Zebra finches are birds which, once they’ve found that special someone, will tend to stick with them. They are what is called ‘socially monogamous’, meaning they form pairs to mate and raise offspring over multiple breeding seasons, but with the occasional sneaky mating with the next door neighbour. Males court females by singing to them. 

Mating with birds that are not your partner, so-called ‘extra-pair copulation’ makes sense for males: the more females a male can mate with the more offspring he will have in the following generation. However, since this is not true for females, why might they also play away from home? It is bad news for a male if she does so: he may end up raising chicks that aren’t his. It also has its disadvantages for the female: if her partner suspects that the offspring may not be his, he may stop helping care for the young. There is also the risk of sexually transmitted diseases.

A zebra finch pair

Scientists have recently proposed two reasons why females might mate with males other than her partner. Both of these possibilities are due to constraints in the birds’ genes. ‘Genetic constraints’ are basically cases where natural selection would love to do a really good job as usual, but is held back by those troublesome little genes. Sometimes what would be optimal in a perfect world just isn’t feasible given the way genes work.

For example, perhaps the genes which are related to males being more promiscuous are also the same genes that make females so. We can imagine a situation where males that were more promiscuous would have more offspring. This would mean that their genes relating to promiscuity would become more abundant in the next generation. They will also be passed on to females where they could result in females also being promiscuous, even if it is not directly adaptive for them to be so.

The second possibility proposed by the scientists for why females might be sexually promiscuous is also due to a genetic constraint: perhaps the genes that affect the female’s response to her partner are the same genes that affect her response to other potential partners. This means that if her response to being courted by other males were selected against, say, making her unimpressed by all types of singing, then it also might make her less receptive to her own partner’s singing. Thus she would not reproduce at all and those ‘singing is unimpressive’ genes would die out.

A zebra finch male singing to a female. She doesn’t look massively impressed. Image: Max Planck Institute for Ornithology

These two hypotheses were tested for the first time recently in a study with zebra finches. Using a captive population of over 1, 500 birds, the scientists were able to determine whether either of these explanations seemed likely. Through cross-fostering (switching the chicks between two pairs of parents before they hatch) the scientists found they were not learning promiscuous behaviour from their mother or father, at least at this early stage in life. They also recorded how receptive a female was to being courted, through allowing males to court her (through singing) and seeing what her response was; she either solicited sex, or responded aggressively. The researchers then went on to look at how much females responded to courtship once they were paired with a male, both from this male and from others. They also collected genetic data so they could see just how often a female’s chicks were not her partner’s.

Some females were very keen to mate with their partners, but not as keen on mating with other males. Other females were more keen to mate with other males than their unlucky partner. As these two things were not correlated, this indicates that the two behaviours (mating with your partner and mating with other males) can be separated from each other. This refutes the second hypothesis put forward by the scientists.

However, males and females that had more genes in common were also more similar in how promiscuous they were. This supports the first hypothesis that female promiscuity could, in fact, exist just because it favours the males (in terms of natural selection) to be promiscuous. So, males that sire more young through being more promiscuous will also have more daughters, carrying these ‘promiscuous genes’. I put this phrase in inverted commas, because in reality it is never as simple as a ‘gene for’ any behaviour – see here to read a bit more about why this is. This finding is exciting, as it is a new way of thinking about why females might be promiscuous when it is not immediately obvious.

This is probably not the only piece to the puzzle, as science is hardly ever as simple as this. However, it does seem likely that this evolutionary constraint is a substantial part of the answer to this question.

Reference: Forstmeier, W., Martin, K., Bolund, E., Schielzeth, H. & Kempenaers, B. (2011) Female extrapair mating behavior can evolve via indirect selection on males. Proceedings of the National Academy of Sciences 108(26): 10608.