Mathews Inc.
By CJ Winand



Determining the RUT . . . Science vs. Moon?


Does the moon, or does photoperiod (daylight length), determine timing of the rut?  This perplexing question has long intrigued bowhunters and was one of the topics addressed at the 24th Meeting of the Southeast Deer Study Group in St. Louis, Missouri.  Along with 14 prominent wildlife biologists from around the country, David Osborn from the University of Georgia presented the findings in his much anticipated paper entitled, “Does Moon Phase Chronology Determine White-tailed Deer Breeding Dates?” 


Needless to say, his presentation was of major interest to the crowd of biologists, managers and hunters.  Under the direction of Drs. Karl Miller and Robert Warren, Osborn compiled the results of deer breeding studies from seven states:  South Carolina, Texas, Mississippi, Missouri, Maine, Minnesota and Michigan.  Total data represented breeding records from over 2,500 pen-raised and free ranging does. 


Data was collected in two ways: One, breeding dates for captive deer were determined by using a tame buck to “heat check” each doe.  When the does were receptive and breeding was preformed, the researchers noted the actual date.  


For free-ranging does, backdated conception dates were estimated by measuring fetal development. Years ago, various biologists developed a crown-rump measurement scale for fetuses.  By measuring the length of fetuses in harvested or road-killed does, biologists can backdate to when conception actually occurred.  The fetal scale is a very important tool because biologists can use it to detect changes in deer breeding dates relative to herd management and habitat quality.  Thus, by comparing known (pen-raised deer) and estimated free-ranging deer conception dates with moon phase, researchers were able to evaluate the influence of moon phase on the timing of breeding.


In one example, Osborn established, October 27th  - that is 6.5 days before the hunter’s moon or the second full moon after the fall equinox (generally in September) as the average breeding date for 1,053 does collected over a 13-year period on the coastal plain of South Carolina.  From year to year, the 13-year average breeding date deviated only an average of 4.6 days from October 27.  However, timing of the Hunter’s Moon varies from year to year, and when conception dates for individual years were compared to the Hunter’s Moon, the deviation was about 11.4 days. 


To put it another way, average timing of conception for individual years years deviated from October 27 by only 4.6 days, but it deviated from the Hunter’s Mooon by as much as 11.4 days.  Although that’s only one example, Osborn found that calendar date, not moon phase, was more reliable in predicting breeding dates in 21 of 22 data sets from different states or physiographic regions.  When asked about the one exception, Osborn speculated that it was an unrealistic artifact of a small sample size.


Some critics point out that crown-rump measurements can vary among researchers.  Additionally, some of the fetuses may have decreased in size due to the formalin solution in which they were preserved.  Other critics suggest that a conception date based on backdating from the date of birth or fawning is a more accurate way to determine breeding date.   However, Osborn believes that fetal aging equations are more reliable than estimates based on fawning dates because gestation length can vary among does as much as 30 days. 


The average gestation in whitetail deer is about 201 days, but captive does at the University of Georgia have delivered healthy fawns from Day 183 to Day 213 of gestation.  Therefore, pregnant does may have 30 days of variation in fawning dates, even when they became pregnant on the same date.  Osborn agrees that conception dates based on crown-rump measurements include some error, he believes that estimates based on fawning dates contain as much, or likely more, error.  The only way to be certain about conception dates is to observe mating.    


The University of Georgia researchers also examined captive herds in Georgia, Michigan, South Carolina and Virginia.  Because the researchers knew actual breeding dates for captive deer, estimates based on fetal measurements and fawning dates were totally eliminated.  One of the most interesting findings of the study was that captive deer with known breeding dates, and wild deer with breeding dates estimated by fetal measurements yielded similar results. 


Osburn concluded, “Therefore, we believe it is not necessary to revise the conventional understanding among deer biologists that breeding dates are primarily influenced by photoperiod (not moon phase) and are relatively consistent among years within a particular population.”  It’s important to note that this research was an objective look at the influence of moon phase on several large data sets.  Additionally, any variation in estimating conception dates was eliminated by reporting population averages, and the error involved with the fetal aging techniques is moot.'s Moon Calendar Application

Java Applet

Hit the top right buttons to use's new Moon Calculator!




Fetal Age Model


What about any differences between northern versus southern deer fetuses?  In 1997, James Kroll and Ben Koerth compared the fetal crown-rump length data from four different fetal age models from South Carolina, Michigan and two in New York.  They determined that, excluding the model from Michigan, which had a small sample size, no real differences existed among the other three.  Kroll and Koerth suggested that the fetal age model developed in South Carolina was the most appropriate.  It should be noted that subspecies and regional differences could differ, but the South Carolina-based model has been the standard and most widely used.  The researchers also pointed out that even with the small sample sized, Michigan fetal age model, it only produced a rut chronology about one week later than the others.


