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The Usefulness of GPS Telemetry to Study Wolf Circadian and Social Activity

Methods


We captured wolves via helicopter net gunning (Barrett et al. 1982) from February 1997 - September 1998. We weighed, measured, and ear tagged wolves, fit them with a GPS radiocollar (Merrill et al. 1998), and released them. We aged wolves by tooth wear (Gipson et al. 2000) or by knowledge of each animal's history. We programmed each GPS collar to obtain a location at regular intervals from 15 minutes to 3 hours (Table 1). If no location was recorded, the collar tried again in 15 and 30 minutes. If all 3 tries failed, we made no further attempts until the next programmed interval.

Table 1.  Details about 10 wolves studied by Global Positioning System telemetry near Little Falls, Minnesota, from 20 February 1997-14 September 1998.
Wolf Dates of GPS collar activity Location attempt interval No. GPS locations No. location pairs spanning one location attempt interval
No. Gender Age
850 F 2+ yr 2/20/97 - 3/9/97 1 hr 327 319
recalculated: 4 hr 79 77
850 F 2+ yr 4/10/97 - 7/9/97 4hr 254 120
840 M Yearling 2/20/97 - 3/14/97 30 min 647 646
860 F 10 mo 2/20/97 - 3/10/97 1 hr 265 252
820 M 10 mo 2/21/97 - 3/13/97 15 min 1,477 1,358
133 M 2+ yr 1/3/98 - 7/9198 3 hr 594 385
229 F Yearling 1/31/98 - 6/18/98 3 hr 569 351
399 M Yearling 2/3/98 - 7/27/98 3 hr 1,120 925
627 M 2+ yr 2/3/98 - 6/17/98 3 hr 715 552
134 F 2+ yr 9/14/98 - 11/15/98 3 hr 385 217

After the GPS collars completed collecting all the data for which they had power, we triggered their release from the animals (Mech and Gese 1992) and downloaded the stored data into a computer (Merrill et al. 1998). In 2 of 13 cases, the release failed, so we retrieved the collars by live-trapping or helicopter capture of the wolves. We plotted data in Arc View© (ESRI, Inc., Redlands, Calif.) and calculated distances between points and Minimum-Convex-Polygon home-range estimates using the Animal Movement Extension (Hooge and Eichenlaub 2000). We measured circadian movement as mean distance traveled per GPS location interval. Distances and rates of travel are minimum estimates because they are straight line measures of routes of unknown lengths between points. Thus, for example, 2 equal distances could represent different actual distances and rates of travel.

Although summation of distances between GPS obtained locations would appear to accurately represent each wolf's movement pattern, this is not necessarily the case. Each time the GPS attempt and both retries were missed, the distances between locations before and after the missed intervals) represents the minimum distance traveled in at least 2 intervals. If not removed from the data set, distances spanning missed intervals could substantially alter the description of a movement pattern. For example, with one wolf, the location attempt and both retries failed in 107 of 812 intervals (13%) before an extraterritorial foray and in 88 of 308 intervals (29%) during the foray. The pattern described below for that wolf was not apparent before we deleted lines spanning missed intervals from the analysis. We evaluated all our wolf movements and rates of travel using only distances associated with programmed intervals (±0.25 or 0.50 hour, including successful retry attempts).


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