“When will the rut occur this year?” It’s one of the most oft-asked questions among deer hunters eager to schedule their precious time off so it coincides with that magical time when otherwise wily whitetails drop their guard and move more during daylight hours. I’m going to share a little secret on how to accurately predict peak rut every year, but first a little background.

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

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

Others believe weather plays a role. An unseasonably warm fall might delay the rut while early cold might accelerate it. If that’s true, what would a changing climate do? There’s plenty of research on that as well, all with a similar conclusion.

Biologists also employ several techniques to determine when peak breeding occurs. One is fetal fawn measurements. They collect samples, and by measuring their size and back-dating by the 200-day gestation period, they can accurately predict when a fawn was conceived. What they’ve found, over and over, is that while the peak breeding period may vary over a broad geographic range, it occurs at the same time every year for any particular area.

Let me re-phrase that for emphasis: It never changes from one year to the next.

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

So how do we explain exceptions that are sometimes observed, like bucks chasing does in October or December? Deer don’t use calendars. Peak rut, which occurs over roughly 7-10 days, is defined as when most adult does are bred; but breeding occurs on a continuum. Some does will enter estrus early, others late. And a poorly-timed warm spell won’t delay the rut. The deer will just move more during the cooler hours of night.

So when will the rut occur this year? The same time it did last year, and the year before, and the year before that.

Bob Humphrey is a freelance writer and Registered Maine Guide who lives in Pownal. He can be reached at: bob@bobhumphrey.com