For better hunting success, it’s critical to understand the role of does in mate selection. It is now being theorized that excursive behaviors by females during the breeding season are a form of female mate choice, or a way to incite competition between bucks and give the female access to a higher-quality mate.
The air is cool on your face as the November winds rustle the leaves covering the forest floor. From your perch high in an old oak you patiently scan your shooting lanes, alert for the first sign that your quarry is approaching.
For months you’ve been dreaming about what scenes might unfold before your eyes as you watch one of nature’s great soap operas — the rut. You have envisioned every scenario: rubs, bucks in pursuit of does, maybe you could even be lucky enough to see a sparring match this year! Yet, have you ever stopped to consider that there might be someone else pursuing these hormonal bucks for which we search so diligently? No, not a fellow hunter muscling in on your territory. Instead, your competition comes from a much less expected source — female white-tailed deer.
By Dr. Stephen Ditchkoff
and Jeff Sullivan
The behavior of female white-tailed deer during the breeding season has long been of interest to researchers. Even during the 1960s and ’70s, biologists monitored penned deer to see how does behaved as they came into estrus, or were approached by rutting bucks. This fascination with the breeding behavior of whitetails has persisted to modern day, with scientists beginning to place collars on deer that track their movements via satellite and can monitor locations as often as once every few minutes!
All of this work has led scientists to believe that does generally adhere to one of two strategies as they enter the breeding season.
The first strategy, known as “sit-and-wait,” has been observed near the peak of the breeding season and is generally characterized by a doe increasing use of her core area — the area where a deer spends most of its time.
For example, if we were to strap a radio collar around your neck and document your space use, we would find areas within your home range that have much greater use. For most people, these areas would be home and work. Does also decrease movement rate and home range size — the area where that animal spends 95 percent of its time. Imagine this as your house plus your route to work and places you visit regularly, such as the grocery store. Essentially, the female is constricting her movements to a small, well-known area.
Such behavior presents multiple benefits, including reduced energy expenditure, decreased risk of mortality and an increased ability to heavily mark the area with scent indicators expressing her receptivity to passing males. However, it is believed that for the sit-and-wait strategy to be effective, females must be a limited resource to the extent that males will exhibit intensive search behaviors.
In other words, there needs to be a low enough density of does that bucks have to go out and search for them if they want to successfully breed.
As a hunter, it should be very easy for you to envision the results of this behavioral strategy — bucks spending a lot of time searching for does. You should also recognize how this behavioral strategy potentially influences your hunting success.
EXCURSIVE BEHAVIOR STRATEGY
With all of the benefits females gain from performing sit-and-wait behavior, it might be difficult to envision the need for a second strategy. However, as hunters, we are well aware that there are deer populations that at times are characterized by a surplus of does, and scientists theorize that these population characteristics lend them- selves to a different female strategy.
In populations with low deer densities or a relatively low abundance of females, a second behavioral strategy that does employ during the breeding season to maximize their probability of being bred has been reported — excursive behavior. This behavior is generally characterized by increased activity leading to an excursion, or brief trip, outside of the home range.
Previous work by the University of Georgia has described these excursions as lasting an average of 24 hours, and variable in distance, ranging from 0.57 to 4.78 kilometers. Additional work currently being conducted by our research group at Auburn supports these findings, and also suggests that female excursions are best described as straight line movements. These traits have led some scientists to suggest that excursive behavior by females might be an effort to maximize the probability of finding a male during the short period that the female is receptive.
ROLE REVERSAL: FEMALE MATE SELECTION
From the doe behaviors long reported in the scientific literature it would seem logical to conclude that when there are fewer females than local bucks are able to breed, does will adhere to a sit-and-wait behavior, allowing bucks to come to them. In contrast, when females are abundant they should be going on excursions to maximize their probability of locating a mate.
Unfortunately, some recent research on the topic has added an element of confusion, as researchers have reported excursions by females under a wide variety of population demographics. However, this confusion has led to an intriguing suggestion: It is now being theorized that excursive behaviors by females during the breeding season are a form of female mate selection, or a way to incite competition between bucks and give the female access to a higher-quality mate.
Due to a general lack of data and relatively poor understanding by the scientific community of the behavior of females as they come into heat, we, with financial support from the South Carolina Department of Natural Resources, set out to examine this aspect of deer biology and determine the movement patterns of females during the period surrounding conception.
To accomplish our goal we darted adult female white-tailed deer and fit each with a GPS collar programmed to collect locational data every 30 minutes. Many of our study animals were harvested by hunters after the rut, enabling us to determine their date of conception by examining the development of their fetuses.
