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Testing Releasable GPS Radiocollars on Wolves and White-tailed Deer


Our tests demonstrated the potential of the 920-gram, releasable GPS collar for research on medium-sized animals such as deer and wolves. The number of fixes we obtained in just 2 weeks from the two Denali wolves would have taken 20 years to collect at the usual rate of about 1 aerial radiotracking location per week. Similarly, conventional ground radiotracking for the deer would have required 18 hr of tracking per day for 38 successive days. The informational value of location data collected over 2 greatly different periods varies, and each method produces different types of information. However, the GPS collars gathered location data much more quickly and in much greater quantity than previous techniques allowed. Furthermore, the ability of the GPS collar to record location data both day and night could add a new dimension to movement studies. Because collection of short-interval data reduces collar longevity, some questions (e.g., about home ranges and seasonal movements) may be best addressed with long-interval data. Short-interval data may be most appropriate for questions about movements during denning, dispersal, or other temporally restricted periods.

We exposed several flaws in the prototype collars, as was the intent of the tests, and the manufacturer corrected the flaws (as indicated by subsequent use of new collars). Collars that made fewer successful attempts than others were probably more often under thick canopy (Moen et al. 1996). Thus additional tests are needed in thick cover during summer. Another factor influencing number of locations is animal activity. When an animal is sleeping or resting, for example, the GPS antenna may point away from the sky. Such a position causes GPS batteries to expend more power per attempt, which shortens GPS life.

The manufacturer indicates that, as GPS batteries weaken, the likelihood of a successful location attempt is constant until the batteries reach a low level. At that point, enough power exists to determine locations, but many failed attempts may occur in sequence. This explains why some collars had longer than expected life (Table 1); towards the end of data collection in these collars, ≤1.3 days passed without successful locations, followed by a few additional successful locations.

We found no evidence that the GPS collars we tested were too heavy for the wolves or deer, and other researchers have used collars weighing 1.08-1.22 kg on wolves (Ballard et al. 1995). Nevertheless, we caution that the heavier the collar, the greater the chance that the collar may affect the animal's activity or movement patterns. We recommend that manufacturers reduce GPS collar weights for wolves to <600 gm because standard VHF collars of that weight have been used for decades with no apparent effects on the wolves or the data.

The primary limitation of GPS data collection at brief intervals is the short period of collar operation. This is likely to improve with advances in battery technology and new software that may conserve battery life. Furthermore, by programming the GPS collar to collect only 1 location per day, one can increase its theoretical potential life to about a year.

Another problem involves possible failure of the collar-release mechanism. Because an entire collection of data may depend on the reliability of this release device, manufacturers should strive to include a redundant release system. Ideally, a GPS collar would collect, store, and transmit data in real time.

For more efficient data collection, manufacturers should thoroughly waterproof collars, test software in the field, and use VHF circuitry that maximizes signals. Ideally, GPS collars would allow remote downloading of data (Rempel et al. 1995) and would incorporate a recapture device allowing researchers to change batteries or collars (Mech et al. 1984, 1990, Mech and Gese 1992). However, the latter 2 additions would require more weight, so they must await developments that reduce weight of the entire package.

Until further refinements and technological advances are made, the 920-gm, releasable GPS collar we tested will prove valuable to many researchers studying the movements and activity of medium-size mammals.

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