To Cheat or Not To Cheat?

Research Blogging / by Dave Munger /

Across the animal kingdom, the decision of whether or not to be faithful to a mate often comes down to Darwinian considerations.

Credit: Hunter-Desportes

Dear Darwin,

I’m seeking the ideal mate, and I just don’t know what to do. I could mate with the biggest, strongest male in my niche, but all the other females are just as interested in him as I am, and I’m worried he won’t give our offspring and me the attention we deserve. I could have a less-attractive male all to myself, but then I run the risk of having inferior offspring. I’m half-tempted to pair up with a runt, but then mate with the hunk on the side. What do you think? Is monogamy all it’s cracked up to be?

— Evolving in Evanston

Dear Evolving,

You said it, sister! True monogamy is actually quite rare among animals. Take birds, for example. While there are lots of socially monogamous birds, pairing up for a season or even for life, true loyalty is much more rare. As Jeremy Yoder, a graduate student who studies symbiosis in moths and Joshua trees notes on his blog, as many as 90 percent of bird species “cheat” on their mates, and about 11 percent of chicks aren’t fathered by the bird the mother has paired with.

As you point out, female birds may be cheating in order to improve the fitness of their offspring, choosing a social partner for loyalty but getting their eggs fertilized by a stronger, more attractive male, and also possibly to reduce the chances of inbreeding. Males are motivated to cheat by the prospect of disseminating their genes more widely. But in the real world, the results aren’t always as straightforward. Yoder points to a 2010 study led by Andrea Townsend and published in The American Naturalist. The researchers found that female crows, while exhibiting social monogamy, did indeed engage in “infidelity,” both with males inside and outside the family group.

But far from being more fit, the offspring that resulted from such promiscuity tended to be more inbred than the “legitimate” offspring of a pair. The only beneficiaries seemed to be the adulterous male crows, who indeed did disseminate their genes more widely.

So for a female like you, cheating might seem like a good option, but it may not pay off as handsomely as you might hope.

— Darwin

Dear Darwin,

I’m a male poison frog, and I’ve been noticing that most if not all other species of frogs seem to be getting much more action than I do. I’ve been loyally tending to just one mate, distributing her tadpoles to tiny pools formed in the forest canopy’s foliage. Every day I check on each of the tadpoles to make sure they’re getting plenty to eat, and call to my mate to provide for them when they’re hungry. It’s a tough life, and I’m beginning to wonder whether it’s worth it. Am I missing out on the kind of fun other frogs are getting?

— Pondering Promiscuity in Peru

Dear Pondering,

Let me guess, you’re a Ranitomeya imitator. I hear this a lot from your species. It can be tough being the only truly monogamous amphibian, but you do get some benefits from the arrangement as well. “GrrlScientist,” an evolutionary biologist and freelance writer based in Germany, described a truly amazing study of your species last month.

Your monogamous behavior gives you a real advantage over similar frogs like R. variabilis. Because both you and your mate care for the tadpoles, they can survive in much smaller pools than R. variabilis, whose parents abandon them after placing them in individual pools. Since their parents don’t feed them, the tadpoles need bigger pools, where there’s plenty of food. You’re able to thrive in territories other frogs won’t touch, but it only works if both parents tend to the tadpoles.

The researchers, led by Jason Brown, also conducted genetic testing on R. imitator offspring to see if their parents truly were loyal. Indeed, they found only one example of unfaithfulness in 12 pairs of frogs. Their work was published in The American Naturalist. So while you do work hard to care for your offspring, you can rest assured that it’s most likely you’re own genes that you’re nourishing.

— Darwin

Dear Darwin:

I’m a male gannet and I’ve had it about up to here with my mate. Sure, she was great with last year’s brood, helping feed and rear them quite successfully. But this season, she’s nowhere to be found. I arrived at the breeding grounds a week ago, but she’s apparently got better things to do. Meanwhile, there’s an attractive young gal two nests over who still hasn’t found a mate. Is it time for a divorce?

— Old Problem in New Zealand

Dear Old:

I’d advise you to hold out a little longer. University of Waikato biologist Alison Campbell examined a study of gannets just last year, which found that while gannets aren’t as loyal as previously believed, the families that stay together do better than the 40 percent who “divorce” each year.

Now, obviously if your mate never shows up, it’s in your interest to pair up with someone else, but Stefanie Ismar and her colleagues found that more chicks were fledged to loyal pairings than to birds that switched partners. The research was published in Naturwissenschaften. The researchers suspect the chicks from loyal couples do better because learning to share parenting responsibilities takes time. Couples who have been successful in the past are better able to handle the vicissitudes of parenting than newlyweds.

So while monogamy is relatively rare in animals, in cases where monogamy has evolved, there are good reasons for it. For more on monogamy, try searching for the term on It’s a great way to learn about how monogamy evolved in some animals—including humans.

— Darwin

Dave Munger is editor of He also blogs at The Daily Monthly. Each week, he writes about recent posts on peer-reviewed research from across the blogosphere. See previous Research Blogging columns »

Originally published April 28, 2010

Tags biology ecology research

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