Research into the triggers that spur interest in does by bucks during the annual reproductive period known as the rut show some interesting findings.

Bucks actually detect a doe’s estrus condition by inhaling fumes containing releaser pheromones.

Contrary to what one might expect, those special odors do not come directly from the doe’s urine.

During preliminary trials conducted at the University of Georgia, researchers tested the response of deer to urine collected directly from the bladders of does known to be in estrus with that of non-estrus urine, buck urine, and several control solutions, including salt water. Their experimental design consisted of using automatic aerosol dispensers to release test material about every 15 minutes into circular areas of lightly tilled soil about three feet in diameter. Then they tallied the number of deer tracks at each site, after 24, 48, and 72 hours, to see if deer were more readily attracted to any of the test solutions.

Results of their study revealed that deer visited about one-fourth of the test sites each day, but the investigators could detect no preference for any of the solutions used. That is, deer came to the salt water just about as readily as they did to the bonafide doe-in-heat urine. Therefore, the researchers concluded that urine collected directly from the bladder of estrous does did not contain those potent releaser pheromones as most deer biologists would have predicted.

In a series of rather involved trials with penned deer, bucks were introduced to does artificially treated with estrous urine, non-estrus urine, estrous vaginal secretions, and water. Although individual responses varied, bucks devoted significantly more attention to does treated with estrous vaginal secretions.

These experiments produced rather convincing evidence that it is not the urinary tract but the female reproductive tract, and associated vaginal secretions during the time of estrus, that is the primary source of pheromones that serve as sexual attractants. These findings were confirmed in subsequent studies.

Given that the releaser pheromones signaling a doe’s estrus condition are in her vaginal secretions, then, the doe does not necessarily have to urinate in order to signal a buck that she is receptive. All that is necessary is that the buck be able to approach the doe close enough to smell her urogenital area.

During most of the year, female whitetails show a strong avoidance for adult males; does rarely let males approach closely until they are nearing estrus. Aside from very subtle posturing differences associated with estrus, researchers have been unable to identify any special pre-estrus visual warnings given by the female to the male that indicates her ensuing willingness to breed. Nor is there any evidence that she vocalizes to announce her estrus condition.

Once the buck can approach the doe close enough to smell and lick her urogenital area and tarsal glands, however, he has some hint that she is at least approaching a receptive stage. Since bucks seem to be able to detect a doe’s approaching estrus at least several hours, if not days, before she will accept copulation, the experienced buck will generally follow behind the doe at a reasonable distance until she is receptive. Sometimes, several bucks may follow along in single file, the most dominant being first in line.

A doe will only accept a male during peak estrus, which lasts for about 24 to 36 hours. If she is not bred, or for some reason does not become pregnant, the cycle may recur in 23 to 30 days. Thereafter, if she remains in peak physical condition but does not become pregnant, on northern range, she might recycle for the third time.

In milder southern climates, however, an unbred adult doe might come into estrus as many as seven times during one season. This considerable difference from north to south in the number of times a doe might come into estrus during a single season is another reason for potentially long whitetail breeding seasons in the South.

Once a buck locates a doe in estrus that is willing to stand for copulation, he stays close and care- fully tends her. He’ll drive away all other deer, including the doe’s fawns of the previous spring, but especially other bucks that also are readily attracted by the sex pheromones emitted by the estrous doe.

The breeding pair may mate several times during the bonding period and might stay together from 24 to 72 hours.

Some researchers suggest that estrus among all but the youngest of related reproducing females should be synchronous, because estrus can be induced by male- produced pheromones. Therefore, if a dominant female and a subordinate female come into estrus at the same time, the dominant doe might displace the subordinate and copulate first. If so, subordinate does are more likely to delay mating or will more readily mate with a subordinate male.

On the other hand, if adult females of a clan come into estrus only a few days apart, a dominant buck might remain with the clan for several days and breed several does within a relatively short period of time.

— John Ozoga is a retired research biologist from Michigan. He has been D&DH’s research editor since 1994.