Aggressive? Blame the testosterone. At least we used to; it turns out things are much more complicated than that. Take Siberian hamsters, for example: They get more aggressive during short winter days, despite producing lower amounts of sex hormones. And when females are given a boost of these hormones, their feisty attitudes don’t change. Now, scientists are beginning to work out why that might be.
Perhaps unsurprisingly, it’s to do with melatonin, the hormone that helps regulate our sleep-wake cycles. Released as the Sun goes down, levels of melatonin rise in the body during darkness, and fade as it gets light again. Bright lights inhibit its production, which is why fiddling with your smartphone at night isn’t a good idea if you want a good kip.
Alongside this well-known role in body clock regulation, it now seems that, at least in hamsters, it has a role in the regulation of aggression. This work actually builds on previous research conducted by the same group at Indiana University, which found that seasonal, winter aggression in hamsters was driven not by the sex hormones from the gonads – ovaries and testes – but the adrenal gland found atop the kidney.
The adrenal gland releases many hormones, but the one of interest in this study is called dehydroepiandrosterone (DHEA), a steroid used to produce androgens and estrogens, the male and female sex hormones. Earlier work showed that in combination with another hormone, adrenocorticotropic (ACTH), melatonin boosts DHEA release and enhances its effects in males.
But this latest study actually contradicts that find, discovering that melatonin can exert its effects on DHEA solo, without ACTH. As DHEA can lead to the production of male and female sex hormones, it turns out that in female hamsters, DHEA can compensate for low levels of one of the estrogens that are experienced during the winter months.
For the investigation, 129 Siberian hamsters were housed in long, summer-like days for a week, then a quarter were switched to short, winter-like days. Some were also given a dose of ACTH. A solitary species, both males and females are highly territorial, so to gauge aggression levels, the researchers devised a setup whereby a hamster would perceive another as an intruder in its territory.
As described in Proceedings of the Royal Society B, females exposed to the winter-mimicking conditions displayed heightened aggression levels with a concomitant rise in both melatonin and DHEA levels. In addition, they became more responsive to the effects of DHEA, and their adrenal glands showed structural changes.
Unsurprisingly, the females that remained in the longer-day environment weren’t as aggressive, but what was interesting was that even the injection of ACTH did not seem to exert an effect on aggression, despite the fact it’s known to increase DHEA levels.
Taken together, it seems that, at least in these animals, DHEA is an important regulator of aggression. Furthermore, this study demonstrates that melatonin acts on the adrenal gland to seasonally switch gonadal regulation of aggression to adrenal.