A Deer Biologist's Dissection of The Rut

By Jason Snavely

We humans often like to complicate matters, even when the facts are very clear. It is human nature to break complex subjects down into simpler, more manageable parts. There’s nothing wrong with this reasoning-based approach, and it actually sets us apart as a species.

The whitetail rut is one subject of particular interest for bowhunters. What are “the facts” of the whitetail rut? Just as the news media has major influence over how we think about everything today, the hunting media greatly impacts our perception of the rut. One popular hunting TV show even manages to create an entire 13-episode season around their interpretation of the phases of the rut!

As a wildlife biologist of 17 years, I regularly field questions regarding all aspects related to whitetails. This is one of the favorite parts of my job, as some really thought-provoking questions drop into my inbox. One such email recently came from a customer of my food-plot seed company, and although the majority of it dealt with his plans for the fall 2018 planting season, it was the last line that really caught my attention. “Looks like an early rut this year,” he wrote. “I hope you’re ready.”

Considering what I do for a living and the amount of time I spend in hunting camps, I’ve heard a version of that comment thousands of times. However, what made me stop and think this time was the fact that the email was sent June 28 — four months before the rut would be in full swing! I paused for a moment and pondered how he knew the rut was coming early while the bucks he would be hunting were still in velvet.

Whitetails are far and away the most researched game animal, and scientific literature is in great supply. Most subjects are studied repeatedly to compare results and “replicate” certain discoveries. This is the perfect example of the scientific method at work. Research supporting what triggers the rut as well as the “phases” of the rut certainly satisfies the scientific process through replication of findings. I’m not saying we know it all when it comes to the whitetail breeding season, because we certainly don’t. Perhaps one day, sound science will legitimately unravel sexy topics such as the potential lunar effects on whitetails. But for now, it’s just not there; although I’ll admit it sure is fun to make predictions each fall with those cardboard moon guides! For now, let’s clear a few things up regarding this magical time of year that causes all of us bowhunters to estrange our families, skip household chores, flirt with divorce, erode sleeping habits and become less effective at work.

Rut Timing

All animal behavior is driven by long-term survival. Although short-term variations such as high or low temperatures may change micro behaviors, something as important as the long-term sustainability of a population (breeding) has evolved around best practices for survival. The whitetail rut is no different. When educating others on the science of the rut, I like to fast forward to when fawns hit the ground and work backwards, because without successfully recruiting fawns into the adult breeding population, the herd is in trouble!

Fawn birth coincides with spring green-up, when habitat conditions are ideal for survival. A combination of warm spring temperatures and rains produces dense understory cover that hides fawns. Nutritious forbs and legumes are abundant for does experiencing the rigors of milk production. Therefore, the broader timing of when a doe is bred is ultimately dictated by this window for greater fawn survival. Timing becomes more critical as you travel north, since fawns must attain a certain body weight to survive the harsh winters. It is for this reason we see less variation in rut timing from year to year in the northern range than we do in the southern range.

Now that we have an end in mind, let’s rewind to the beginning. In temperate regions, where most of us reside, rutting activity is triggered and controlled by changes in the ratio of the number of hours of daylight to darkness in a 24-hour period. Biologists refer to this as the photoperiod. Shorter day lengths in late summer are registered in the pineal gland, triggering a gradual increase in melatonin levels and a series of hormonal fluctuations. These changes also trigger velvet shedding and the rutting behavior that follows.

Changes in day length define the window during which breeding can occur. However, exactly when breeding occurs can be affected by factors such as doe age/health and herd characteristics such as density, buck age structure and adult buck-to-doe ratios. As a deer manager, I believe this is where the subject gets murky. Hunter harvest and habitat quality have a great impact. An area where many bucks are taken by hunters will exhibit less rutting behavior and a less intense rut than an area where hunters only harvest mature bucks. Since we all hunt deer in areas with varying population characteristics, comparing notes on the narrow window of breeding activities (i.e. what phase of the rut are we in?) is a classic case of comparing apples to oranges. Simply put, we don’t all experience the same phase of the rut at the same time.

Age and body condition of the does where you hunt affects when they enter estrus, ultimately affecting the timing of the rut. Research has shown adult does in good health breed earlier than first-time mothers. Delayed breeding dates can negatively impact a successful fawn crop in the spring. Generally, does are receptive for a 24-48-hour period, depending on the local buck density and how likely she is to interact with a buck (or bucks) during that time. Ideally, all does are bred during that first estrous period, hence the reason biologists prefer that plenty of bucks are available in the herd.

Breeding activity also varies from property to property, for reasons beyond the scope of this column. In general, it’s safe to say the synchrony of the rut, although heavily influenced by herd characteristics and environmental factors, is affected by changes to day lengths. Biologists refer to whitetails as short-day breeders.

Understanding the complexities of the rut, I always advise clients to break it down into this simple concept: if the season is in where you hunt and your hectic lifestyle allows time for you to get in the woods, GO! Don’t wait for a TV show to tell you it’s time to climb! If you’re seeking the best days to take your vacation time, I’m fairly certain we all know, from past hunts, when that occurs. If you pay attention to when fawns hit the ground in the spring, on the property you hunt, back dating 200 days will accurately lead you to your next sick day(s)!