The breeding season, commonly called the rut, takes a differential toll on female and male white-tailed deer. Does are fertile only 24 hours every 28 days.
If does are not impregnated during that time, they come into estrus again in about 28 days. Bucks, on the other hand, are on call 24/7 from October through January.
When not in estrus, does live in small family groups consisting of an adult female and her female yearlings and fawns. As the matriarch’s estrus period approaches, she undergoes hormonal changes that bucks can detect in her urine.
In September, before the rut actually begins, bucks court does by following them. At this time females do not permit bucks to approach closely. Usually a buck stays 50 yards or more behind a doe before she enters estrus.
He monitors her condition and approachability by sniffing her urine. If more than one buck trails a particular doe, they will spar until one establishes dominance. That buck will follow the doe until she comes into estrus.
In the days before estrus, a doe’s urine scent changes to indicate that she will soon be approachable. When the 24-hour estrus window opens, the buck approaches more boldly and stays at the doe’s side until she willingly accepts the buck’s advances.
At this point, a buck and doe can be considered a mated pair, if only for a day. A buck usually tends a doe for the entire estrus period to prevent other bucks from mating with her. He eats with her, beds down with her for a few hours, and periodically tests her willingness to mate by nosing her rump.
DNA tests show that a buck’s vigilance is not always perfect. Occasionally fawns from a doe having twins or triplets have DNA from more than one male.
If a doe does not become pregnant during her first estrus, she gets a reprieve for about 28 days. At that point she once again comes into heat and becomes the subject of intense attention from bucks.
Until a doe gets pregnant, estrus cycles continue into February, giving each female multiple opportunities to mate. Consequently, the vast majority of adult does ultimately get pregnant.
Among bucks, a clear dominance hierarchy develops in September. Larger, older bucks, especially those with larger antlers, dominate smaller, younger ones.
Smaller bucks avoid larger ones to conserve energy and avoid injuries that might occur during a fight. A 98-pound weakling knows better than to challenge a macho buck.
When confrontations occur between near equals, the response of dominant and subordinate bucks is predictable.
The lowest form of confrontation is a direct stare. Subordinate bucks simply look away and back off.
In a more aggressive encounter, the dominant buck will hold its head high or low. A low-head posture indicates the dominant buck is ready to chase, while a high head posture indicates a dominant buck is ready to rear and fight.
Encounters such as these occur soon after bucks lose their antler velvet, so in September bucks spend a lot of time posturing to establish dominance.
During this period, bucks often spar in what seem to be simple pushing contests. Bigger bucks almost always win these head-to-head shoving matches. The vast majority of physical encounters are ritualistic and safely resolved.
As does come into heat in October, bucks become more solitary and turn their attention to the opposite sex. Dominant bucks become preoccupied with a single doe. This explains why hunters often observe a big buck following a group of does and why drivers should always expect a trailing buck when a group of does crosses the road.
A buck’s preoccupation with a doe in heat takes him out of the competition for other does. This is how subordinate bucks are able to gain access to does in heat.
After mating, the rut continues for healthy males. It’s an exhausting three or four months. During that time, a buck can lose 25 percent of its body weight.
Complicating white-tail breeding biology is that does in an area come into heat at approximately the same time. Over the course of just a few days, most of the does in an area are available for just 24 hours.
This also helps insure that more than just a few dominant bucks get to breed.