A team of researchers from Indiana University in Bloomington, Indiana, have published a finding in today’s online edition of the Proceedings of the Royal Academy B that suggests that female hamsters exposed to short winter days tend to be very aggressive to their male counterparts.
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The study is a collaboration between the Indiana University Bloomington College of Arts and Sciences’ Department of Biology and the Department of Psychological and Brain Sciences; and led by a Ph.D. student in biology, Nikki Rendon.
The ultimate knowledge gained from the study could be applied to promote research on treating aggressions in people.
"This study reveals a ripe area for research," said Nikki Rendon. "The results show for the first time that melatonin acts directly on the adrenal glands in females to trigger a 'seasonal aggression switch' from hormones in the gonads to hormones in the adrenal glands – a major contrast to how this mechanism works in males."
Professor of biology at IU, Gregory Demas, and Dale Sengelaub, professor of psychological and brain sciences were part of the study. Lauren Rudolph, postdoctoral fellow at the University of California, Los Angeles, also participated in the research as an undergraduate student in Sengelaub’s IU lab.
Rendon first observed that shorter winter days made animals such as hamsters to be aggressive; and Demas investigated to confirm that the restlessness and aggressions that hamsters experience during short winters is not caused by changes in their sex hormones, but rather from the adrenal glands located above their kidneys.
Melatonin is a hormone released by the human body in very dark places and at night but it virtually disappears during the day. And the hormone dehydroepiandrosterone or DHEA, a sex steroid linked to aggression in animals and maybe humans is released by the adrenal gland. And that is why sport administrators ban the use of DHEA by athletes.
"This study, which builds upon our previous work investigating the connection between short days and aggression in males, shows noteworthy hormonal mechanisms in females and provides important new insights into the role of sex in these mechanisms," Demas said.
In conducting the research, the scientists exposed 130 Siberian hamsters with an adrenal system similar to humans, to long days over a week period; and also exposed 45 others to short days over 10 weeks. They were then placed together to determine their aggression in defending their territory against one another, and the researchers monitored their time of attack, number of attacks, and length of attacks to rate their aggression levels.
The researchers discovered that female hamsters exposed to shorter days had higher levels of melatonin and DHEA, and they were more aggressive even in the face of physical changes in their adrenal glands. But other 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," Rendon said.
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"By conducting this research on females, we are increasing our understanding of hormones and social behavior in a field currently dominated by discussions on testosterone regulating aggression in males," she added.