Measurement and Data Collection in the Field
A Thirty-Five Year Study of Black Bears
In 1969, the National Park Service asked the question, How many bears live in Great Smoky Mountains National Park (GSMNP)? Researchers at the University of Tennessee have been on a quest to answer that question ever since. Black bears, also known by their scientific name Ursus americanus, were once abundant throughout North America. In the Southeast, habitat loss and overexploitation during the late 1800s and early 1900s severely reduced their range to only 20% of the historic range. During the late 1960s bear managers became very concerned about declining bear populations in the southern Appalachians, so researchers started to study bears to better understand how many bears there are, how they live, what they eat, where they hibernate, how far they range, how old they get, and a host of other questions.
The initial scientific studies stretched into a unique, long-term study, which has been ongoing, as of this writing, for almost 35 years. We have gained tremendous insights into the behavior of individual bears as well as the population as a whole. Fortunately, bear populations in the southern Appalachians and elsewhere have recovered tremendously since the 1960s.
In order to study the bears and obtain a population estimate, we perform whats known as mark/recapture experiments (see Snake Mark/Recapture Study-2015 for more information on mark/recapture studies). A mark/recapture experiment involves capturing an animal and giving it a unique mark, such as an ear tag with a number. Over the course of the trapping sessions, recaptures of previously marked bears are noted accordingly. The ratio of the number of new captures to the number of recaptures can be used to estimate how many animals are in the study population. The actual analysis of the data and the mathematical computations can be very complex, but the data collection method has basically remained the same.
For the summer of 2002, my crew and I collected data from the same trap lines that have been set in previous years. We worked a total of eight trap lines over the course of the summer, with lines paired up so that two lines are in operation during any given trapping period. Trap periods consist of 15 days of fieldwork. The first day involves hiking out to our designated trap locations and building the traps. Each trap line has approximately seven traps. In crews of two, we check the traps daily for bears, regardless of weather. This summer, our crew consisted of four field technicians: Roy Smith, Bethany Bostrom, Jason Lupardus, and myself. Trapping bears is not easywe typically hike 7 miles a day in rugged terrain while carrying about 30 pounds of gear on our backs. After the 15-day trap session, we get a four-day break, and then we start over again on two new trap lines until all eight lines are completed.
If all goes well and we get lucky, a bear will be waiting for us at one of our traps. The first thing we do is assess the situation and get an initial impression about the state of the bear and the surroundings. Then each member of the crew will estimate how much the bear weighs, and we compare our estimates and come up with a consensus. We need to approximate the weight because the next thing we do is to immobilize the bear, and the tranquilizer we use is dosed according to weight. The tranquilizer is administered using a long metal pole with a syringe mounted on the tip. One person gently distracts the bear by talking to it and waving her arms, while the other person gives the drug in the hip muscle with the syringe. Within a matter of minutes, the bear is asleep and we get to work caring for the bear and collecting important data.
Taking Critical Measurements
Many different measurements are taken for each bear we catch. Using a flexible measuring tape, we take measurements such as total body length (from nose to tip of tail), shoulder height, head width, length between ears, length of head, length of ears, forearm circumference, neck circumference, chest circumference, and more. These measurements, called morphometric measurements, give us information on the size of bears and how they grow, and we can note differences in measurements between males and females. From these measurements, we have developed a mathematical formula that, given an individual’s chest, neck, shoulder, and total length measurements, can actually predict how much a bear will weigh.
Estimate a Black Bear’s Weight
Researchers have developed a formula (Eason and van Manen, unpublished data) to estimate a black bear’s weight (in kilograms) based on several body measurements: the animal’s total length (TL), its height at the shoulder (HAS), its neck circumference (NC), and its chest circumference (CC). Use the weight predictor spreadsheet and the table of morphometric measures below to try it out (please note that the table includes data from black bears we worked with in 2002). The spreadsheet will derive the weight estimation for you; all you need to do is enter body measurements from the Morphometric Data table.
[The above article is authored by Katie Settlage and first appeared on FieldTripEarth in November 2002.]