Moon phases, hunting and the rut: Something to debate at deer camp

Illustration by Adelaide Tyrol

By DAVE MANCE III

Deer hunters, like professional athletes, are always looking for an edge – it’s the nature of the pursuit. And so we’re susceptible to superstition, alluring gadgets, marketing campaigns.

A classic combination of all three is the moon table – a chart that tells you when the best hunting days are based on the moon phase. These charts were a sporting magazine staple in the early days. In the print world, they have largely gone the way of the Marlboro Man, but you can now download an app which uses the moon to help schedule your hunting vacation.

Whether deer movement is affected by moonlight is an intriguing question. But because it’s hard to isolate the moon from all the other phenomena affecting deer behavior, I can’t imagine how you’d go about proving or disproving any particular theory. Scientists have conducted radio-collar studies with small groups of deer trying to gain insight, but the samples were so small, and the data so ambiguous enough, that it was just hard to tell.

The idea that the moon triggers the rut, however – a subset of the deer/moon information – doesn’t really hold up under scrutiny. Mammologists have long held that the rut is triggered by declining daylight in autumn, which has nothing to do with the moon phase.

Over the past decade, studies in Pennsylvania, Illinois and New Brunswick all reaffirmed the idea by examining fetuses extracted from road-killed does over the course of multiple years. Since we know a whitetail has a gestation rate that’s pretty close to 200 days, and fetuses by and large have consistent growth rates, they can be to determine the date of conception. Researchers found remarkable consistency in the date of the average peak of the rut from year to year, regardless of weather conditions or the moon.

This idea that day length triggers the breeding cycle (scientists refer to this as photoperiod) makes intuitive sense in a northern climate, since in most years, deer need to synchronize birthing with the seasons. If a fawn comes too early, it’ll be too cold and the mother won’t have adequate nutrition. If it comes too late, the fawn won’t be able to accumulate enough body weight to make it through winter.

However, it’s important to consider photoperiod as a general rule, not the end-all-be-all factor. For example, bud-break in trees is another natural phenomenon based partly on photoperiod. But the bizarre spring of 2012 – when buds broke two weeks earlier than normal after a week of 70-degree March weather – showed us that trees take cues from temperature, too.

Hard-and-fast rules are so much more satisfying than general rules, yet nature is full of the latter. This provides an opening through which one can cast doubt.

Looking at the New Brunswick fetal-deer study which ran for 9 years, the average peak of breeding fell within a seven-day period every single year. On eight of the nine years, it fell in the same 4-day period in late November. But a few outliers were breeding from mid-October through mid-December each year. On a graph the data resemble a bell curve, so somewhere an astute hunter was watching a doe in heat in October and wondering what gives.

In such a case, it’s human nature to seek alternate explanations.

I asked Rod Cumberland, the New Brunswick deer project leader for the study, why some does came into heat early or late, and he told me that “reproductive readiness is governed by nutritional plane.”

It’s conjecture, but his thought was that the early-breeding does were ones who were not impregnated the previous year – for whatever reason – and did not lactate all summer. Because they were in better physical condition, they were ready to breed earlier.

He said that the vast majority of does who bred during the peak rut were females that carried a fawn or fawns the previous year. His theory goes on to suggest that the late breeders were a combination of females who missed connecting with a buck during the first estrus, and doe fawns who matured and became receptive in December.

Dave Mance III is the editor of Northern Woodlands magazine. The Outside Story is assigned and edited by Northern Woodlands and is sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation.

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