The Moon and The Rut

I’ve long known the moon has an effect on wildlife behavior. I probably first noticed that effect in fishing, especially the moon’s influence on the timing of spawning. The old-timers in Florida, where I fished a lot during my youth, would confidently predict that thus and such fish would spawn on this or that full moon. As near as I could tell, they were usually right. I came to accept as fact that bass in that part of the world would spawn on the full moon of March, shellcrackers on the full moon of April and bluegill on the full moon of May. 

When I first started deer hunting, I didn’t think much about the moon but that quickly changed. The first thing that came to my attention was that distinct ups and downs in daylight movement appeared to be correlated to moon phases. Even as I was trying to figure that out, I graduated from college and became the managing partner of Burnt Pine Plantation, a 13,000-acre deer hunting operation in Central Georgia. The plantation became a veritable laboratory for whitetail study, and we collected data on everything imaginable. How the moon figured into deer movement, particularly rutting activity, was one of the things that fell under our scrutiny.  

I soon began to suspect that there was a strong correlation between the full moon and the timing and intensity of rutting activity. I frequently observed rutting activity during all hours of the day during the full moon, making me suspicious that at least daytime rutting activity was enhanced by this moon phase. In time, night surveys, personal observations and a profusion of night-born running deer tracks, a sure sign of bucks chasing does, provided evidence of heavy nighttime rutting activity around the full moon. As data from the plantation swelled, I began to notice that a disproportionate number of mature bucks were killed near the full moon, strongly suggesting a link to the rut. This was especially so when the full moon hit near our traditional peak-rut dates but less so when the full moon hit a week or more either side of the magic November 3 to 9 window. With tangible evidence to rely on, we began to successfully predict heightened rutting activity around the full moon when it hit within a week of the traditional rut dates. Then, years into our program, we finally got hard proof that even breeding, not just ritual rutting activity, and the full moon were linked to a degree. 

Georgia regulations call for a weeklong either-sex season between Christmas and New Year. This is when the plantation harvested the majority of the does necessary for management. Having a harvest universe of a hundred or more does allowed us to gather solid data on breeding dates. This was done by backdating the fetuses to determine when breeding actually took place. Over time, we saw clear, though modest, year-to-year shifts in peak breeding dates, really more like “bulges” forward or backwards within the same traditional rutting timeframe. It didn’t take long to recognize that these shifts generally corresponded to shifts in full moon dates. That provided the hard proof we needed to verify what we had already observed in the field and in our harvests. The full moon not only affected the intensity of the ritual rutting activity, it also had a direct, albeit modest, impact on the timing of whitetail breeding. However, from all we saw, the full moon’s impact on ritual rutting activity was greater than on actual breeding. In other words, the full moon would shift rutting activity further its way than it would shift breeding per se. 

Since that discovery years ago, more research and time in the field have shed additional light on the full moon’s effect on rut timing and intensity. I’ve had the opportunity to test the theories across the country in a wide variety of circumstances. Today, I’ve come to accept certain relationships between moon phases and rut timing and intensity as something I can apply to hunt planning with a high level of confidence. 

Full Moon And The Rut

I said earlier that each moon phase could be viewed as lasting essentially seven days, the day of the phase plus three days before and after. That’s certainly a logical way of looking at it, but it doesn’t quite fit in the case of the full moon and the rut. The fact is that the full moon’s effect on the rut seems to be greater toward the middle and end of the phase than at the front. The upswing in activity appears to start right before the actual full moon and continues a few days after. We’ll look at a possible explanation of why that is so a little later. 

Also, our discussion here is aimed at hunting application, so we’ll be mostly talking about ritual rutting activity as opposed to actual breeding. When important, I’ll try to make the distinction. And, we’ll be assuming all things are equal, which they seldom are. Outside factors other than the moon also affect the timing and intensity of the rut, as we’ll see next month. 

Now, the impact of the full moon on the rut. We’ll start with this: The full moon intensifies rutting activity when it coincides with our weeklong traditional peak of rut. This is the best possible rutting scenario and results in an intense, well-defined rut of normal duration centered around the traditional peak-rut dates. While it is true that the full moon’s effect can be slightly different when it hits toward the front of our traditional peak as opposed to the back, that is splitting hairs more finely than necessary. Suffice to say that small shifts can take place in the timing and intensity of the rut depending on where within the peak week the full moon falls. Generally, I prefer an early full moon to a later one for reasons that should soon become apparent. 

