Partner retention as a mechanism to reduce sexual conflict over care in a seabird

The costs of caring for offspring are predicted to lead to an evolutionary conflict between parents, where each parent benefits if the other provides the majority of care. However, in many biparental species, breeding partners remain together for multiple breeding attempts and their respective future reproductive fitness prospects are therefore intertwined. Since an increase in current care by a long-term partner reduces that partner's future investment and longevity, individuals do not automatically only benefit when their partner provides care. We test whether selection will favour individuals that reduce the burden of care falling on long-term partners, thus decreasing evolutionary conflict over parental care. Using a semi-natural, captive colony of black-headed gulls (Chroicocephalus ridibundus), we show that benefits of long-term partner retention can indeed reduce sexual conflict. Long-term partners have less intense courtships and are more behaviourally compatible, and individuals in long-term pairs spare each other's resources by increasing their own parental investment compared to those with new partners. Lastly, we demonstrate that high partner compatibility in newly-formed pairs can select for commitment to the pair bond. Our results highlight that compatibility benefits of long-term partnerships can increase selection for mate retention and increase parental investment.