Wednesday, November 21, 2012

Competitive Females

Paula Broadwell, the aggressive competitor.
Photo from her Facebook page.
By now, you’ve probably heard all about Paula Broadwell, the woman that seduced the notoriously disciplined CIA director, four-star US Army general, husband and father, General David Petraeus. What kind of a woman might be able to sway a man that has such admirable self-control? Broadwell was Petraeus’ biographer, a West Point graduate with a Harvard graduate degree, an Army Reservist thrice recalled to active duty, a fitness champion, Ironman triathlete and even a machine gun model. Her accomplishments are clearly impressive, but maybe the key comes down to her competitive nature. I mean, she did send several threatening e-mails to an attractive socialite and Petraeus family friend, warning her to stay away from her (other) man.

When we think about competing for mates, we generally think about males competing for females and breeding territories with horns to duke it out, or elaborate feathers to show off, or dance-offs to demonstrate their physical abilities. But females often have to compete for the high-quality males and breeding territories too. And many of the concepts that apply to males competing for females have been found to also apply to females competing for males.

A dark-eyed junco thinking
"What you lookin' at?".
Photo by Kristal Cain.
As much as we know about males competing with one another, we know surprisingly little about females competing with one another, although they clearly do. Kristal Cain and Ellen Ketterson at Indiana University sought out to shed light on female competition and its effect on breeding success. They did this with female Carolina dark-eyed juncos, a socially monogamous songbird species in which both parents care for the young. They were curious whether more aggressive females would also have other competitive traits, like large body size. They also wondered whether aggressive females would have better breeding success.

The researchers caught female juncos to measure and put identifying leg bands on them. They then released them and spent their nesting season looking for their nests. When they found a nest, they identified whose nest it was by the female’s leg bands. The researchers tested how aggressive females were towards competing females by placing a caged female within 3 meters of a subject’s nest and watching to see if she swooped at the caged female. Then they kept an eye on the nest to see if the chicks all survived until they fledged (left the nest on their own) or if the nest was destroyed (usually by a predator) before the chicks fledged.

A female junco in full-on attack mode. Photo by Kristal Cain.
Females that were more aggressive towards “competing” females tended to be bigger and had chicks that were more likely to fledge. Now, if this were a story about competitive males, we might think big aggressive males with more successful chicks might have higher testosterone. Alternatively, low testosterone is often found in males that are better fathers. But these are females… Does it even make sense to talk about testosterone in females? Of course it does! Turns out, males don’t have a monopoly on testosterone; females have it too.

The researchers drew blood from the females and then gave them a “testosterone challenge” by injecting them with a hormone called gonadotropin-releasing hormone (or GnRH for short). GnRH is a trigger that causes a series of biological events that result in the gonads producing more hormones, including testosterone. The researchers then drew a second blood sample to measure how much testosterone levels changed in response to the GnRH injection.

More aggressive females produced more testosterone in response to the GnRH injection than did less aggressive females. This same effect has also been shown to be true of males behaving aggressively towards each other. I guess males and females really aren’t all that different, eh? But interestingly, females that produced more testosterone in response to the GnRH challenge also had more successful nests.

It’s important to keep in mind that these results are correlational. Maybe testosterone makes females bigger and more aggressive and better mothers. Or perhaps having a temper increases your testosterone production. Or maybe some other hormone that increases in response to GnRH (there are many) is responsible for the effects. In any case, females that are bigger and more aggressive and have more successful offspring also produce more testosterone in response to a GnRH injection.

Paula Broadwell shows off her aggressive abilities in this KRISS ARMS video
(gif'd by Michael Pakradooni).
As far as we know, no one has given Paula Broadwell a testosterone challenge, but she undoubtedly has a number of correlated competitive traits. Paula Broadwell is a competitive, physically fit, attractive parent who has shown that she can out-compete the spouses of high-quality mates… But then again, so is David Petraeus.

Want to know more? Check this out:

Cain, K., & Ketterson, E. (2011). Competitive females are successful females; phenotype, mechanism, and selection in a common songbird Behavioral Ecology and Sociobiology, 66 (2), 241-252 DOI: 10.1007/s00265-011-1272-5

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