One of the classic images from evolutionary theory is of two males smashing each other up to compete for the chance to fertilize a female. Scientists have spent over a century studying this kind of male competition, but new discoveries reveal that competition between females is just as important.

The problem is that female competition and aggression don't always look like the male versions. As Paula Stockley and Anne Campbell argue in a special issue of The Philosophical Transactions of the Royal Society B, male competition is showy – we can see fantastically armored beetles, the colorful tail of the peacock, and the often deadly fights between many male mammals.


But what we can't see are the ways that female mouse hormones spike at certain times of the year, causing them to become intensely aggressive toward other females. And until a great deal of observation had been done, scientists also hadn't seen the way female apes murder each other's babies.

Today, we have decades of data showing how competition between females, including human women, affects sexual selection and the evolution of a species. Stockley and Campbell introduce the new science of female competition with a selection of essays in the Royal Society B that pull back the curtain on the often hidden drama of female aggression.


Low-Cost Competition

One of the big themes in these studies is the fact that females invest so much energy in reproduction and child-rearing that they are unlikely to expend enormous amounts of energy on competition. Instead of smashing each other, they engage in low cost forms of competition, often resolving the question of who gets to breed and who doesn't without ever fighting.

This isn't to say that there aren't many species where females do fight – the spotted hyena is a good example. But on the whole, females tend to preserve their energy for reproduction. And there are a couple of interesting ways they do it.


First of all, keep in mind that these conflicts are motivated by the same things that motivate male conflicts in sexual selection. Females want access to mating partners, and to resources for their offspring. But how can one female prevent other females from mating without killing them? In many cases, they do it by teaming up.

Animals from insects to mammals cooperatively raise their young. High status females have more children than low status females, who are often their sisters. It is simply a better use of energy for the low status females to accept their positions within the group and have fewer children.

You can even see this pattern among some groups of humans, such as the Mosou of southwestern China, where many generations of sisters live together and their husbands live elsewhere – only visiting their wives at night. The older sisters tend to have more children, and do more farm work, then the younger ones. As Stockley and Campbell put it, "reproductive conflict among females [is] more readily resolved to restraint among subordinates rather than escalated physical contests with their dominant."


Aggression Without Violence

Females tend to threaten each other with social isolation rather than violence. Among social animals, being cast out of the group can mean death, or very few chances to mate. Among humans, perhaps the most social animals we know, the "mean girls" phenomenon is a perfect example of low energy competition. Nobody is beaten, but we know for sure who has lost the battle.


The problem with talking about humans, of course, is that we are not wild animals. As Stockley and Campbell are careful to point out, humans have been so influenced by culture that it's very hard to tell if a lack of overt aggression among women is an evolutionary or cultural artifact. Because so many women are culturally trained to tamp down their aggressive urges, it's impossible to call their behavior "natural."

So it's difficult to draw many conclusions about how early human women might have behaved 50,000 years ago based on how they behave today.

Flexible Behavior

To return to the world of animals, it's interesting to note that females seem to be aggressive in a much more flexible way than males. As I mentioned earlier, female mice go through periodic spikes in aggressive behavior, partly as a result of hormonal shifts.


Females also become more aggressive when they are defending their young . That's likely because the hormone oxytocin, released during and after pregnancy, isn't just a "love hormone" to spur mother/child bonding. It also governs aggression. So in the wake of pregnancy, when oxytocin levels are high, females are simultaneously more nurturing and more likely to go ninja on your ass.

Overall, however, female aggression and competition are more subtle than the male forms. Females assert dominance, and come to control the gene pool, by fostering cooperation. This is especially true among social animals who live at a high population size. Of course, cooperation isn't all happy "friendship is magic" stuff. A cooperative female may cede her reproductive privileges to another.

It's a low cost solution to a problem as old as life itself. Put another way, just because females don't grow giant horns doesn't mean they aren't ripping each other's hearts out.


Read all the papers about female competition and aggression in the Special Issue of Philosophical Transactions of the Royal Society B

Annalee Newitz is the editor-in-chief of io9. She's also the author of Scatter, Adapt and Remember: How Humans Will Survive a Mass Extinction. Follow her on Twitter.