What happens when the full moon hits outside the magic rut window? It acts like a magnet on the rut, skewing it toward the full moon. To illustrate what happens, imagine a symmetrical bell-shaped curve of rutting activity when the full moon coincides with the traditional peak of rut. (In reality, a rutting curve would be “front heavy” since the rut, especially ritual activity, starts rather suddenly, peaks early and then tails off gradually.) When the full moon hits outside this peak window, imagine that the bell-shaped curve stretches, flattens and shifts toward the full moon, with the peak now lower and off center. The end result is that the full moon alters the timing, duration and intensity of the rut … but only within limits. The further away from the peak-rut window the full moon falls, the less attraction it exerts on the rut, which makes sense remembering that daylength is the macro-timer. Conversely, the closer to the ideal time, the greater pull the full moon has on the rut. 

How far away from the peak rut can the full moon exert influence? Remember, the furthest the full moon can fall from a specific date is roughly two weeks. In the case of our weeklong peak window, the full moon can hit as far away as about 11 days from either the beginning or ending date of the peak-rut window. That’s close enough for the full moon to always exert some influence on the rut, i.e., spread it out and reduce its intensity, but from what I’ve seen, its power is considerably diminished when it falls a week or more away from the traditional peak dates.   

Ok, here comes a bit of a curve ball: The effects of the full moon seem to be somewhat greater when it falls before the peak rut dates rather than after. Actually, this is logical. Since daylength is the primary timer, passing up the macro-timer to get to the micro-timer doesn’t make much sense. When the full moon occurs in front of the peak rut dates, the micro-timer (moon) has more of an open field and first shot at influencing the rut. (We also may be able to pin the greater influence of an early full moon on a biological explanation soon to follow.). The closer the fore side full moon falls to the peak dates, the more the rut maintains its classic intensity, duration and form, even though it does shift forward at the cost of backside activity. The further away (up to the week or so limit we talked about), the more the rut slides forward, spreads out, lowers in intensity and drops off on the backside, all of which cut into hunting opportunity. The changes in the rut we’re talking about here are more than enough to cause a wise hunter to rethink his usual rut-centered timing. 

A full moon hitting after the traditional peak rut can delay and retard rutting activity, but it doesn’t have as dramatic an impact as one falling before. Why? Daylength, the primary controlling factor, has had a chance to come into play before the secondary factor, moonlight, arrives on the scene. Plus, new data strongly suggest that the full moon’s effect on the rut is delayed a few days before completely kicking in, pushing a late moon’s effect even further away from the peak window, thus weakening it. (We’ll talk about this shortly.) It seems to me that for an aft full moon to have an effect similar to an early moon it has to hit three or four days nearer the traditional peak. Whatever the degree, when a full moon hits less than a week after the traditional peak rut, rutting activity starts slower, drops in intensity and carries on later than normal to overlap the full moon. 

Darkness, Light And Melatonin – The “Sleeping” Hormone

Why does the full moon have such an effect on the rut? The most obvious answer is light. The full moon illuminates the night, allowing deer to carry on their rutting activity in the quiet of the evening undisturbed by man and unhampered by darkness. True, deer can see much better at night than we can, but on dark nights, they still have visual limitations when trying to carry out the acrobatic maneuvers required in the rutting ritual. 

I have long been convinced that most whitetail breeding takes place at night. After all, how many bucks have you ever seen in the actual act of breeding? Of the countless bucks I’ve seen in the wild, I have only witnessed a double handful in the act of breeding. That alone leads me to believe that most breeding takes place at night. Certainly, daytime rutting activity can be intensified by the full moon, but I think what we see during the day is just a carryover from hot and heavy nighttime rutting escapades. If that is so, it doesn’t take much of a stretch to imagine that better visibility from more light is part of the reason. But, I believe it goes deeper than that.

So much varying and even conflicting stuff has come out recently on the effects of moon phases on deer activity that it’s hard to separate the wheat from the chaff. Certainly, some of the best and most scientific information on the effects of the moon on whitetail breeding was published in the book Solving the Mysteries of Deer Movement by biologists Dr. James Kroll and Ben Koerth. In their book, James and Ben delve into a possible scientific explanation for why the full moon triggers breeding activity. In their findings may lie some key answers to the moon/rut relationship. Here, I’m going to try to give you a hunter’s perspective of what goes on and what you need to know to turn this information into big bucks. 

