Making Sense of the Madness: Describing Buck Movements During the Rut.
By: Aaron Foley, Randy DeYoung, David Hewitt, Mich Hellickson, Ken Gee, Mitch Lockwood, Karl V. Miller and David Wester
Hunters spend countless hours trying to pattern buck movements during the rut, attempting to decipher the “code” that unlocks the secret to buck locations. Some hunters use trail-camera photos to decide where to hang a treestand in hopes that the buck will return. Other hunters believe that buck movements are simply random and chaotic; thus they may set a blind with a good view, hoping to see some antlers moving through the brush. Both approaches sometimes meet success.
Given the seemingly erratic and unpredictable movements of whitetail bucks, it is quite remarkable they are able to locate and mate with nearly all does in a population in only two to three weeks, especially considering that does almost always outnumber bucks, and does are in estrus for only 24 to 36 hours! How are bucks able to do this? Do they have search strategies?
We, as humans, employ search strategies all the time. Suppose your vehicle is running low on fuel. It would be more advantageous to drive directly to the nearest town where a gas station is likely to be found than to cruise around the countryside randomly in hopes of finding a pump before the engine dies. Obviously a strategy based on prior knowledge of doe locations, rather than searching randomly, would be an advantage for bucks.
In 2008, scie
ntists at th
e Caesar Kleberg Wildlife Research Institute partnered with King Ranch, Texas Parks and Wildlife Department, Samuel Noble Roberts Foundation, QDMA, and the Warnell School of Forestry at University of Georgia to initiate a five-year project with the goal of describing rut search patterns. On a 10,000 acre tract on the King Ranch in South Texas, we captured bucks and attached GPS radio-collars programmed to record locations every hour from November to February.
The deer density was 25 deer per square mile with a balanced sex ratio (one buck per two to three does), and deer harvest was minimal. During the 5-year study, we captured and collared 101 bucks, including eight 1.5 year olds, 33 2.5 year olds, and 60 3.5 or older. Most bucks collared were 3.5+ years old because harvest on the ranch is limited to bucks 4.5 years and older.
We categorized the males into three age groups (1.5, 2.5 and 3.5+) because mature males have been through several ruts and may have different search strategies than young males that may be inexperienced as well as subordinate. Returning to the analogy of a driver running low on gas, a teenager driver may search for a while before finding a gas station whereas an experienced driver is likely to know exactly where a gas station is. Furthermore, a teenager with little cash is likely to pump $5 or $10 of
gas at a time, while an adult with a good job can afford to fill up. The experience and greater resources probably explain the fact that mature bucks have been found to sire 70 percent of fawns in populations with good age structure.
We separated the breeding season into four two-week long phases based on conception dates of fetuses obtained from does harvested at the King Ranch: early (November 18 to December 1), peak (December 2 to 15), late (December 16 to 29), and post (December 30 to January 12).
Each rut phase reflects the relative abundance of receptive females, which may affect movement patterns exhibited by the 3 age classes. For instance, during early rut, when few does are ready to be bred, some experienced males may not bother searching whereas some younger males may excitedly search for females.
We assessed 4 movement parameters – movement rate, search intensity, a search strategy termed the “Levy walk”, and focal areas. Let’s spend some time defining the four parameters.
Movement rate is simply the distance moved per hour. We expected males to increase movement rate during peak rut when most females are receptive. Mature bucks have greater fat reserves (more gas money) than young bucks and may have the ability to spend more time searching for does versus foraging. Therefore, we anticipated higher movement rates from mature bucks relative to younger bucks.
Search intensity is the proportion of area used within a buck’s home range. Increased search intensity indicates a buck used more area within its home range. A search intensity of 80% indicates a buck was nearly everywhere within its home range - as if the young driver in search of fuel used many roads to find the gas station - whereas a search intensity of 30% indicates the buck was within the same area repeatedly, just as an experienced driver knows which road leads to the gas station. We expected lower search intensities from mature bucks relative to younger bucks because mature bucks may be more experienced and familiar with their surroundings whereas young bucks may search new areas within their home range.
The Levy walk, which basically means random searching, is optimal for finding dispersed resources. This search strategy has been observed in many different animals, from bees to marine mammals. Without getting into the complicated statistics, the Levy walk in our case represents bucks that exhibited movement patterns indicative of searching for unpredictable resources. In other words, these bucks may have had difficulty locating does or did not know in advance where does are likely to be. Conversely, a buck who knows where does are likely to be would exhibit a “Brownian walk” movement pattern. We anticipated Levy walks to be common during early, late, and post rut when relatively few females were receptive. We also predicted that mature bucks, due to their experience, would be more likely to be Brownian walkers - knowing where to look for does - relative to young inexperienced males.
Focal areas are areas within buck home ranges that had relatively high use and were revisited frequently. Bucks that return to a location repeatedly during the rut are attracted to these areas for some reason. We evaluated the change in number of focal areas, number of visits, size of focal areas, whether the focal area was revisited regularly, and location of focal areas relative to focal areas of other bucks. The map on the right shows a buck repeatedly visiting two focal areas.
