Relationships, monogamy, and infidelity are not just of importance to humans. Many species of animals are “socially monogamous”, meaning they will raise offspring as a pair. In birds, this means that parents will tend a nest together, protecting it and bringing the chicks food. Despite this, infidelity is common among supposedly monogamous birds - more than 90% of 200 “monogamous” bird species will mate outside of their primary relationship. While male birds having multiple mates tends to be self-explanatory – male birds who have more than one partner will have more offspring – female infidelity is more puzzling. Non-monogamy can be risky for a female bird. If her regular mate notices that too many of the offspring in the nest are not his, he will leave the nest, leaving the female and her chicks vulnerable. So why risk it? This study investigates potential reasons why.
While infidelity is a more controversial subject in humans, it can provide some benefits in animal societies. This study utilised evolutionary models to analyse the effects of female infidelity on cooperation in the wider community. Female birds commonly mate with a neighbouring male and will raise the chick in her home nest. Her male partner will protect and collect resources for the nest, including the chick that is not his own offspring. Competition for resources and nest territory would typically encourage aggressive behaviours toward neighbouring males. This study found that since males also had chicks outside the nest, they behaved more cooperatively and less aggressively as this would benefit their offspring as well. If a neighbouring male collected resources for his nest, he was also collecting resources for another male’s offspring.
While this study focuses on birds, it has some broader implications about the evolution of cooperation and mating patterns. Mating outside the pair can increase cooperation, as offspring are raised throughout the community. This helps explain why species that typically do not engage in group behaviour (such as hunting together) can develop stable communities. Infidelity is generally not an effective strategy for cooperation and group stability in humans, but other species have effectively used this strategy. This study shows that cooperation can evolve under surprising conditions, and different mating patterns can drive cooperative behaviour. Entry by Jess Naramore