Once we were able to determine the date of conception, it was a simple matter of analyzing the movement data of does around their specific dates of conception. With the moderate deer density and balanced sex ratio of the deer population on the property where our study was conducted, previous research would suggest that a sit-and-wait behavior would be expected. However, once we finally had the opportunity to examine the complete data set, we found some interesting trends.
We found that movement rate (distance moved per half hour) and probability of activity (the probabil- ity that a deer was actively moving rather than bedded) of our does both steadily increased approximately 33 percent during the 40 days prior to conception. However, approximately five days prior to conception until five days post conception, move- ment rate and probability of activity decreased slightly, and then steadily increased again until 15 to 20 days following conception.
This trend is best demonstrated when looking at the daily average movement rate of our study animals (Figure 1). The increase in activity leading up to conception suggests that females were engaging in behaviors that would result in increased exposure to males, and by increasing movements the females were able to broadcast their reproductive status over a wide range. Additionally, we observed several females performing excursions outside of the home range in the days prior to conception (Figure 2). By increasing movement rate and traveling outside of the home range, the females were alerting the maximum number of bucks that they were ready to breed.
The steady increase in activity following conception was unexpected, yet very interesting. We had hypothesized that a female would be best served by reducing activity rates once conception had been achieved. Not only would this allow her to save energy for the demands of the upcoming pregnancy, but reduced movement would also decrease risk of exposure to hunters.
However, we postulate that the observed increases in activity after conception were due to the female not knowing that she has successfully conceived, and are the result of her preparing for a subsequent estrus. Females that are not bred will enter a second estrus approximately 28 to 30 days following the initial estrus. This is one reason why deer herds with incredibly unbalanced sex ratios experience such a long and protracted breeding season. By increasing movement post conception, it appears that females are once again maximizing the probability of encountering a mate, and maintain this elevated rate of activity until pregnancy hormones confirm that they have successfully conceived.
While both the trends in movement rate and the observation of excursions near the time of conception provided strong evidence that females were performing excursive behavior to attract mates, we found additional support for this conclusion when we examined female space use. We found that does increased both their home range and core area size as they approached conception, with peak values seen during the seven day window immediately surrounding conception (Figure 3).
This shows us that not only were does moving at a greater rate lead- ing up to conception, but that they were doing so over a much greater area. By increasing the range of their movements, females increased their likelihood of encountering additional males, and consequently, provided themselves the opportunity to attract a higher quality mate than might have been able to locate her within the smaller home range. Not only is this increase in core area and home range size the exact opposite of the constriction observed when the sit-and-wait strategy is performed, but when combined with increases in movement rate and the observed conceptions it helps paint a clear picture of how does are behaving leading up to conception.
The findings of our research have led us, and hopefully you, to the conclusion that does perform- ing excursive behavior in populations where they would be expected to have little trouble attracting a mate via the sit-and-wait strategy are doing so as a means to attract a higher-quality mate. In essence, these does are performing excursive behavior not out of necessity, but as a way to play an active role in mate selection, a change from what many hunters and researchers have long believed.
Yet, we must be careful not to make conclusions beyond what these data show us. While it might be tantalizing to believe that these excursive behaviors are targeted at specific bucks, like our hunts so often seem to be, that seems to be unlikely because bucks also are quite mobile during the rut. Instead, we believe that excursions are an attempt to incite competition by alerting more males of the female’s presence, allowing her to breed with the best of those attracted.
While our findings leave some questions, with the additional insight they provide into the behaviors of white-tailed deer researchers can look even deeper into behaviors we once never knew existed.
With this new information in hand, take a moment to think back on your hunts during ruts long past. Remember the way you felt as the tranquility of the forest was broken by a doe crashing through the branches while your trophy 10-pointer was in hot pursuit. Remember the surge of blood through your veins, and the beads of anxious sweat that built on your brow.
Remember, if you can, the swelling of his neck as the buck so eagerly pursued his quarry — that doe in estrus. Now, with this scene unfolding before you once more, ask yourself, were things truly as they seemed? Or maybe, just maybe, had the real hunter already won and was simply leading her prize back home. This fall when the rut is on and you return to your woodland paradise, remember these words as you anxiously await the deer of your dreams: You might not be the only one hunting for him.
— Jeff Sullivan completed his M.S. degree in 2016 at Auburn University researching white-tailed deer under the supervision of Dr. Steve Ditchkoff. He is currently employed by USGS at the Patuxent Wildlife Research Center.
— Dr. Steve Ditchkoff is a professor in the School of Forestry and Wildlife Sciences at Auburn University. He manages the deer research program at Auburn and has been conducting research on white-tailed deer for 25 years.
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