Short Winter Days May Trigger Aggression Hormones in Females

Short winter days can get anyone stir crazy. Now, though, researchers have found that short winter days actually trigger aggression hormones differently based on sex.

Earlier research has shown that wintertime aggression in hamsters arises not from sex hormones in the gonads-ovaries in females and testes in males-but rather the adrenal glands, which are located on top of the kidneys. Melatonin is a hormone that rises in the body during darkness and lowers during daylight. The hormone from the adrenal gland is dehydroepiandrosterone, or DHEA, which is a sex steroid shown to affect aggression levels in mammals, birds, and possibly humans.

In this latest study, the researchers found that melatonin acts directly on the adrenal glands in females to trigger the release of DHEA, without the need for pituitary hormone. DHEA appears to actually compensate for low levels of estradiol, which is a form of estrogen, that occurs during the winter.

More specifically, the researchers found that female hamsters exposed to shorter days had increased levels of both melatonin and DHEA in addition to higher aggression scores. They also had physical changes in their adrenal glands. Females exposed to longer days did not experience these changes.

"It's growing increasingly clear that sex hormones play an important role in controlling aggression in both males and females-but females, human and non-human, are traditionally vastly understudied in the sciences," said Nikki Rendon, one of the researchers, in a news release. "By conducting this research on female, we are increasing our understanding of hormones and social behavior in a field currently dominated by discussions on testosterone regulating aggression in males."

The findings are published in the journal Proceedings of the Royal Society B.

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