It sounds so romantic, the idea that swans mate for life. If one dies, its mate also dies—of a broken heart. How faithful. How tragic. How so not true.

I hate to burst your bubble, but swans are not the faithful mates we imagine them to be. Instead, the birds are Casanovas, rakes, rogues, and cheats. In the dark of night, they sneak away from the nest and visit the swan next door. And it’s not just the males. In fact, in at least one species, the females cheat more than their mates.

This is in spite of the males’ significant contribution to parenting, providing food, fighting off potential predators, and even investing more time incubating the eggs.

We’ve known for a while that when a swan is widowed, so to speak, it mourns for a bit and then finds a new mate. But we assumed that it was a case of “until death do us part.”

It turns out, we’ve been fooled by the birds’ seemingly faithful behavior. With the advent of DNA testing, the truth comes out. According to a well-publicized study of Black Swans in Australia, one in six cygnets are hatched out of “wedlock”! That’s a whole lot of sneaking going on!

Swans aren’t just having affairs. They also get divorced. Pairs break up for a number of reasons, but especially when nests fail. Animals define success by the number of descendants they produce. They can’t afford to be sentimental if it means they die childless.

And it’s not just swans. There is a long list of birds we have assumed were monogamous, including albatrosses, petrels, geese, Sandhill Cranes, Black Vultures, Ospreys, eagles, pigeons, and some owls and parrots. Yet the more research that’s done, the more we realize that looks are deceiving. Even birds that only mate for one season tend to fool around on the side.

 

We can’t blame it all on one gender. Males may find it easier to find time for a quick coupling, but in many birds, it’s the female who wanders. Either she mates with another male, or she leaves an egg or two in someone else’s nest. No one is innocent.

According to a 1988 article by ecologists Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye,

Increasingly, ornithologists and behavioral ecologists have come to view monogamy as part of a “mixed” reproductive strategy in which matings may occur outside the primary pair bond, but both members of the pair still contribute substantially only to the care and feeding of the young from their own nest.

And that was our understanding 30 years ago. With new tools, we are now realizing how true this assumption is.

Birds (and other animals) will do whatever it takes to maximize the number of offspring they leave behind. Extra-pair bonding is one way to hedge your reproductive bet. If your mate proves to be less than optimal, perhaps this other one will have better genes.

It’s kind of sad. I loved the noble idea of a majestic swan committing to its mate for life. It turns out to be one more example of anthropomorphizing animal behavior. On the other hand, as we celebrate Valentine’s Day this week, I now appreciate my monogamous spouse even more!

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The answer to last week’s quiz was Northern Pintail.