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
