Behavioural compatibility, extrapair copulation and mate switching in a socially monogamous parrot

Extrapair sexual behaviour may facilitate mate switching in monogamous species by allowing individuals to appraise and acquire a different mate. An individual benefits from mate switching if the new mate enhances the switcher's reproductive success more than the current mate. Several factors, including mate compatibility, may influence whether another individual represents a better option than the current mate. In a large, communally housed, captive colony of cockatiels, Nymphicus hollandicus, we tested three related hypotheses. (1) Members of pairs that show low mate behavioural compatibility should be more likely to engage in extrapair sexual behaviour than members of pairs with high mate behavioural compatibility. (2) Individuals that pursue extrapair sexual behaviour should be members of pairs with a high likelihood of reproductive failure. (3) Individuals that engage in extrapair sexual behaviour should later pair with their extrapair partner. We operationally defined pairmate relationships as more compatible when they involved lower within-pair aggression, less distance between mates and greater within-pair allopreening responsiveness, behavioural synchrony and copulation frequency than the opposite pattern of behaviour. Pairs in which neither pair member engaged in extrapair sexual behaviour had significantly higher behavioural compatibility scores than pairs in which at least one member did so. Every individual that engaged in extrapair copulation subsequently re-paired with their extrapair partner. This extrapair copulation and subsequent mate switching effectively dissolved relationships that had poor reproductive prospects and established new, reproductively successful relationships. This result suggests that extrapair sexual behaviour facilitates mate switching in cockatiels, and that behavioural compatibility is important in mate retention and divorce.