We’ve established that the timing of the whitetail reproductive cycle is tied to light, primarily daylength and secondarily moonlight. We have seen how deer at a particular locale are genetically ingrained to enter breeding readiness at certain daylengths. We have seen that the full moon tends to “bend” the rut its way. But why? How does it work? How do deer measure light? What happens biologically to trigger a response? 

We know that deer have a very sensitive light receptor called the pineal gland. This gland measures the light (actually, the amount of darkness) registered through the eyes and “communicates” its findings to the body through the production of a hormone called melatonin. We know it as a sleep inducer, but it also plays an important part in the regulation of day/night cycles and seasonal rhythms. Interestingly, darkness, not light, increases the production of melatonin. In whitetails, melatonin, or to be more accurate, the absence of melatonin, has a role in the release of sex hormones, including those that lead to breeding readiness. Now keep this in mind: Darkness increases melatonin production; light reduces it, and the absence of melatonin at the right time of year can release sex hormones, trigger estrus in does and initiate breeding. 

Since melatonin is produced in greater quantities when it is dark, the production of melatonin increases on the new, or dark, moon. The greatest effect of that increased production comes into play during the days following the new moon, meaning during the first quarter. As the nights get brighter and brighter moving toward the full moon, melatonin production drops. James reports that artificial insemination studies have proven that the reduction of melatonin levels in does during the rutting window will trigger the onset of estrus. The removal of melatonin actually sets into motion a series of hormonal events that takes a few days to play out before bringing on estrus and subsequent breeding. So, the increased light, resulting in reduced melatonin, accompanying the full moon can initiate estrus in does and spark a flurry of rutting activity around and immediately following the full moon. This explains a number of things. 

First, it explains why the full moon has such a dramatic impact on the rut – its light indirectly initiates estrus in breeding-ready does, the number of which depends on how close to the ideal breeding dates (daylengths) the full moon hits. If the full moon falls too far away from those peak dates, fewer does are in breeding readiness. If it hits closer, more does are ready and triggered into estrus. Thus, we see the workings of the pattern discussed earlier. 

It is important to note that once the increased light of the seven-day full moon phase begins to reduce melatonin levels, it takes a few days before does enter estrus. In that delay, we find a possible answer for why the best rutting activity seems to crank up around the mid and latter part of the full moon phase. The delayed response of the does also explains why a full moon hitting before the traditional rut dates has a greater impact on the rut than one falling after – the few days delay in coming into estrus puts breeding on the fore moon closer to the ideal daylengths and further away on the aft moon. 

There also may be a practical understanding of deer movement coming out of this. I’ve known for years that deer feeding movement during the new moon tends to be good during the prime early morning and late afternoon hours. I’ve also known that daytime feeding movement during the full moon tends to be poor. Remember, whatever else melatonin is, it is also a sleep enhancer, and an excess of it has been found to cause mood swings, tiredness and that “hung over” feeling the following day. In short, high doses of melatonin can dull daytime activity. Is it possible that melatonin production, which increases on dark nights, promotes more rest and less movement at night during dark moons? If so, is it also possible that the light of day brings on renewed activity, especially during the prime early and late hours, perhaps by reducing melatonin levels or simply because the deer didn’t move much at night? Conversely, is it possible that reduced melatonin production on bright nights results in less resting and more moving at night? Consequently, after an active night, wouldn’t it be logical for the deer to rest more during the day, perhaps only to grab a bite around midday, as they seem to often do on full moons? 

Perhaps more important to this discussion, all this would also explain why rutting activity intensifies during the full moon and is often so visible during midday hours. It’s not much of a stretch to figure that a full moon during the rutting window results in a greater number of hot does; bright nights of active rutting; resting after daybreak; midday rutting flurries by charged up, competitive bucks; resting again late in the afternoon before an hot night on the town. There is a certain straightforward logic in all of it. 

Well, that’s a look at how the sun and moon may factor into the timing of rutting activity, but there is still more to the puzzle. Next month, we’ll examine some of the other factors that affect rut timing and intensity … and your hunting strategy.

(Editor’s note: This article was excerpted from David Morris’ new book, Advanced Strategies for Trophy Whitetails.”)