“When will the rut occur this year?” This is one of the most frequently asked questions from deer hunters eager to plan their precious free time to coincide with that magical moment when otherwise wily white-tailed deer let down their guard and move more during the day. I’m going to share a little secret on how to accurately predict the peak of the rut each year, but first some context.

Seasonal changes in deer physiology and behavior, such as when bucks shed their velvet and dissolve bachelor herds and when does become receptive to suitors, are triggered largely by changes in day length. Decreased daylight triggers physiological changes like an increase in testosterone. This, in turn, causes the aforementioned transitions.

Some people believe that moon phases play a role; and as the timing of fall full and new moons varies each year, so should the timing of the rut. Biologists have studied for decades what influence, if any, the phase of the moon has on deer behavior and always come to the same conclusion: none.

Others think the weather plays a role. An unusually warm fall could delay the rut while an early cold could accelerate it. If this is true, what would be the effect of climate change? There is also a lot of research on this topic, all coming to a similar conclusion.

Biologists also use several techniques to determine when peak reproduction occurs. One is fetal fawn measurements. They collect samples and, by measuring their size and backdating the gestation period by 200 days, they can accurately predict when a fawn was conceived. What they found time and time again is that although the peak breeding period can vary over a wide geographic range, it occurs at the same time every year for a particular area.

Let me rephrase that for emphasis: it never changes from year to year.

There is a good reason for this, especially in northern deer. Selective pressure pushes the reproductive peak into a relatively narrow time window. If fawns are conceived too early in the fall and then born too early in the spring, there will not be enough food available for the nursing mothers; too late and they won’t have time to grow enough to survive their first winter. Nature continually tests the limits and this produces exceptions over the long term, but fawns conceived closest to the peak period have the best chance of survival.

So, how can we explain the exceptions sometimes observed, such as bucks running after does in October or December? Deer don’t use calendars. The peak of the rut, which occurs over approximately 7 to 10 days, is defined as the time when most adult females are mated; but reproduction occurs on a continuum. Some goats will enter estrus early, others late. And a poorly timed heat wave won’t delay the rut. The deer will simply move more during the cooler hours of the night.

So, when will the rut be this year? The same time as last year, and the year before that, and the year before that.

Bob Humphrey is a freelance writer and registered Maine guide who lives in Pownal. He can be contacted at: [email protected]

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