This timing is important, considering whitetail behavior, like other animals, is driven by long-term survival. With the possibility of a harsh winter ahead, fawns must attain a certain body weight to survive North Dakota’s leanest months.
While what triggers the rut in fall is often debated, the primary driver, Jensen said, is photoperiod.
“Over eons, deer have evolved to increase their testosterone production base upon photoperiod, the shortening of the days,” he said. “Essentially, that triggering of shorter days causes chemical changes in the brain which in turn causes the production of testosterone.”
The secondary driver appears to be the condition of the doe herd, Jensen said.
“If does are in real good condition, they tend to go into estrous earlier,” he said. “If they’re not in good condition, if they’re very young or very old, those animals go into estrous later and then that shifts the timing of the rut.”
Jensen said the peak of the rut for whitetails is generally between Nov. 15-20.
“For white-tailed deer, the breeding season is highly synchronous,” he said. “Generally, the peak when fawns are born is June 6. Seventy percent of fawns are born within plus or minus two weeks, and over 90% are born within four weeks of that date. So back dating to this time of year, that puts it at about the 15th to the 20th of November.”
For mule deer, it tends to be a little later and slowly tapers off, finally finishing up with a few animals actively breeding in late December.
There are a number of basic rut behaviors bucks exhibit this time of year, from posturing to show dominance to fighting with other bucks.
Another behavior also familiar to hunters who have spent enough time on the ground during the deer season is the lip curl, seen when a buck tilts its head back as if watching something pass overhead, while curling its upper lip and exposing teeth.
While the lip curl is easy to remember and rolls off the hunter’s tongue, biologists call this “flehmen” behavior.
When a doe approaches estrus, Jensen said, she provides clues to suitors as to her readiness. Some of the clues are behavioral and some are chemical – the later found in urine she deposits during frequent stops.
And this is where the lip curl comes in.
The purpose of the lip curl is to expose the scent from the urine to an olfactory organ called the vomeronasal, located on the roof of the mouth near the nasal passage. This organ aids the buck in his evaluation of the doe’s reproductive stage and willingness to mate.
If the message he receives is clear, the buck either follows the trail of a doe, or continues his search. It’s a time-consuming preoccupation that replays itself every fall in the deer world.
“The rut is really an energy drain on bucks. They don’t feed. They’re moving. They’re constantly checking. They’re fighting with other males,” Jensen said. “So, by December, they’re pretty well sapped and a lot of those dominant bucks may die in January because they are exhausted.”
