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One flew over the cuckold’s nest: a birds-eye view of female infidelity

Wild zebra finch pairs keep their eyes peeled for opportunities to cheat. Simon Griffith

Infidelity between sexual partners is ubiquitous – almost as prevalent as the tight and long-lasting social bonds that couples form.

But thanks to a recent German study of Australian zebra finches, a cheating partner now has a new excuse: they can blame both their male and female ancestors.

It now seems that the genes driving promiscuous behaviour have a long history of being shaped by evolutionary selection in both males and females.

The costs of infidelity can be high, with both males and females likely to desert an unfaithful partner, or at least reduce their investment into the partnership and any resulting offspring. Despite the risks, both males and females regularly cheat on each other.

The evolutionary forces underlying such behaviour have been the focus of research over the past three decades. The potential reward for a male that cheats on his partner is the opportunity to sire additional offspring with another female, without having to invest any parental effort into those offspring.

Throughout the ages, it is well known that high-status males such as kings, politicians and more recently rock stars have produced many offspring outside the marital bed (but of course such behaviour is more widespread).

The difference in the number of offspring sired by such men, compared to those who remain faithful to their partner, is a very powerful evolutionary force – first described by Darwin in 1871 as “sexual selection”.

Successful cheats will, on average, leave more descendants (and therefore genes) in following generations than those males who remain faithful. Cheats will probably also produce children with their own partner and be less likely to be cuckolded themselves.

A male that is attractive enough to succeed in gaining extra-pair copulations with other females is likely to be highly valued by his own partner and she is unlikely to cheat on him.

Be still my cheating heart

In contrast to the male side of the story, the benefits of infidelity are not as obvious for females. An unfaithful female is unlikely to increase the number of offspring that she produces, and she will still have to invest as heavily in offspring sired by an extra-pair male.

The costs of desertion by a partner who suspects he has been cuckolded are also probably higher to a female because she is likely to have to bear the full cost of rearing offspring on her own.

Females typically receive nothing from extra-pair males other than sperm, so it has been suggested that the main benefit to a female from an extra-pair coupling is to gain good genes from that male for some of her offspring.

But despite much work, there has been very little evidence that the extra-pair offspring are better than within-pair offspring. There are already some theories as to why females are as likely as males to engage in risky extra-pair behaviour (and they typically are because “it takes two to tango”).

Wolfgang Forstmeier and his colleagues in Germany have added an exciting new angle to the debate by showing that there is a genetic correlation between promiscuous behaviour in males and females. Their findings suggest that some of the same genes drive cheating behaviour in males and females.

Strong selection on the behaviour in one sex will see the behaviour being expressed in the other sex simply as a by-product of the fact that all of us carry genes from both our mother and father.

A gene that makes a male particularly successful is likely to be passed on to his daughters as well as his sons.

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In the study, Dr Forstmeier and his team analysed thousands of hours of video footage of male Australian zebra finches (Taeniopygia guttata) courting females. The team were interested in whether the males’ advances were rejected or accepted by the female, often resulting in a successful copulation.

The team were able to demonstrate the key “genetic correlation” in the propensity to engage in extra-pair behaviour.

They did so by studying more than 1,500 individuals over five consecutive generations and tracking the behavioural similarities between male and female relatives in a large pedigree.

This excellent and highly detailed study tells us how evolution works in sexually reproducing animals. It shows us that the genetics underlying behavioural traits are often very complex, and our ability to study the Darwinian selection of such traits is difficult.

While males and females are very different in both structure and behaviour, many of the genes that determine male characters will also be carried by females (and vice versa).

This means that as genes move across generations and spend time in either male or female individuals, natural selection will often be trying to pull them in different directions.

In their study, Forstmeier and colleagues show that the genes that help to determine promiscuity in males and increase their success in producing descendants, are quite costly when carried by females.

As a result many females in the population are participating in behaviour that makes little sense for them but may ultimately improve the success of their male descendants.

This study provides a fundamental understanding about how evolution and genetics shape sexual behaviour in socially monogamous animals. The study is of great relevance to humans and any other animals that form long-term pair bonds for reproductive purposes.

_Why do you think people cheat on their partners? Is “it’s only evolution, darling,” an acceptable excuse? Leave your views below. _

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