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Wednesday, July 27, 2011

Black Bear Trail Research PART III: Stride and Straddle Results

This is the third post regarding the black bear research my class conducted this summer in Massachusetts. Please review the first post here and the second post here. Today's post will concentrate on the measurements we took of the ritualized portion of the trail.
Measuring the straddle or width
of a black bear trail
(Massachusetts. 7/11)
Before I present results, let's make sure we all agree on terminology. "Straddle" is the same as the trail width. "Stride" is a little more confusing. Some authors measure stride from one track to the next track left by that same foot. For example, you could measure from a right rear track to the next right rear track. Other authors measure stride in a walk from one track to the next track. In the case of a direct register walk like our black bear, this would mean you are measuring on the diagonal, from a right track to a left track. Finally, some authors measure from the front of one track to the front of another track and some measure from the rear to the rear. In theory, this shouldn't matter. The key would be consistency. Check the book you are using for that author's particular methods.

As the photo to the left shows, we used Popsicle sticks and toothpicks to outline the deep, habitually used footfalls (I went into a little detail on an earlier post as to how we decided on the placement of the track within the deep impression). Then a wooden ruler could be placed on the edge of two tracks in order to properly measure the straddle or width of the trail. For the stride, we measured from the rear of one footfall to the rear of the next. In other words, we measured the distance from a right foot to a left foot, not a right to a right. Some authors would call this a step rather than a stride. We only measured a small portion of the habitualized trail. All measurements are in inches:
Stride (or step): 27, 26.75, 23.25, 26.5, 18
Straddle (or width): 15.75, 14.25, 17, 15.5, 17.38, 15.5

For a direct register walk, Rezendes (1999) gives a range of 17-23 for stride and 9.5-14.5 for a straddle. Elbroch (2003) posts numbers only slightly different at 17-25 and 8-14 respectively. Three of our five stride measurements exceed even Elbroch's larger figures. In addition, five of six straddles are above what would be expected. These results alone are interesting as they suggest the bear is not walking in a "normal" walk when creating these trails (or at least this one segment of this particular trail). However, there is a greater significance to these numbers that Nick pointed out to us while we were in the field.

Think: As stride increases, straddle should decrease. In other words, as I stretch out my steps, I would also be narrowing the width of my trail. Think of how an animal's trail looks in a walk compared to a run. A running animal can leave a trail that is only as wide as its tracks. Elbroch gives strides and straddles for a trotting bear as 27-37 inches long and only 6-10 inches wide. See what I mean? A bear's foot is about 5 inches wide, so a trotting bear is almost stepping in a straight line. But the results we obtained showed the opposite. The stride was elongated AND the width was increased. Visualize what the bears must do in order to use these trails. They are stretching their limbs farther than in a normal walk as well as placing their feet wider than they normally would. As I attempt this across my living room floor, I feel very uncomfortable and am reminded of a reptile rather than a mammal.
FLCC Black Bear Management Class 2011
(Paul's Ash Tree, 7/11)
What is the significance of this? Unknown. But if the other trails Nick and Val work with are similar, it sure seems as if the bears are modifying their normal gait when creating these trails. Why? Does this increase the chances that the bear will be able to straddle a tree and deposit scent? Is it a visual display? Does it increase muscle strength? So many questions, so few answers. I think that is the essence of research...


  1. Can one determine the size, gender, or purpose based on the marks on the tree? Such as the depth of the marks for the size or gender, or length of stride up the tree for the purpose of the climb, say a longer stride if the bear was in a hurry to get up the tree if it were spooked or shorter if climbing for food or shelter.

  2. Excellent questions, Marshall. You can certainly tell cub scratches from adult. As for climbing scratches, the theory is that when a bear is climbing a tree, it leaves relatively short scratches that are paired "widely" on the trunk... in other words, the paws are even (paried) but are far apart on the trunk so the bear can climb up. As they descend, the scratches are longer, lilke they are putting on the brakes. I am not sure that works in reality all the time, but it makes a nice theory :)

  3. One of your students called me to "talk bears" and alerted me to your blog.

    I used to be a locomotion biomechanics researcher, studying a variety of quadrupeds as well as humans. It is known in humans that altering either straddle ("step width") or stride length during walking or running increases the metabolic energy cost of walking. So, the bears are likely expending extra energy to do their "cowboy walk".

    I camera trap, with an obsession with getting photos of bears marking their territories. I've always assumed that the "cowboy walk" had something to do with the fact that the bear was urinating as he walked. I don't know where a bear's scent glands are, and whether a cowboy walk might let the urine run over some scent glands before it hits the ground...

    I have my cameras out waiting for the males to climb up in elevation to their denning territory. They always mark all the whammy trees on their fall journey.

  4. Thanks KB! Alyssa did a great job with her presentation. We appreciate your help. Interesting idea about the urine... Looking forward to your future bear posts.... JVN


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