by Katie Settlage
The den work this year was a bit frustrating, but it also brought some new and exciting experiences, as can be expected when it comes to field work. It was frustrating because I didn’t have much success in locating dens. I monitored a total of 21 collared bears this winter, and I only ended up visiting 4 dens! Here’s what happened to the rest of those collars:
- 7 of those collars were not transmitting anymore. Their batteries were dead, or possibly the female had traveled outside the range of our ability to hear the collar.
- 4 were dropped collars. A collar may be dropped when the spacer wears off, which we expect to happen after 2-3 years. Sometimes spacers wear out too early, though, and that is what happened to 2 of the collars this year. Sometimes the spacer and the collar are chewed up by the cubs or yearlings while they wrestle with their mother. That happened to the other 2 dropped collars.
- Then I had 5 bears that were on the move when I tried to locate them! This is pretty surprising considering that I was doing most of my monitoring of the bears in February and early March—during prime hibernation time. However, there may be an explanation. Just like last winter, this winter was fairly mild, with several days of 60° F weather in February and early March. Even more notable is the fact that the southern Appalachians experienced a moderate hard mast failure in the fall of 2003.
A hard mast failure is when the major nut producing trees, such as white oak (Quercus alba), fail to produce much of a crop. Bears rely heavily on the nut crop in the fall to put on a fat layer in preparation for denning. The fat layer is important because it is needed not only for her survival, but it is also critical if she is to produce cubs and nurse them through the winter.
Female black bears have a fascinating reproductive strategy to deal with the uncertainty of the fall mast crop. They practice what is known as delayed implantation. Delayed implantation refers to the fact that even though the female breeds in the summer, her fertilized eggs do not implant into her uterus until late fall. The fertilized egg floats around the uterus and, if the female gets enough food in the fall to support a pregnancy, the egg will implant and develop normally.
|What other animals utilize delayed implantation? Why do you think some of these animals developed that adaptation?|
But if she does not find enough food, then the egg is simply reabsorbed—otherwise the cubs that would be born would die at a very early age. This strategy allows her a better chance to survive the winter and reproduce again next year. The fall of 2003 was a poor year for mast production, so we suspect that many of the female bears we monitored did not produce cubs. If a female does not have cubs, then she does not necessarily need to stay in the den continuously through the winter.
Hibernation serves two main functions for black bears: it reduces the amount of energy the bear expends during the time of year when food is scarce, and it provides females a secure place in which to give birth. So, if the weather is warm and a female does not have offspring, then she may get up and stretch her legs on a winter walk. We feel that it is safe to assume that the 5 bears that were moving this February did not have cubs.
That leaves me with the handful of bears that we actually did locate in the den. Of the 6 left, 1 was sleeping in a brushy daybed instead of the more typical tree den or rock crevice. Locating her was one of the more interesting days this winter. Her signal was slow, indicating that I might be looking for another dropped collar. When I got close to the signal, I was looking straight into the thickest, brushiest patch of laurel and greenbriar I had ever seen. I thought it was very inconsiderate of her to drop her collar for me to retrieve in there! I was crawling on my hands and knees, making very slow progress, but I knew by the sound on the receiver that I was very close to the collar. Then to my surprise, the collar pulse quickened, and as soon as it did it started to quickly fade in strength. As it turns out, the collar was attached to a bear, and she had been snoozing by herself in the brush until I scared her off. I was surprised at how fast she got out of that thicket, because I never even saw her, and it took me a good 10 minutes to extract myself from the brush.
The other four dens were more typical. All were in large hollow trees. One of the trees could not be climbed, so we were not able to tell if she had cubs or yearlings. Of the 4 that could be climbed, 2 had cubs. Each of those 2 females had at least one cub, but it is possible they each had 2. Often it is difficult to tell between one and two cubs because the mother usually lies with the cubs hidden under her chest, where they are warm and can nurse easily.
Even though I did not find many dens this year (we typically visit between 10-15 dens each winter), it was still a good den season with many new and exciting experiences. For instance, I got to perform some aerial telemetry this year. It was quite a rush to be up in a small airplane, circling over the Smokies and listening to the bears sleeping down below. I really enjoyed seeing the topography from a bird’s-eye view. I also had the pleasure of taking 3 newspaper reporters out with me to look for dens. The reporters were very interested in our work, and it was fun to take them out and share this experience with them and with the people that will read the stories.
About the author:
Katie Settlage is a Graduate Research Assistant at the University of Tennessee at Knoxville.
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