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What Happened to the 2021 Rut?

The 2021 deer hunting season left many whitetailers with questions and data now proves it wasn't a “normal” year in some places.

What Happened to the 2021 Rut?

Many factors affect the timing and duration of the whitetail’s annual breeding period, including geographic location, buck:doe ratios, nutrition and moon phase. (Photo by J. Edwards Photography, Shutterstock)

Every year, the Internet is abuzz with field reports from whitetail hunters about the status of the whitetail breeding period, known as the “rut.” Often, hunters weigh in with their thoughts on the intensity and status of the rut — some arguing the rut hasn’t begun and others claiming it’s already over.

As the 2021 deer season comes to an end, I’ve been asked by countless frustrated hunters why the rut was so unpredictable this year. That’s not surprising, as the 2021 season was perhaps one of the worst and most frustrating throughout North America. My own hunting ventures this year support this claim.

Yes, there were some great bucks taken across the continent, but for the most part, hunting was a hit or miss proposition. Yet, as early as July, I was warning about the potential for a poor hunting season; and, it turned out that I was right — not because I am clairvoyant, but five decades of research studies have clearly identified factors that cause the problem.

Tracking the Mating Season

For years, it has been frustrating to hear so many erroneous perceptions of what constitutes the rut, so let’s begin with a brief primer on what the rut really is! According to Wikipedia: “The rut is the mating season of certain mammals, which includes ruminants such as deer, sheep, camels, goats, pronghorns, bison, giraffes and antelopes, and extends to others such as skunks and elephants.”

The key words here are “mating season.” It is not defined as a day or even as a few days, rather a “season!” In whitetails, the rutting season extends a varying number of days and includes the following stages: pre-rut, scent posting, combat, chasing, breeding and conception, and finally physiological recovery. Each of these stages has to occur in a proper sequence for a successful reproduction at a level that will allow recruitment equal to or greater than mortality.

Ideally, the shorter the time involved in actual breeding/conception, the better for the species. Average gestation from our data, covering almost 50 years, is 198 days. So, a doe conceiving on Nov. 15, 2021, will fawn around June 1, 2022. The tighter the fawning period, the less impact of predators, so we try to manage deer herds to have a short, concise conception curve, which in turn produces a short, concise fawning period.

When you hear a hunter talk about the rut, is he/she talking about all the stages of the rut cycle, or just one or two of the individual stages? I submit that the average hunter considers the rut when chasing occurs. It certainly is an exciting time, with bucks and does running all over everywhere, creating quite a show in the process!

what happened to the 2021 rut
Managers should strive to balance deer herds to a 1:2 buck:doe ratio to reduce the overall duration of the rut, preventing prolonged “trickle rut” breeding. (Photo courtesy of Dr. James C. Kroll)

It is my observation that hunters then view the breeding/conception period as “lockdown.” Then the “cleanup” occurring at the end of the breeding/conception period is considered a “second rut!”

The frustrating situation — more and more the rule rather than the exception nowadays — is when the above, natural rut cycle deteriorates into what is known as a “trickle rut.” I define a trickle rut as actually being a prolonged breeding/conception period, created by does that either do not find a buck on her first estrus cycle, or one that comes into estrus late due to physiological stress (usually created by poor nutrition during the nursing recovery period).

Believe it or not, does often begin cycling in late September, and repeat estrus every 25-27 days for as long as March. This is the cause of those spotted fawns you see during the fall hunting season; something never intended by nature. To understand the why of these things, the next step is to fully explain how the rut cycle comes to be, as well as factors that can disturb the normal rut.

Understanding Rut Timing 

Why do whitetails have a rut? The first obvious answer is that they need one to reproduce! A precisely timed breeding season guarantees the production of fawns at a time of the year that will maximize recruitment. In reality, it is more the timing of weaning that affects when deer breed than any other factor. Once weaned, fawns are virtually on their own, and the amount of highly nutritious vegetation available at that time is critical.

How do whitetails determine when to rut? There are animals that tend to breed during a specific time of the year, and those that breed pretty much all year. Those that have a distinct breeding season do so because they need to produce offspring during the best conditions for survival of said offspring.

Most of the temperate climate deer fall into this category, and each species has its own breeding period. Tropical deer (Axis, Hog Deer and Rusa) live in a relatively constant day-night world, and they tend to not have specific breeding seasons. This gives us a clue about how whitetails determine the time of year.

The challenge was to develop a way to “know” when it is the time of the year to breed, then adjust it to improve survival of offspring. Animals do not have calendars or clocks, yet they have developed an innovative way to perform this task — by measuring length of daylight.

Long ago, ancestors of vertebrate animals had a third eye (parietal eye), still somewhat visible in lizards. If you look at the top of a lizard’s head, you can see it as a small light-colored dot between the two true eyes. In lizards and snakes, this organ is for light reception, and connects to a part of the brain, called the pineal body of the brain. Stimulation from the parietal eye affects timing of breeding through hormone production.

Although mammals no longer have this organ, there still is stimulation of the pineal gland from the eyes. The pineal gland is a neuroendocrine gland which synthesizes and secretes melatonin. When it is dark, the pineal gland and other tissues produce melatonin. In turn, the deer’s brain monitors the concentration of melatonin in the blood and interprets the length of night in so doing.