Looking For Love in All the Right Places
After analyzing the data, we detected high variation in movement patterns among bucks, which likely indicates individualistic behavior. This is no surprise to experienced hunters. Some bucks were seemingly nomadic and roamed across the landscape, whereas other bucks seemed to stay within very small home ranges throughout the rut. However, we were able to detect several trends in patterns.
During peak rut, yearling bucks (1.5 years old) moved 24-28% less than the other age groups, 2.5 and 3.5+. This suggests that yearlings may be conserving their doe-search effort until later years, when they are better able to compete with other bucks. Or, yearlings may be doing the best they can by hanging out near does until a mating opportunity arises, as evidenced by long periods of time yearlings spent within focal areas relative to older bucks. While yearlings averaged 11 hours per visit, older bucks spent 5 to 6 hours per visit.
The 2.5 year old males had very similar movement patterns to mature bucks; however, 2.5 year olds were more likely to be Levy walkers (random searchers). In contrast, nearly all mature bucks were Brownian walkers, as if they knew where to look for does. Because 2 year old males were Levy walkers, we interpret this as 2 year olds having difficulty locating does due to inexperience - they did not know where to find a doe - or because of competition with mature bucks. Recall the study population had a high proportion of mature bucks; therefore, 2.5 year old bucks likely faced stiff competition.
Search intensity was surprisingly low for all bucks at 20-40%, which indicates bucks generally used the same areas repeatedly rather than exploring widely within their home range. We analyzed the focal areas and found that during peak rut, most bucks had 2-3 focal areas within their home ranges, and these were visited often but briefly. This suggests bucks were homing in on specific locations within their home ranges, staying a few hours, and moving on to the next focal area, and later repeating the entire pattern. Interestingly, about 60% of these bucks returned to their focal areas periodically, usually every 24-28 hours. Because does are in heat for 24-36 hours, we concluded that these focal areas were associated with doe core areas. Bucks probably revisit these focal areas to check does using behavioral and scent cues.
Interestingly, we detected several instances of gaps in repetitive visitations to focal areas, which may reflect occurrences when a buck was able to locate a receptive doe and isolate her from other bucks. Further, we know that during peak rut, bucks are intolerant and aggressive towards other bucks. We found that focal areas of many different bucks overlapped, which strongly suggested that does are within these areas. It appears that bucks are daily revisiting 2-3 focal areas in their home ranges, spending about 5-6 hours in each, to assess the receptiveness of does.
We assume bucks are revisiting these focal areas during the rut to check does. That assumption relies on whether does reduce movement rates to increase likelihood of being located by males. In other words, for this to work, does must make their locations predictable to bucks. Some previous studies have found does perform excursions - brief forays outside of their home ranges - or increase movement rates during peak rut, whereas other studies have shown that does reduce movement patterns.
We analyzed movement patterns of 5 does that were captured on the study site to assess whether their locations would be predictable to bucks revisiting discrete areas. Because physically mature does are more likely to be bred than young does and may exhibit different movement patterns as a response to rut search activities, we separated the females into 2 age groups (1.5-2.5 and 3.5+). While our sample size is small, we did document most females decreased movement rates by 2-36% from early to peak rut. Also, all mature females had a single core area and the size their core areas (37-59 acres) were slightly smaller than the size of male focal areas (44-82 acres). This observation suggests that mature doe locations would be predictable during the rut, which would enable bucks to locate does periodically and check their receptiveness.
Finally, we detected several bucks had focal areas near or overlapping the location of doe core areas. The overall movement patterns of does appear to fit with our hypothesis that males revisit discrete areas repeatedly to assess receptiveness of the predictably located females.
Furthermore, the intervals between visitations are a very good strategy for allocation of a buck’s time and resources. If a buck assessed the doe too frequently, he could lose out on the opportunity to locate additional does. Too long between assessments, and the buck would miss the mating opportunity.
What does all this mean?
For us scientists, it was very exciting to quantify rut search strategies that could never be fully studied through direct observation. Whitetails just spend too much time hidden in thick cover. The data also remarkably document the apparent use of “spatial memory” in white-tailed deer. Bucks remember where does are located and spend most of their time in one or more focal areas, or traveling in between.
However, additional research is needed to better understand the tactics bucks use to locate receptive does. Do scrapes assist with conveying information between the two sexes to make doe locations predictable? How do differences in age structure, sex ratios, and hunting pressure affect search behavior exhibited by bucks during the breeding season? For instance, high deer density may result in less obvious periodic movements exhibited by bucks, since they are in a “target-rich environment”. Or, high hunter presence may cause female locations to be less predictable.
However, one old adage that comes to mind and seems most fitting:
Hunt the does during the peak rut.
In addition, if you see a buck in the same area more than once during peak rut, he will probably be back…hopefully during legal hours!
For detailed information about the research, see:
Foley, A. M., R. W. DeYoung, D. G. Hewitt, M. W. Hellickson, K. L. Gee, D. B. Wester, M. A. Lockwood, and K. V. Miller. 2015. Purposeful wanderings: mate-search strategies of male white-tailed deer. Journal of Mammalogy 96:279–286. DOI: 10.1093/jmammalogy/gyv004