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Male Dispersal: Maybe they do ask for directions?

Posted: June 5, 2015

The mystery of male dispersal has vanished with the advent of the radio- and GPS- collar. Now we can determine the who, when, and how far for those boys that hit the road.

In 2002, we embarked on an ambitious project to capture and radio-collar 600 male deer over 3 years. It was unknown whether we could reach this lofty goal, but as Thomas Miller said “confidence is a battle half won before you commence it.”

After 3 years, we captured 543 bucks, not 600. But confidence is also the companion of success (Unknown Author), and we were indeed successful!

Most of those bucks (454, to be exact) were less than 1 year old when captured in winter, so we expected many of them to disperse in the summer and fall.

Dispersal is a one-time movement from a natal (where born) home range to a different adult home range. For our research (and most studies), we say an animal disperses if there is no overlap between natal and adult home ranges. Here is an example of a deer that dispersed. This deer was born on the south slope of Blue Mountain near Carlisle, PA and dispersed to the northwest near the Juniata River.

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Migration, on the other hand, is a movement that is repeated over time. Deer in more northern latitudes will migrate to wintering yards and then return to their summer range. There is no overlap between their summer and winter range. In the West, deer will migrate to lower elevations in winter and return to higher elevations in summer.

So what does male dispersal look like in Pennsylvania?

It turns out that about 75% of male deer disperse as 1-year-olds. In 2002, half the dispersal occurred in spring (May-June) and the rest occurred in early autumn (September-October).

The average dispersal distance is about 5 miles. But there was one ambitious yearling that dispersed 25 miles.

What did we learn from these 340 road trips? That dispersal in males is a fast and furious process. Half the males disperse in <12 hours and almost all of them take <24 hours to reach their new home.

And they seemed to know where they were going. We had GPS collars on 9 males that dispersed and obtained locations every 2.5 hours. On a scale of 0 (random movement) to 10 (straight line), these males score an 8.1.

In fact given the speed at which they disperse, we were lucky to get 3 locations on one of these males when they were traveling between their natal and new adult home ranges. When males disperse they do it quickly and in a straight line.

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Ok, so they go fast and in a straight line. How do they know where they are going? Are there sign posts? Maps? GoogleEarth? Deer only use the Earth part of GoogleEarth. Landscape features affect what direction and how far deer disperse. In the Ridge and Valley region of central Pennsylvania, deer are more likely to disperse parallel to the ridges.

Roads and rivers often define our travel directions and they in some respect do the same for deer. These hard lines will shortstop deer dispersal. A deer is more likely to disperse away from a road and more likely to stop his dispersal movement before crossing a road. And it’s not just interstate highways but any state route (two-lane road) is big enough to trigger this stop. The image below is for Armstrong County. Four different deer illustrate this perfectly dispersing away from roads and stopping on the near side of the next road.

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Cool stuff! But does this have any significance to those outside the world of wildlife biology? You bet it does.

For the hunter that sees a button buck on his property in early May, there is a 75% chance he’ll be gone by the archery season. In contrast, if you see him with his first set of antlers in the archery season, there is a 75% chance he came from somewhere else. You can’t stop their movement which means you can’t stockpile button bucks on your property.

Think “genetics” of “big bucks” are limited to where those big bucks are seen. Think again. Seventy-five percent of their boys hit the road every year and they know where they are going.

-Duane Diefenbach

Previous post on this topic: Oh The Places to Go!

Next post on this topic: So Why Did the Deer Cross the Road (or not)?

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