It's the rutting season. Red deer hinds will be gathering, and the stags have but one thing on their minds. A mature male that has netted himself a harem is very dedicated. He practically stops eating, focusing instead on keeping his hinds near and his competitors at bay. If you're a red deer stag, one of the ways you make sure that your adversaries know you mean business — and that you're big — is roaring.
While female red deer prefer the deeper roars of larger stags, roaring also appears to be part of how stags size one another up, before deciding whether or not to get engaged in a full-on physical fight. Most confrontations are settled without locking antlers. In male red and fallow deer, the voicebox or larynx is very low in the throat — and gets even lower when they roar.
Straplike muscles that attach to the larynx contract to drag it down toward the breastbone — lengthening the vocal tract and deepening the stag's roar. Deepening the voice exaggerates body size. Over generations, stags with deeper roars presumably had more reproductive success, so the position of the larynx moved lower and lower in the neck. When a red deer stag roars his larynx is pulled down so far that it contacts the front of his breastbone.
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