White-tailed deer dispersal has been discussed and studied ad nauseam.
Or at least it seems that way. Type “white-tailed deer dispersal” into Google and in 0.42 seconds you are presented with 154,000 results.
Dispersal has had its fair share of coverage on this blog as well. Most, if not all, of the attention is usually given to yearling males. With ¾ of them leaving home, it’s no wonder. But what about the other half of the population?
We talked about the forgotten half in a previous post. You might not have even noticed. This is usually the case for these flowers of the deer population. Often overlooked because they don’t grow bones from the top of their head or make a mass exodus from where they grew up.
But someone noticed. That someone was a graduate student, Clay Lutz. Clay is a sensitive kind of guy and decided to give the ladies the proper attention they deserve. After all, where would any of us be without our mothers?
Clay found in Pennsylvania that about 12% of females disperse, they can go long distances, and they can take a long time doing it (as long as 55 days!). So the ones that leave take their time and aren’t afraid to keep looking for the right spot.
Why would females disperse? It’s risky business. I mean you could die! So the Ultimate cause must be pretty persuasive. For males, current theory indicates they disperse to minimize inbreeding and competition for mates.
If this were true for females, we would surely see more than 12% of them disperse.
Clay got to work trying to find out what made these girls tick. First order of business was to check out other published studies. Was there a pattern? Did Pennsylvania does behave similarly to deer elsewhere? Are there any relationships to deer density, landscape characteristics, road networks, etc.?
Looking to the past, Clay found that dispersal rates for females varied from a low of 4% to a high of 49%. But for the majority of populations, fewer than 20% of females dispersed.
Ok, so we know that Pennsylvania is not an anomaly. Less than 20% of yearling females disperse. But that doesn’t answer the question of why? Why would 12% of yearling females risk death to disperse from their natal range?
Clay did what all good researchers do and formulated a hypothesis which was female dispersal is the result of competition for space before and during fawning season.
Females seek to isolate themselves during this time. Ozoga et al. (1982) found that crowding of females disrupted maternal behavior and increased mortality of fawns. Isolation during the birthing period strengthens mother-offspring pair bonds and is a defensive strategy against predation.
So mom excludes her yearling offspring as a way to increase her fitness (passing on her genes to more surviving offspring). Male yearlings take this exclusion as an opportunity to increase their fitness by moving to an area with less related females (decrease inbreeding). The majority of female yearlings stay put. What benefit does the minority gain by dispersing?
Let’s look at another hypothesis. Research in New York found that when a group of female deer where eliminated from a local area, this area had reduced deer densities for 3-5 years. It was anointed the Rose Petal Hypothesis. Picture a dominant female in the center surrounded by her female offspring whose home ranges overlap to some degree with their mother, like the petals of a rose.
If you “pick” one or more of these roses off the landscape, you can create an area of low deer density because of the very low female dispersal rate. No new females move into the area to “grow” another rose.
This is great news for managers. Why? Take urban deer management for example. According to the Rose Petal Hypothesis, removal of several female groups would create a void of deer in a place where you don’t want them. Or forest management – before a timber harvest is conducted, removal of the “roses” would allow enough time for regeneration before the “roses” return.
Wonderful! Let’s get to work. The only problem is that the Rose Petal Hypothesis has a footnote. That footnote was added in 2010 almost 20 years later. Other research couldn’t replicate the experiment. Despite all attempts, maintaining a localized area with lower deer densities was impossible. Why?
Clay’s research helps to explain.
When deer densities are low (<10 deer per sq. mile of forest) like in the Adirondacks where the Rose Petal Hypothesis was developed, there are fewer deer and limited female dispersal (<10%). However, with higher deer densities (which is the norm everywhere south of the Adirondacks and in any urban setting), there is simply too much female dispersal to ever create a localized area of low deer density. The roses just grow too darn fast.
As deer densities increase, all those roses create a thorny situation for yearling females. Their dispersal rate increases as a way to increase their fitness. By establishing an adult home range in an area with fewer females, they have higher social standing and less competition for fawning areas.
And a chance to start their own flower garden.
-Duane Diefenbach and Jeannine Fleegle
Previous post on this topic: Female Dispersal? Really.
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