Birds are considered exemplary monogamists: ask any person on a street about examples of true husband-and-wife commitment and you’ll likely hear lots about swans, sparrows, ducks and loons. It is not surprising taken that usually these species are seen in excessively monogamous circumstances: in most cases as pairs of males and females carefully rising several nestlings, not interfering with other couples. But the reality of the bird world is much less ideal. Cuckoldry – or extra- pair paternity – is by far the most common reproductive strategy observed in birds, found in nearly 90% of all bird species.
Why do females want to betray their husbands and risk loosing their precious paternal care? After all, if the actual, social partner realizes that he has been cuckolded – he may abandon the female at any stage of nestlings development, leaving the mother with almost no chances of successfully raising the chicks alone. By engaging in multiple sexual relations females also risk been exposed to many sexually-transmitted diseases, seriously deteriorating their future reproductive performance. In the presence of so many risk
factors, potential benefits of actually engaging in extra-pair copulations must be really serious. It turns out that extra-pair partners’ genes are the precious that females are after.
Genes may seem to be a very abstract idea to females that are just about to make their very important sexual choices. But genes provided by the father can improve survival of their offspring and actually change the outcome of an evolutionary game that had been taking place in the kingdom of animals since its very beginning. The theory of “good genes” predicts that – by choosing the right partners – females ensure that the best, the most advantageous genes meet in their offspring. Such genes may provide nestlings with better and stronger immune response, or better tolerance to oxidative stress – or may encompass male offspring with the sexiest, the most attractive sexual ornaments. In general “good genes” enhance offspring’s condition, increase their fitness and ensure their greater evolutionary success.
If genes are so essential why females can’t just choose the right “good genes” bearers at the very beginning? Well, as anything precious – super sexy owners of “good genes” are usually outnumbered by average or inferior males, i.e. bearers of worse (or the worst) version of genes. Some females just have to breed with these inferior candidates – and the only way they can improve the quality of at least some of their offspring – is cuckoldry and seeking for additional copulations.
Such genetic benefits seem to depend strongly on the conditions the offspring experiences in the given breeding season. Sometimes cuckoldry provides the offspring with a very substantial survival benefit – but sometimes it has no effect whatsoever. In other words – females have to perform continuous “calculations” whether, in given conditions and taking into account the quality of their partners, it pays off to make this difficult step and engage in extra-pair copulation. So even in evolution the relationships get complicated.
Luckily, human females, or women, do not have to face similar dilemmas. Without any doubts our behaviour has been shaped by similar evolutionary processes – however nowadays our sexual behaviour is shaped mostly by cultural evolution rather than darwinia, biological selection. We still may have such behaviours deep within ourselves, dormant and silenced by social norms – but culture and ethics are the forces that count the most in human societies.
About the Author
Dr Szymon Drobniak: Post-doc at the University of Zürich, Switlerland and assistant professor at the Jagiellonian University in Kraków, Poland. He did his PhD studying quantitative genetics and breeding behaviour of birds in Sweden. Currently his research focuses on the evolution and role of colour in nature. He love travelling and is also a successful graphic designer, totally in love with designing books and scientific illustrations.