Abstract

Infidelity is common among bird species with biparental care and social monogamy, but we still do not know well why females take part in extra-pair behaviour. Males are expected to gain fitness from siring extra-pair offspring, because extra-pair fathers do not expend resources on parental care. This is, however, not the case for females who raise the resulting extra-pair young, and who may risk retaliation from their mate, and other potential costs, posing the question of why females take part in extra-pair matings. The indirect benefits hypothesis offers an explanation: by cheating, females obtain “good genes” that are better, or more compatible, for their offspring. However, this hypothesis is not well supported empirically, evidenced by two contradictory meta-analyses on the topic, and active discussion in the field. Recently suggested, novel, testable hypotheses provide a fresh perspective. These so-called non-adaptive hypotheses do not require female infidelity to be adaptive per se, but rather explain female infidelity as resulting from intra- and intersexual antagonistic pleiotropy. These hypotheses suggest sexually antagonistic pleiotropy, behavioural spill-over, and social network effects to be important. I will present results from my own long-term wild house sparrow population, and from experiments with captive sparrows.