Then the brain does something remarkable; it calculates the length of the day as a reverse of melatonin concentration. There are a lot of theories about what exactly causes the whitetail rut when it happens, but deer research has shown a relationship between melatonin and sex hormones. Increasing melatonin in the blood as nights get longer stimulates sex hormones.

In our breeding research, we have used melatonin implants to determine when deer breed. Since light suppresses melatonin production, any significant amount of light during the night, such as a full moon, should have an impact on this process. Our research has suggested full moons in October (Hunter’s Moon) and November (Beaver Moon) can serve as primers to the breeding cycle. The problem is that the timing of full moons varies from year to year!

What Else Affects Rut Timing?

The melatonin/sex hormone control of whitetail breeding is one thing, but there are confounding factors that can frustrate this system. These include the physiological condition of the deer coming into fall, moon phase timing and demographics of the herd.

Temperatures play a role in affecting whether or not deer move in daylight hours. You may have noticed some does come into the fall in their winter coats, while others keep their usually reddish coats much longer. This is a response to nutritional stress, caused by late nursing or poor range conditions caused by drought or over-browsing.

what happened to the 2021 rut
The author has carefully collected data from 2005 to 2020 in the northern LP of Michigan showing conception dates. As the graphs show, the distribution of conception dates does indeed move about the fall months of October, November and December. (Graphics courtesy of Dr. James C. Kroll)

A doe will not breed until she has shed her summer coat and grown the winter replacement coat. Likewise, bucks in poor condition, may delay velvet stripping and have low testosterone levels in their blood. This alone will serve to prolong the start of breeding.

Moon phases, as I noted above, are not static within any month from year to year. For example, the full Hunter’s Moon and Beaver Moon in 2021 were on Oct. 20 and Nov. 19, respectively. These same full moons occurred in 2019 on Oct. 13 and Nov. 12; and, in 2017 they were on Oct. 5 and Nov. 4.

Demographics impact when whitetails breed. Natural deer herds are those with natural age and sex structure; that is, ones that have all age classes of bucks present and at a sex ratio of about 1:2.

Many years ago, I wrote in North American Whitetail about our discoveries regarding signposts and staging areas. We even managed to synthesize the pheromones produced by bucks deposited on signposts; and we demonstrated a reaction by bucks to artificially treated signposts. These “social” areas, which are frequented both by bucks and does, can have a significant impact on priming of breeding activity.

There is precedence in domestic animals like goats for male pheromones affecting estrus in females. We have documented visitation by both bucks and does at signposts, the latter even rubbing their heads on signposts. In areas where there are virtually no mature bucks, normal breeding behavior is altered and disrupted by their absence. Further, poor buck:doe ratios lead to prolonged estrus cycling, producing the infamous “trickle rut.”

Given these factors, individually and collectively, and their potential impact on the timing of the rut in whitetails, it is no wonder we are seeing decreased predictability in rut timing.

Real World Examples

We work and have worked with many landowners over the range of the whitetails. We not only advise these landowners, but also conduct research on their properties. Two of these are perhaps the best managed properties in the U.S., Turtle Lake Club and Grand Rack Club in the northern portion of the Lower Peninsula of Michigan.

Turtle Lake has been managed by Wayne and Luke Sitton for more than 20 years; and over that time, they turned a deer herd decimated by 100 years of over-harvest of bucks, as well as a high Tuberculosis infection rate, into a top-notch deer hunting paradise, with almost no TB!  

Further north, Adam Osmun, manager at Grand Rack, has taken a deer herd that annually migrated out of the Club, and created a resident herd on high nutrition and great demographic structure.

Our research in the Northern LP has been multifaceted, but some of our most important data come from the annual necropsies (animal autopsies) we have conducted now for 17 years. Under state permit, we collect does in February, and conduct a complete necropsy on each, to determine health and reproductive performance.

what happened to the 2021 rut
By state permit, the author conducts necropsy research on Michigan does to determine the time of conception. This research is vital in determining the overall health of whitetails by predicting peak fawning times. Shown here, Wayne Sitton (left) and Adam Osmun (right) begin the procedure. (Photo by Dr. James C. Kroll)

To determine when the does were bred, we remove fetuses, then measure the crown to rump length of each; using a statistically reliable measure, we estimate the conception date for each fawn. This allows us to produce a “rut graph” for each year of our study. Here’s a question, if a doe conceives on a specific date, can we not say with great confidence that she was with a buck on that day?

You may have heard or read that the “rut never changes!” I graphed the distribution of conception dates for Turtle Lake in 2005, 2010, 2015 and 2020. As you can see from these graphs, the distribution of conception dates does indeed move about the fall months of October, November and December!

Furthermore, when we graphed the number of days involved in breeding, the range was 11 to 95 days. One of the strategies used both for Turtle Lake and Grand Rack is to significantly improve buck:doe ratio, reduce numbers of deer and substantially increase year-round nutrition using food plots and good forest management practices.

I analyzed the breeding distribution data, to assess our goal of tightening the length of breeding through the above measures. There was a definite relationship between number of days in breeding and the year of management! And there are two years with abnormally longer breeding times, both during years of extreme drought.


This research in Michigan, as well as our work in many other geographic locations led me to the firm conclusion rut timing can be improved through management. Yet, management cannot change the timing of the full moons!

Also, realistically, there are many states where such measures either are not practical, not possible or not supported by state agencies! Yet, understanding why the rut occurs the way it does certainly can explain why you are frustrated the next time it happens.

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