Most all deer hunters get amped about the rut and breeding phase of season. Understanding what triggers estrus in does and how breeding is dictated by fawn survival rates is obviously a huge assist in nailing when to be in trees. We’re about to show you the whys and hows of doing just that.
This is Chapter 9 of Steve Bartylla’s free online book, Understanding Mature Bucks.
Now it’s time to start looking at deer breeding phases. To me, no other topic really does as good of a job showing how both cruel and protective of the species Mother Nature is.
Over the years, I’ve heard of all sorts of stuff that supposedly triggers estrus (their fertile cycle) in does. A cold snap, the moon and sex ratios are the three repeated most often. Every study I’ve seen on back dating fetuses in road killed does, all point to the same area having the majority of does enter estrus in the same time frame, in that specific area, year after year.
The pineal gland is believed to be responsible for detecting the amount of daylight in each 24-hour period. As days start getting shorter, it signals the body to start producing winter coats. As the days start lengthening, it spurs shedding, for example. In fact, that one gland is believed to cause young deer to stop growing each early fall, shifting to fat building and a ridiculous amount of other changes in the deer’s physiology.
In bucks, a big one is that it queues the increased production of testosterone. In many ways, testosterone is the fuel that drives their engine. Its rise is what queues velvet to die, antlers to harden, inspires increased rubbing, scraping, fighting and drives the search to breed, just to name a few. In turn, dropping levels of testosterone are what cause aggression to drop, antlers to shed and new antlers to start forming, and that’s just hitting the high notes.
As it applies to breeding and does, when the amount of daylight in a 24-hour period is reduced to a certain amount, the does enter estrus. Now, there are factors that impact when that will be for a specific area. For example, one would understandably believe that estrus comes earlier in the far north woods, as we’ve had it drilled into our heads so often that cold triggers breeding. In reality, northern Alberta’s peak breeding phase is actually later than northern Wisconsin’s which is a bit later than southern Wisconsin’s. Why? I firmly believe it’s because northern Alberta has a later spring than northern Wisconsin, which has a later spring than southern Wisconsin. You want horrible fawn survival rates? Drop them in snow/cold or before spring green up.
Fawn survival rates are based on many factors, including experience of birth doe, her place in the hierarchy, quality of fawning cover, predator numbers and so on. However, all of those factors pale in comparison to the timing of fawn drop in the north. In any of the northern states and provinces, drop that fawn too early and it’s dropped before spring truly arrives, resulting most often in death within the first week or two of life.
On the flip side, the later that fawn is dropped the smaller it will be entering winter. In areas with significant, long-term snow cover, that often means dying of starvation (it takes more energy to heat one body unit the smaller the body is, while it also is more work to walk through snow the shorter a deer is, while not allowing them to reach as high for the woody browse that is typically responsible for sustaining their lives). Fawn drop timing is key to higher fawn survival rates, and Mother Nature gave deer a formula for hitting that sweet spot, and that is the amount of daylight in a 24-hour period, with natural selection over countless generations favoring those that hit that sweet spot, further cementing the process.
Things get far “messier” as we travel south. In most southern areas, the deer there were actually transplanted from northern states, often coming from more than one area. At the same time, deer in these southern states have their own challenges, but timing fawn drop with spring just isn’t one of them, or at least nowhere near as serious of a consideration as for their far northern counterparts. The result is that, generally speaking, the less intense winter is, the more spread out the rut will be. Mother Nature isn’t penalizing wide-ranging breeding dates with frozen newborns or starving them over their first winter.
Now, just because peak breaking in central Wisconsin occurs between Nov. 5 and Nov. 15 each year doesn’t mean there aren’t does being bred in October and December. It merely means that’s when the majority of does will enter estrus.
Generally speaking, the healthiest does will enter estrus first, a few days to as much as a couple weeks before the rest. At the same time, overly stressed does tend to come into estrus a few days to a couple weeks later. Weather has zero impact, outside of some biologists believing that extreme storms may actually somehow trigger does on the cusp to be delayed a couple days, though I sure can’t confirm nor deny that. Same with the presence of mature bucks supposedly triggering estrus a few days earlier than if there is a lack of mature bucks.
WATCH: THE BEST WAY TO CALCULATE YOUR BUCK-TO-DOE RATIO
Lastly, for today, I have hunted and managed some of the most ridiculously skewed sex ratios nature can provide. I’m 54 years old, have literally lived outside all fall since I was 10 and I’ve never once seen an adult doe that was “missed” the first rut and came into estrus 28 days later (which they will, if not successfully impregnated the first time). YES, that CAN happen. I just have never seen it and most of the serious hunters I know haven’t, either. I know a couple of hunters who have (and I’m sure a few of you have), but it’s rare in my world.
The reality of breeding is that an estrus doe will stand for any buck that happens to be there. It doesn’t matter if he’s a 1.5-year-old spike or a 6.5-year-old stud. Heck, even a portion of healthy nubbin bucks are capable of and will mount an estrus doe, if given the chance. At least as it applies to the dirt I’ve been on, this idea of a “second” rut being made up by all these missed does is fantasy.
When a doe nears estrus, she typically starts actively looking for bucks, if she isn’t already being chased. When you see that doe cruising through, wafting her odors for all to smell with her tail, squatting to pee more than once, seemingly wandering aimlessly, that’s most always an estrus or near estrus doe trying to drum up attention. She can almost always find some buck to take care of it for her.
Doe fawns must reach certain physical and physiological thresholds to breed that first fall. In farm belt states, most fawns are bred that first fall. Even in the Upper Peninsula of Michigan, with shorter growing seasons, less nutrition and harsher weather, on average years, around 5+% of doe fawns will be bred. That percentage goes up with favorable weather/growing seasons and down to as low as zero with unfavorable conditions. Because the fawns must meet those thresholds to be physically able to enter estrus, they tend to enter estrus days to over a month after peak breeding.
Finally, a reminder that I’m a northern boy. That’s where most of my experience is and what I understand best. What I can say for sure is that the farther one goes south from Canada, the more stretched and twisted this all gets, as I firmly believe that it’s Mother Nature/winter itself that is really dictating this process, due to positive (fawn survival) and negative (fawn death) reinforcement/natural selection, carrying over countless generations. As a final side note, with our weather patterns getting crazier by the year, these past 10ish or so years, I’ve also noticed the entire rut cycle getting more twisted and drawn out, as the negative reinforcement just isn’t consistent, which can also be seen by a steady increase of spotted fawns I’ve been getting in deer season.
Read Chapter 1: Whitetail Tendencies
Read Chapter 2: Whitetail Home Ranges
Read Chapter 3: How Deer Use Core Areas
Read Chapter 4: When Core Areas Shift
Read Chapter 5: Seasonal Shifts
Read Chapter 6: Family Group Dominance
Read Chapter 7: Male Dominance
Read Chapter 8: Deer Population Dynamics