There are red stags roaring for sex out there. All pumped up with testosterone, trying to gather a harem of hinds and prepared to fight off the competition to get them. Most of the time the stags just stand on the horizon, posturing to each other, but if necessary they'll try to nut each other into submission, if they don't get tangled up in each others antlers first.
It's a queer noise they make, much more of a groan than a roar, not unlike a cow in calf. It's as if when roaring was handed out, dogs and bears got the consonants, so they go 'rr...rr...', and the stags got the vowels, so they can only muster 'oa... oa...' The result sounds more wretched than fierce, but presumably to a hind it's as sexy as Tom Jones.