The hypothesis that breakdowns in monogamy – aka cheating – could be a strategy used by female birds to strengthen defenses against predators has been confirmed via a study on pied flycatchers.
Some evolutionary psychologists have spent decades claiming it is natural for males to try to father children with many mothers, while females should be choosier and only pick the best fathers. Scientific evidence has moved on, however, revealing animal dating strategies are far more complex and varied than initially acknowledged.
The latest example comes from pied flycatchers (Ficedula hypoleuca). Although these birds usually raise their young in pairs that look straight out of a 1950s sitcom, DNA evidence reveals the male doing the work to feed the young often is not the biological father. Researchers found a way to test an evolutionary explanation, and published the confirmation in Proceedings of the National Academy of Sciences.
Adult birds put great effort into protecting their young, but some predators are so much larger or more fearsome that two birds need help to defeat them. It has been suggested that a female who has shown multiple neighbors a good time may have a support network to call on in a crisis. Indeed, blue tit (Cyanistes caeruleus) populations where extra-pair paternity is common are less likely to lose their entire brood before the young reach adulthood, but the link between these two measures has not been confirmed.
The team tested the idea by setting up nesting boxes in threes. By timing installation, males nesting in Box A got the opportunity to mate with their neighbors, but those housed in boxes B and C were denied the same chance. Stuffed predators were made to appear to threaten nests B and C, and Krams and co-authors watched the responses.
Females showed no interest in helping out their neighbors when threats appeared, and neither did males from boxes B and C. The box A males, however, knowing there was a chance some of the neighboring broods were their own, often joined the defense of those young. They did not, however, neglect their duties in protecting their own nest.
It seems the males had some inkling of the success of their extra-pair mating, turning up much more often when one of the threatened young actually was theirs than when they had mated with the female without result. Sadly, we may never know what males from boxes B and C thought about their surprisingly helpful neighbor.
Early evolutionary theorists theorized that female infidelity would diminish their partner's incentive to help raise the young, and therefore must be disadvantageous and rare. They were much more comfortable with examples like the pied flycatcher's other behavior, where a male will establish a second nest, and then go back to their first partner.
However, genetic testing revealed how frequently paternity lies outside the pair, particularly among bird species. In the extreme case of Australian magpies, the majority of young are not fathered by the male doing half the feeding and guarding, forcing a search for more subtle explanations.
As with all examples of animal mating behavior, care should be taken before drawing implications for humans – what works for one species often doesn’t apply to others. Although it is common for officially monogamous humans to have sex with people other than their partners, having children under these circumstances is remarkably rare, particularly compared to birds. Nevertheless, the work raises a possibility to consider when studying human sexuality, rather than endlessly relying on a limited sample of stereo-reinforcing animals.