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How are testosterone levels related to aggression in male vertebrates?

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red deer stags (Cervus elaphus) — Aggressive behaviour A pair of red deer stags (*Cervus elaphus*) competing for possession of a female in the rutting season.

In male vertebrates, there is a well-established, complex relationship between circulating levels of androgens like testosterone and aggressive behavior. Across many species, aggression levels generally fluctuate in tandem with natural changes in testosterone. While castration typically reduces aggression, experimental administration of testosterone can restore it.1

This relationship is multidirectional; while testosterone influences aggression, participating in a fight can also alter hormone secretion. Winners often experience increased testosterone, whereas losers may show decreased testosterone and elevated cortisol.1

Beyond these immediate, "activational" effects, testosterone and other hormones exert "organizational" effects during early development. Early exposure to gonadal steroids shapes brain structures, making them more sensitive to the aggression-facilitating effects of testosterone in adulthood. This developmental programming contributes to natural variations in aggression among individuals of the same sex. Furthermore, other factors, such as social structure complexity and nongonadal hormones like vasopressin, also modulate these aggressive responses.21