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Key points
- Animal aggression is not limited to rivalries among males.
- Brain estrogens often play a larger role than brain testosterone in regulating aggression.
- The adrenal glands and the brain itself can be the source of steroid hormones that regulate aggression.
Toxic masculinity is a major societal concern. It is said that testosterone from the testes makes men overly aggressive. But the biology of hormones and aggression is not that simple.
A symposium within this past week’s conference in Lisbon, Portugal of the International Society for Neuroethology highlighted this complexity. Researchers reported on experiments revealing steroid hormone regulation of aggressive behavior in diverse non-human vertebrate animals, where the molecular and cellular causes can be studied more effectively than in people. Of course, we can’t be sure that these findings from non-human animals apply consistently to humans, but they are at least intriguing.
Aggression occurs in all sexes and seasons
Speakers explained that in the fish, birds, and rodents they studied, aggressive behavior, often in the context of defending territory, commonly occurs in both males and females, meaning that aggression is not just a masculine trait. For species that breed only in one season, aggressive behavior can also occur in the non-breeding season and can actually be highest then, so aggression is not just about fighting for mates either. During the non-breeding season, the testes in many species shrink to a small fraction (one-tenth or less) of their breeding-season size and produce almost no testosterone. But steroid hormones (a group that includes testosterone) are produced in other locations in the body, including the adrenal glands and the brain.
Brain estrogens increase aggression
In many vertebrate species, it is actually not testosterone or any other androgen (“male sex hormone”) that regulates aggression. Instead, estrogens (“female sex hormones”) play the key role in increased aggression.
Androgens and estrogens are very similar chemically. Testosterone can be converted to estradiol, the main estrogen, in a single chemical step, facilitated by an enzyme called aromatase that is common in the brain. The location and amount of aromatase within the brain determines where and how much testosterone is converted to estrogen. Brain estrogens then stimulate aggression.
Adrenal gland hormones can be converted to estrogens in the brain
Some of these animals are more aggressive in the non-breeding season because their adrenal glands increase the production of another androgen, called DHEA, which is converted to estrogens in the brain. (The human adrenals also produce extra androgens prenatally in congenital adrenal hyperplasia, leading to intersex characteristics in people with no Y chromosome and no testes.) Other animals produce estrogens within the brain from other steroid molecules. Estrogens typically increase aggression by acting on cells in a small structure called the hypothalamus that regulates many fundamental biological drives, including hunger, thirst, and sexual behaviors.
In short, all sexes can be aggressive. Estrogens often affect aggression more directly than testosterone does, and the source of steroid hormones regulating aggression can be the adrenal glands and/or the brain, not just the gonads (the testes and ovaries). Thus, our simple ideas about testosterone from the testes being responsible for aggression may be misleading.
