Mate fidelity improves survival and breeding propensity of a long-lived bird.

Abstract

Evolutionary and behavioural ecologists have long been interested in factors shaping the variation in mating behaviour observed in nature. Although much of the research on this topic has focused on the consequences of mate choice and mate change on annual reproductive success, studies of a potential positive link between mate fidelity and adult demographic rates have been comparatively rare. This is particularly true for long-lived birds with multi-year, socially monogamous pair bonds. We used a 26-year capture-mark-recapture dataset of 3,330 black brent Branta bernicla nigricans to test whether breeding with a familiar mate improved future breeding propensity and survival. We predicted that experienced breeders nesting with a new partner would have rates of survival similar to familiar pairs because long-lived species avoid jeopardizing survival since their lifetime fitness is sensitive to this vital rate. In contrast, we expected that any costs of breeding with a new partner would be paid through skipping the subsequent breeding attempt. We found that unfamiliar pairs had lower subsequent breeding propensity than faithful partners. However, contrary to our expectations, individuals breeding with a new mate also suffered reduced survival. These results add to a small number of studies indicating that a positive relationship between mate retention and adult demographic rates may exist in a diverse array of avian species. Given these results, researchers should consider costs of mate change that extend beyond within-season reproductive success to fully understand the potential adaptive basis for perennial social monogamy. We caution that if mate retention enhances survival prospects, improvements in annual reproductive success with pair-bond length could be a secondary factor favouring perennial social monogamy, particularly in species with slower life-history strategies. Furthermore, some cases where annual reproductive success does not improve with pair-bond duration, yet multi-year pair bonds are common, could be explained by benefits afforded by mate fidelity to adult vital rates.