Home range characteristics of a subordinate predator: selection for refugia or hunt opportunity?

1. What they did

Cougars are often called “habitat generalists”, yet little is known about what shapes where they set up their home ranges. In the Southern Yellowstone Ecosystem of Wyoming, cougars share the land with wolves and bears, which dominate them, and with human hunters. The authors asked whether cougars choose home ranges for safety (refugia such as forest, rugged terrain, and distance from roads and wolves), for good hunting, or for both.

• Capture and collaring. Each winter, from November to March, they used trailing hounds to tree or bay cougars, then immobilized them and fitted Very High Frequency (VHF) radio collars (2001 to 2005) or Global Positioning System (GPS) collars (2006 to 2011). GPS collars took four to eight locations a day.
• Home ranges. They mapped annual 95% home ranges and 50% “core areas” for adult resident cougars, and tested whether sex, collar type, the amount of forest or “hunt opportunity” explained home range size.
• Hunt opportunity. Counting prey across such large areas is hard, so they used a map of how likely a cougar was to make a kill at each spot, built from cougar kills found in the field in an earlier study.
• What is inside each home range. They placed random points inside each home range and across the whole study area, and compared hunt opportunity, terrain ruggedness, distance to roads, distance to the centre of known wolf pack territories, and land cover.

2. What they found

The cougars

• 67 annual home ranges from 28 cougars: 9 females and 5 males with GPS collars, and 16 females and 5 males with VHF collars. Seven cougars wore both collar types in different years.

Home range size

• Sex and collar type explained home range size at both scales. Male home ranges were 1.9 to 3.3 times larger than female ones (for example, GPS male 95% home ranges averaged 567 km² and GPS female ones 210 km²).
• Home ranges from VHF data were larger than those from GPS data (the text gives 1.4 to 1.9 times; see the keywords for the table values).
• The amount of forest partly explained the size of core areas. Hunt opportunity did not explain home range size.

What home ranges contained

• Hunt opportunity: highest in female core areas and lowest in GPS male 95% home ranges. Female home ranges had more hunt opportunity than the study area as a whole; GPS male 95% home ranges did not differ from it.
• Wolves: home ranges (GPS data) were further from the centres of known wolf pack territories than expected, and males were further away than females.
• Roads: all home ranges were closer to roads than expected. Male 95% home ranges were further from roads than female ones.
• Ruggedness: terrain ruggedness hardly differed, being only slightly higher than the study area in some cases.
• Land cover: home ranges mostly mirrored the study area. All had less meadow than expected; most had more shrub; female VHF core areas had more forest; females had less barren ground than males.
• Apart from size, home range features from VHF and GPS data were remarkably similar.

3. What they concluded

• Cougars in the Southern Yellowstone Ecosystem choose home ranges that give good hunting and keep them away from wolves, rather than ones with more refuge such as forest or rugged ground.
• Distance from wolves was the only safety feature they selected. Their home ranges were closer to roads than expected, where the authors assumed they were more exposed to human hunters, likely because roads follow the low valleys where prey gather in winter.
• Females chose better hunting areas than males, supporting the long-held idea that females pick home ranges with enough prey for themselves and their young. Because wolves also focus on prey-rich areas, this may bring females closer to wolves.
• Avoiding re-established wolves likely limits good habitat for cougars, with possible effects on young cougars’ survival and dispersal and on the population.
• VHF data give larger home ranges than GPS data, so sampling methods need to be standardized. Because home range features were so similar, earlier VHF-based cougar studies still hold value.

4. How tracking was or was not used

The paper does not report people tracking cougars. Cougars were found for capture with trailing hounds, which chased them up a tree or onto a rocky outcrop. The “hunt opportunity” map was built from cougar kills located and verified in the field in an earlier study (Elbroch et al. 2013), not in this one.

6. Other technologies, skills or innovations

• VHF collars (Telonics) located by triangulation from the ground or from fixed-wing aircraft (Program LOAS), and GPS collars (Telonics, Televilt or Vectronics); all collars had mortality sensors.
• A “hunt opportunity” map built from a kill-site resource selection function, used as a stand-in for prey availability.
• Vector ruggedness measure from a digital elevation model, and Gap Analysis Program land cover at 30 m resolution reclassified into five classes.
• Seven cougars collared with both collar types in alternating years, allowing a direct comparison of VHF and GPS home ranges.

7. Implications for conservation (from the paper)

• Competition with recolonizing wolves likely limits the availability of good cougar habitat, with implications for young cougars’ survival, their success in dispersing and cougar population dynamics.
• Cougars are a trophy species hunted in Wyoming, and their home ranges lie closer to roads than expected.
• Home range sizes vary widely with sampling method, so methods need to be standardized.