Spring is the season most often associated with courtship, but for many mammals that’s an activity for fall. Their offspring do best if born soon after winter has ended, when they have time to grow before the return of harsh weather. As a result, mating must take place in the fall for births to occur in the spring after months of pregnancy.
This fall breeding season for hoofed mammals is called the rut. This is the time of year when antlers and horns become essential – males use their headgear when they compete for females. Spectacular clashes can occur: bull elk, for example, locking antlers in a contest to see who will prevail and get to mate with a nearby female. The rut begins at different times in the fall for different animals. Pronghorn are among the first; their rut begins in mid-September and can last well into October.
Pronghorn don’t have antlers – they have horns: horns with a forward-facing fork or prong as in their name. Antlers are bone and so are horns, but unlike antlers, horns aren’t shed. Horns have a bony core that is covered with a nail-like sheath; in pronghorns, that sheath is formed by fused hair that grows to become the outermost tips of the horn. Both males and females have horns, but a female’s horns are much smaller, typically not forked and only a few inches long (a male’s horns can be a foot or longer in length).
The horns are mostly for show, but can become weapons if two males are closely-matched. Bucks establish territories in March, long before the rut begins. They select the best grazing lands that will attract does, many does, ultimately a harem that they will defend against other males in September. Usually, all that is needed is a snort, a stare, or a brief chase to discourage a potential rival. But animals of similar fitness may come to blows, clashing their horns until one of them leaves. Young males or old bucks past their prime are rarely able to claim a harem.
The rut ends by late-October and gradually the bucks become social again, mixing together with many females in large herds for the winter. Their contests ended, the bucks shed the outer sheath of their horns but not the horns themselves – a new sheath will begin to grow over the bony core in January or February. By July the new sheath is fully formed and ready for the upcoming rut.
Pronghorn are animals of the open plains making their rutting behavior very visible, unlike whitetail deer that rut in November and December in woodlands. Look for them in extensive grasslands west of the Missouri River. Animals at Wind Cave National Park, Custer State Park and The Nature Conservancy's Whitney Preserve will be the most tolerant of spectators; pronghorn at other locations often keep their distance. In September, watch for pronghorn bucks guarding groups of females on the prairie, among flowering goldenrods and asters in grasses turned russet for the fall.