Promiscuous Finches

Promiscuous Finches Zebra Finch (Taeniopygia guttata)
Photo Credit: Peripitus [GFDL, CC-BY-SA-3.0 or CC-BY-SA-2.5-2.0-1.0], via Wikimedia Commons

Promiscuity-related genes in finches may exist to solely benefit males.


Infidelity genes. I’m Bob Hirshon and this is Science Update.

Like humans, birds called zebra finches are socially monogamous—but sometimes stray. For males, philandering makes sense, since they can sire unlimited offspring. But females can lay only so many eggs, and need stable partners to help raise the chicks. Now, behavioral ecologist Wolfgang Forstmeier and his colleagues at the Max Planck Institute for Ornithology in Germany may have explained why the females cheat anyway. Forstmeier’s team found that the same genes promote promiscuity in both male and female finches.

So it kind of changes the view how we see female promiscuity—in the past, people have said it has to be adaptive, otherwise it shouldn’t exist. And now you can say, well, it may as well be maladaptive, because they inherit this predisposition from their fathers.

He says the genes’ benefit to males could simply outweigh their cost to females. I’m Bob Hirshon, for AAAS, the science society.

Making Sense of the Research

This study addresses a common assumption about the sexual behavior of many animals: namely, that promiscuity benefits males more than it benefits females. This varies greatly depending on the species in question's mating and parenting systems. But in general, the thinking goes like this: Sperm are “cheap,” meaning they're produced in huge quantities at very little energetic cost. Human males, for example, produce trillions in a typical lifespan. So there's no real drawback to “wasting” sperm on as many female partners as possible. In fact, a promiscuous male can father an almost infinite number of offspring, with a new female every day, if he were so inclined. As a result, for males of many species, the best strategy for passing on one's genes may be to be promiscuous—at least until the females start punishing promiscuous males by refusing to mate with them.

In contrast, eggs are “expensive;” females produce a limited quantity in their lifetime, whether they're human egg cells or bird eggs. Comparatively, each egg requires a lot more energy to make than a sperm cell, or millions of sperm, for that matter. Furthermore, females can give birth to only a limited number of offspring, because every birth requires time to gestate a fetus or incubate eggs. Therefore, females are assumed to value quality over quantity in choosing mates. And if females of a species prefer to mate with males that devote more time and resources exclusively to them, that may create social pressure for males to avoid philandering, or at least avoid getting caught.

Zebra finches, as you heard, are socially monogamous—in other words, they tend to settle down with one “official” partner and raise chicks together. Male finches sometimes try to sneak around and mate with other females, which makes sense, according to the logic above. But females participate in those extra-marital affairs too. Why would they? It's unlikely to increase the number of offspring they have, and could potentially alienate their male partnet, who might stop providing food and other resources to chicks that might not be his. 

As Forstmeier explains, many scientists had just assumed that promiscuity must benefit females in some as-yet-unidentified way. But Forstmeier's research suggests that it doesn't have to. He found the same genes related to promiscuity in both males and females. That means that it's possible that the genes just happen to benefit males more than they hurt females, so in the end, they're more likely to be passed on to future generations than not, even through female birds.

In contrast, if different genes related to promiscuity were in males versus females, that would imply that cheating benefits females, at least in terms of reproductive fitness, in some specific way. Otherwise, one would assume that the female promiscuity genes would become less widespread over time, or die out completely.  

The findings may help explain why females cheat on their partners, even if it's slightly detrimental to their reproductive success—in zebra finches and other species as well.

Now try and answer these questions:

  1. Why is promiscuity generally considered more advantageous—from a reproductive standpoint—for males than for females?
  2. What question did this study set out to answer? Be specific.
  3. What conclusion did the authors draw from the fact that the same genes influenced promiscuity in both males and females? Why?
  4. Do you think these findings might apply to humans?

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