Accuracy of Estimating Wolf Summer Territories by Daytime Locations
Demma, Dominic J., Mech, L. David, The American Midland Naturalist
We used locations of 6 wolves (Canis lupus) in Minnesota from Global Positioning System (GPS) collars to compare day-versus-night locations to estimate territory size and location during summer. We employed both minimum convex polygon (MCP) and fixed kernel (FK) methods. We used two methods to partition GPS locations for day-versusnight home-range comparisons: (1) daytime = 0800-2000 h; nighttime = 2000-0800 h; and (2) sunup versus sundown. Regardless of location-partitioning method, mean area of daytime MCPs did not differ significantly from nighttime MCPs. Similarly, mean area of daytime FKs (95% probability contour) were not significantly different from nightime FKs. FK core use areas (50% probability contour) did not differ between daytime and nighttime nor between sunup and sundown locations. We conclude that in areas similar to our study area day-only locations are adequate for describing the location, extent and core use areas of summer wolf territories by both MCP and FK methods.
Very high frequency (VHF) telemetry during daytime has been used to locate and observe wolves since the late 1960s (Mech, 1973). Telemetry projects typically locate wolves when conditions permit flying and observation of animals (Mech, 1973, 2009; Van Ballenberghe et al, 1975; Fritts and Mech, 1981; Peterson et al, 1984; Ballard et al, 1987; Fuller, 1989; Wydeven et al, 2009). These locations are used to estimate wolf-pack home ranges (usually MCPs or FKs) or territories. Because no study we are aware of has compared wolf spatial use during the day with that during the night using any method, wolf territories calculated using VHF locations might only be representative of wolf space use during daytime.
Global Positioning System (GPS) collars became available for wildlife research in the 1990s (Rodgers and Anson, 1994), and are now commonly used for wolf research. Because they can automatically collect large amounts of location data around the clock and in all weather conditions, they can provide an unbiased estimate of 24 h wolf-territory area and location, movement patterns (Merrill and Mech, 2003), prédation behavior (Derama et al, 2007), kill rates (Sand et al, 2008; Webb et al, 2008) and wolf home-range size (Mills et al, 2006) . However, we are unaware of any GPS-based comparisons between wolf home ranges determined at night versus day. Smith et cd. (1981) calculated coyote (Canis latrans) minimum convex polygon (MCPs) from VHF locations collected during daylight, half-night and full-night tracking periods, and concluded that home range sizes determined from >3 nights of hourly locations were considerably larger than those determined from daylight locations.
Because a large body of extant wolf information exists that relied on daytime VHF locations, an assessment is needed to determine if wolf location data collected only during the day represent only the extent of daytime use or whether these data represent both day and night use. Thus we used GPS telemetry to determine how daytime wolf locations compare to those of nightime locations and thus to assess the suitability of using wolf locations obtained by the more conventional daytime methods to characterize a wolf territory. Studies comparing results between different home-range-estimation methods have been published elsewhere and were not the focus of this study.
We conducted our study during Jun. through Aug. of 2003-2004 in a 2100-km area in the Superior National Forest (SNF) of northeastern Minnesota (48°N, 92°W). Nelson and Mech (1981) provided a detailed description of the study area. Wolves occurred throughout the study area at densities of 30-36/1000 km2 during the study (Mech, 2009). White-tailed deer (Odocoileus virginianus) occurred at densities of 12-15/10 km (M. H. Dexter, Minnesota Department of Natural Resources, unpublished report) and constituted the major prey of wolves in the area (Frenzel, 1974; Nelson and Mech, 1981, 1986), primarily fawns during summer (Van Ballenberghe et al, 1975; Nelson and Mech, 1986; Kunkel and Mech, 1994). …