Warblings about paternity: the divide between narrative and truth

There are stories we like to tell ourselves about how birds live their lives. They’re models of loyalty and integrity when it comes to relationships, we like to imagine. They form pair-bonds for life, raise their offspring together, and remain faithful and monogamous, with any interlopers chased off efficiently by the vigilant male partner. Just like us then…right?  Well, maybe not. It turns out things might not be as simple as that, in either case.


Taking paternity uncertainty
a step too far? A reed warbler
feeds a common cuckoo chick.

In a study published in PLOS One this May[i], researchers from the Konrad-Lorenz-Institute of Ethology in Vienna tried to put the stories to the test. They used a caged male reed warbler to simulate an intruder situation, watching how the male-female ‘couple’ reacted to its presence. Some of the results were expected: the male of the pair tried to chase the intruder away. However the males reacted with seemingly much more aggression when their female partner was present, especially if she tried to approach or encourage attention from the other male. It seemed that at least part of the aggressive behaviour was about putting on a show for her benefit. According to Herbert Hoi, one of the researchers, “it only pays for the male to show off when the ‘babe’ is watching him.”[ii] Some even attacked the female herself instead of the intruder. Not sounding like such nice guys anymore.

The other big question, of course, is how well these strategies actually work — what happens when the eggs are laid and hatched? The researchers addressed these issues as well. They looked at whether after the intrusion, the males were inclined to desert or be less nurturing towards the subsequent offspring, given that an element of ‘paternity uncertainty’ had been introduced. Surprisingly, they were in fact just as keen to feed and care for the nestling chicks despite the possibility that they might not be the ‘real’ father — in some cases they seemed to actually make more effort.

In an unexpected twist, it was the females who seemed less caring towards their own chicks in the aftermath of an intrusion. The researchers suggested this might be because they perceived their male partner as ‘weak’ after he failed to properly chase off the intruder, and as a result became less invested in the whole enterprise. Apparently if the unfortunate male is going to put on an aggressive show, he’d better see it through, otherwise it will not only be wasted effort, but counter-productive in terms of the impact on the family.

But even when he gets it right, does it work particularly well? The researchers also did some genetic testing, and noted that in the population they studied as a whole, extra-pair paternity — offspring being raised by the wrong father — was ‘relatively high’ despite the best attempts of the males, with approximately 21% of nestlings turning out to have been conceived outside the pair bond. The male aggression to the intruder did tend to reduce the probability of extra-pair bond conceptions, but that 21% figure suggests it isn’t the most effective strategy. Maybe that’s because those warblers are pretty good at sneaking about too. Illicit copulations are ‘rarely observed’, being described rather unromantically as ‘usually short and hidden events.’ All in all, not quite the picture we might have imagined.

This shows, perhaps, the dangers of projecting our assumptions and cultural norms onto other species. Often, it turns out, things are more complicated than we imagined. But should that warning go a step further? Might we be also guilty of a bit of cognitive dissonance between assumptions and reality when it comes to human behaviour? It seems likely that we too have a bit more complexity to us than the traditional narrative would suggest.

You occasionally hear the ‘one in ten’ figure quoted for the human version of paternity uncertainty. That is, one in ten families with a child inadvertently being raised by the ‘wrong’ father.  That’s probably a bit of an exaggeration. A study in 2009 suggested that the figure was more like 1 in 25[iii] —perhaps still surprisingly high. However as the study just searched records for children with the same surname but different DNA to their father, it doesn’t necessarily tell us anything about whether the father is aware of the facts. Again the dominant cultural narrative tells us that no man would knowingly tolerate such a situation. But real life and real relationships can be more messy and complicated.

To some extent these assumptions come from a too-simplistic interpretation of the ‘selfish gene’ trope: the idea that individuals will only invest time and resources in caring for their own offspring, motivated by the need to pass on their genes. But there are enough adoptive families, step-families and blended families to prove that parenting is about more than DNA. There are enough examples in nature of social groups where individuals help care for offspring not directly related to them to demonstrate that nurturing doesn’t have to depend on genetic relationships.  And that, perhaps, is even more encouraging than the idealistic narrative of the perfect family: for both the warblers and for us.

[i] http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0062541#abstract0

[ii] http://www.vetmeduni.ac.at/en/infoservice/current-issues/presseinformation/presseinfo2013/press-release-05-16-2013-whos-your-daddy-infidelity-and-paternity-in-reed-warblers/

[iii] http://www.sciencedirect.com/science/article/pii/S0168952509001334

Sarah Byrne

Sarah Byrne is a mathematical biology PhD student at Imperial College London. Her science writing and fiction can be found at sarahbyrne.org

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