How can testosterone affect the neck muscles that stags use for roaring?

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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 the context of aggressive behavior, it is important to clarify that testosterone does not merely influence psychological states; it also physically alters the body to support competitive signaling.

In stags, rising levels of testosterone cause the neck muscles required for effective roaring to enlarge. This physiological change is a direct example of how circulating hormones can influence the specific physical structures and signals that animals use during aggressive behaviour and conflict situations.1