Roaring is the currency of the rut. Stags roar at rivals for days before either risks antlers, and hinds listen closely to the result.
The anatomy is unusual. In a red deer the larynx is slung loosely and can be retracted far down the neck by strong sternothyroid muscles, in extreme cases nearly to the sternum. Lengthening the tube above the sound source lowers its formants, the resonant peaks that a listener uses to judge the size of the animal producing a call. Playback experiments by Reby, McComb and colleagues found that hinds and rival stags both attend to formant spacing, responding more strongly to roars synthesised to imply a longer vocal tract.
This is size exaggeration rather than deception without limit, since the vocal tract cannot exceed the neck that holds it. What remains open is how tightly roaring rate and quality track actual fighting ability, as against a stag's condition on a particular day.