Joe Hamilton, Founder of the Quality Deer Management Association was the primary author on the fetal age model from South Carolina.  For management purposes, his research determined no meaningful differences between male and female fetuses, single versus twin fetuses, or fetuses from does ranging in age from 1.5 to 3.5 years.  Additionally, fetal measurements from fresh fetuses did not significantly differ from those preserved in formalin.  The average shrinkage of the fetuses was only 6 percent.  Hamilton also looked at fetus weights, heart girth, abdominal girth, head length and various other measurements to determine his fetal age model.  He found that no other measurements were better than the crown-rump in computing the fetal age model.  It’s also important to note that Hamilton’s data were derived from pregnant does methodically collected at weekly interval (other fetal scale models have used samples collected randomly from road-killed and hunter-harvested does).  Some of the other models were completed by interpolation, which filled in the gaps when there wasn’t any actual measurement for the specific stages of development.   


As a side note, Hamilton used a vasectomized buck to determine if does were receptive.  When this sterile buck “went on point”, a fertile buck was introduced with the “hot” doe and the exact time of breeding was recorded.  After this, the buck was removed and the doe was not in contact with another fertile buck.  The period of receptivity was determined to be 24 hours.  One exception was a doe that bred every time a fertile buck was present up till her fourth month of pregnancy and when the buck’s antlers were in velvet!  I believe they call this “cervid nymphomania?”  That’s all I’m going to say about that!


Moon Models


In attempt to be fair, I contacted Jeff Murray to get his thoughts on these data.  Back in 1994, Murray was the first in the country to correlate moon phase and positioning with deer movements.  Murray’s Moon Guide is based on the moon rising about 51 minutes later each day.   Once a hunter knows the predictable moon peaks, he can construct a sound game plan to put himself in the right place at the right time.  Murray’s annual “Moon Guide” has been a standard tool for many hunters throughout the country.  In 1996, Murray discovered that the moon also holds the key to finalizing the rut dates for any given year and, thus, the Murray’s Rut Guide was born. 


Given all the letters of support Murray has received on his Moon and Rut Guides, I asked Murray his thoughts on Osborn conclusions.  Murray said, “I believe every method researchers have used to "test" the moon's effect on rutting whitetails is flawed.  If the makeup of every study group is not uniform in term s of buck to doe ratio, the results will be skewed.  The main reason is the biostimuation effect mature bucks can have on nearby does.  The issue isn't limited to how many bucks, but I’m definitely suspicious of drawing conclusions of penned deer that live in close proximity to one another”. 


Murray continued, “Regarding measuring fetuses removed from road-killed does in hopes of back-dating conception dates, the basic technique is imprecise, at best.  The main problem is erratic fetal growth, particularly toward the end of the cycle.  This is common among warm-blooded organisms, including humans”.


 Another acclaimed rut predictor is one produced by outdoor writer Charlie Alsheimer and wildlife biologist, Wayne Laroche.  These men predict that the second full moon after the fall equinox – The Hunter’s Moon - sets the trigger on whitetail breeding.  Alsheimer / Laroche contend that fetal scales and backdating provide only estimated breeding dates in a one to two week window.  Therefore, they feel comparisons made with these data are suspect. 


Either way, it sure would be interesting if Murray and Alsheimer/Laroche would combine their data with Osborn’s data?  Hopefully, some day this will occur?   Until then, both Murray and Alsheimer/Laroche’s theories conflict to some degree with what Osborn and other wildlife biologists have been saying for years - that photoperiod (decreasing amount of daylight), not moon phase triggers the whitetail breeding cycle.




Any worthy biologist will tell you that anecdotal data from hunters have value, but that biologists need quantitative data from scientific research to draw sound conclusions.  Obviously, buck-to-doe ratios, a herd’s age structure, weather conditions, the general condition of the does, genetics and hunting conditions all play role in the rut.  Just how much?  Who knows?  But maybe this is where both sides can find common ground?  As Alsheimer told me, “The rut equation is most likely 80 percent photoperiod with the remaining 20 percent heavily influenced by the Hunter’s Moon.”      


Overall, I really believe we may be simply splitting hairs with this topic.  Although the subject is very interesting, who cares?  If the moon guides work for you, then by all means, use them.  The moon raises many questions simply cannot answer and many people, myself included, believe the moon must have something to do with deer movements and the rut.  Just how much is the real question?  As bowhunter Steve Keithley from Maryland says, “In my parts of the country, the rut will be in and around the second week of November, so if you have 10 minutes to hunt, then hunt!” 


I agree.  Whether photoperiod or the moon phase deserves credit, the rutting action might be happening in your woods, but only 5 miles away it could be nonexistent.  Sound familiar?  Bottom line is, you’ll never know unless you’re in the woods!


C.J’s Summary:

Web sites such as this one erupt every time the subject of rut timing comes up.  Are we simply splitting hairs comparing the moon with traditional photoperiod dates?  I think so.  If you subscribe to the moon theories, one must remember that all full moons are not created equal, from month to month or from year to year.  One year the Hunter’s Moon could be 17 degrees off the horizon and 118,000 miles away from the earth, while next year it could be 28 degrees and 210,000 miles away.  Do those moons have differing effects?  Who knows?  When it’s November, follow my lead.  Tell your boss, you don’t feel well and can’t SEE … coming to work, that is.  It works every time!  See ya in the wood

  What is your Opinion?

  • Sitka Gear