Cougar den site selection in the Southern Yellowstone Ecosystem

1. What they did

Dens shelter cougar mothers and their helpless kittens for up to about 8 weeks, and most kitten deaths happen before 3 months of age. Yet little was known about how cougars choose dens. In the Southern Yellowstone Ecosystem of Wyoming, cougars share the land with wolves, black bears and brown bears, which dominate them, and with human hunters. The authors expected females to choose dens in rugged terrain away from roads, and in thickets or other cover that would hide them and their kittens.

• Finding dens. In winter, cougars were treed or bayed with trailing hounds, then fitted with Global Positioning System (GPS) or very high frequency (VHF) radio collars. As part of a study of what cougars eat, the team searched places where a collared cougar had stayed put (8 hours or more on GPS, 24 hours or more on VHF). Dens were found when the searchers came across newborn kittens.
• Den areas (third-order selection). For each den, they drew a 2 km circle, the “den area”, based on how far mothers moved while attending dens. They compared random points inside these den areas with random points in the rest of each female’s home range: terrain ruggedness, habitat type, aspect, distance to roads and water, and “hunt opportunity” (the chance of a cougar making a kill there, from an earlier study of 687 cougar kills).
• Den sites (fourth-order selection). At 20 dens and at random sites in the same home ranges, they measured canopy cover, downed logs, rocks and trees along 10 m lines in each direction, and how well the site hid an animal, using a 2 m concealment board viewed from 10 m away.

2. What they found

Dens and birth timing

• They documented 25 dens between 2002 and 2013, 20 of them from 15 females with full measurements.
• Of 23 dens described, 17 were in deadfall (a criss-cross of fallen logs), 2 in caves among boulders on scree slopes, 3 in brushy thickets, and 1 in a fairly open clump of young fir trees in forest.
• Births were clumped in summer: 56% of the 25 dens began in June or July. The earliest birth was 20 May and the latest 3 November, which suggests courtship from late February to early August.

Den areas

• The best model held only terrain ruggedness, and it carried 99.8% of the model weight. Against the authors’ expectation, den areas were in less rugged terrain than the rest of the home range.
• Den areas did not differ from the rest of the home range in hunt opportunity, water, distance from roads, habitat or aspect.

Den sites

• Compared with random sites, den sites had more canopy cover (about 65% against 34%), many more downed logs, and much more concealment. All three differences were significant.
• Rocks and tree size did not differ significantly in the text (although Table 2 marks trees as significant).

3. What they concluded

• Female cougars in this area choose the den site itself far more strongly than the wider den area. The pattern differs from other carnivore studies, where den selection showed at more than one scale.
• The results support a summer birth pulse, as reported for other cougar populations, and the common description of cougar dens as being in “impenetrable vegetation”.
• Either females do not pick den areas very different from the rest of their home ranges, or the study did not test the right variables. Findings may not apply outside the northern Rocky Mountains; Florida panthers do select particular habitats around dens.
• Forest cover and criss-crossed logs may hide mothers as they come and go, and protect kittens from predators, weather and extreme temperatures.
• A next step is to link kitten survival to den sites and den areas.

4. How tracking was or was not used

Tracking played a supporting role. Cougars were found for capture with trailing hounds. Dens were found by searching on foot at places where collared females had stayed put, and confirmed by finding newborn kittens. The hunt opportunity layer came from an earlier study’s 687 verified cougar kills. The paper does not describe reading tracks and signs at dens.

5. Tracker or observer certification

Not reported. The paper does not say who handled the hounds or searched the sites. A. Kusler, J. Fitzgerald and J. Kay are thanked for field help at dens and random points.

6. Other technologies, skills or innovations

• GPS collars (Telonics, Televilt or Vectronics) set to take 4 to 8 locations a day, and VHF collars (Telonics), all with mortality sensors.
• GPS clusters (groups of locations where an animal stayed) used to find dens.
• Vector ruggedness measure for terrain, mapped in a Geographic Information System (GIS).
• A concealment board (2 m tall, 50 cm wide, in a grid of 10 cm squares) and a spherical crown densiometer for canopy cover.

7. Implications for conservation (from the paper)

• Complex structure at the den site seems most important to female cougars. Land practices that promote and protect downed wood and heavy structure on forest floors will best give cougars suitable den sites.
• If kitten survival proves lower in dens with less cover, which may relate to forest thinning, fire suppression and weather, managers could build den characteristics into land management.