair offspring. The number
of broods initiated was also positively correlated with extra-pair
males, probably through increased opportunity for extra-pair males to
sire offspring over a longer breeding season. Our results provide an
empirical example of a behavioural trait, beneficial to female
fecundity, that is linked with extra-pair behaviour. These empirical
results support the intrasexual pleiotropy hypothesis as a driver of
female extra-pair behaviour.