Updated: Jun 23, 2021
Usually, when we talk about dispersal among whitetail deer, we talk about male dispersal, which occurs in the fall. The infatuation with male dispersion is for obvious reasons, namely our fascination with antlers – in deer hunting, bucks get all the glory. However, another type of dispersal happens among whitetails, and it doesn’t involve the boys, and it doesn’t happen in the fall. This one is all about the girls, and it’s happening right now.
The first known study of female dispersal among whitetail deer was done in Illinois in 1970. It found that approximately 13% of does relocated during the months of May, June, and July. Since that study, more extensive research has been done on this phenomenon, primarily thanks to the affordability and reliability of radio collars.
In Pennsylvania, researchers at Penn State University began tracking female deer in 2005. “Between 2005 and 2011, we captured 277 8-month old females and fitted them with radio collars,” writes Duane Diefenbach in his post “Female Dispersal? Really?” on The Deer-Forest Blog (ecosystems.psu.edu/research). “The graduate student heading up this project, Clay Lutz, discovered some interesting things about female dispersal.
“Clay was able to monitor 229 of the 277 juvenile females during the dispersal period and found that 12% (27 females) dispersed – compared to about 75% of males that dispersal rate is low. But he found quite variable dispersal rates among WMUs. Dispersal rates ranged from 9% (WMU 2G) to 32% (WMU 2D). That 32% is almost half the rate of males!”
Given these statistics, one could imply that dispersal is generally higher in agricultural areas, which also have high population densities. During Lutz’s study, population densities were estimated at 7.6 deer/square kilometer in WMU 2G, 10.8 in WMU 4B, 17.3 in WMU 3C, and 19.4 in WMU 2D (Journal of Mammology).
Especially interesting is the way that females disperse compared to males. Males have a tendency to do it and get it done. They make up their minds to relocate and walk in pretty much a straight line until they find a new residence, usually one to five miles away. Not so with females.
In his blog post, Diefenbach writes that “The average dispersal distance was 11.2 miles, with the longest distance being 32.8 miles. But this is where it gets interesting. Dispersal was not straight. Even though the average straight-line (“as the crow flies”) dispersal distance was 11 miles, the average path it took to go those 11 miles was 32 miles.”
Not only that, but the actual time it takes for females to disperse is much longer than the males. Whereas males typically relocate in less than a couple of days, females typically take about 14.5 days to find a new home. In Lutz’s study, the longest a doe took to disperse was 55 days!
One would think that the longer it takes a deer to disperse, and the farther it travels, the more risky that dispersal will be. That’s certainly the case when females disperse. In Lutz’s study, 7% of dispersing females perished during their journey. That’s really high compared to males, which have a 1% mortality despite the fact that they disperse at much higher rates.
Also interesting is the age bracket of females that relocate. Whereas dispersal among males is typically done as yearlings, females tend to be slightly older. Most females don’t disperse until they are ready to have their fawns. This could help explain why they risk their lives to find new territories.
The goal of most relocating females is to find solitude while giving birth and raising their fawns. A 1982 study by John Ozoga et al. found that when a female population in an area is too high it effects maternal behavior as well as makes them more susceptible to predators (Journal of Wildlife Management). In other words, instinct tells these females to isolate themselves from the rest of the herd in order to achieve maximum fawn survival. An added benefit to isolation is a stronger mother/fawn bonding experience, which is also proven to increase offspring fitness.
From a management perspective, female deer dispersal is troubling. Chronic Wasting Disease (CWD) is a hot topic among biologists and hunters alike, and dispersing deer can carry the disease into new areas, females even more so because they travel much farther while relocating. This makes the disease much harder to contain, which is also why we see such large CWD containment areas in certain Pennsylvania WMUs. Although CWD may have only affected one small area, the containment area must allow for dispersal of deer that reside where the disease has been found. Also, because dispersal is directly related to population densities, herd reduction in those areas is often a necessary precaution.
It’s also interesting to note that roads and rivers can serve as barriers to dispersal – at least temporarily. One deer in Lutz’s study retreated to its prenatal range after reaching a heavily-traveled highway. Three weeks later, though, she left home again and this time found a way to cross the highway unscathed. This female eventually crossed a wide river before encountering another highway which forced her to turn back yet again. This time she stopped at the river and followed its course for a long distance before reaching her final destination. The journey took 12 days.
According to most research, female dispersal occurs between the first of May and early July. Occasionally, one will disperse earlier or later in the year, but the bulk relocate during those 10 weeks. So if you see a doe on the move this time of year, you might consider waving goodbye. Chances are at least some of them won’t be coming back.
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