Thermal Characteristics of Mountain Lion Dens

1. What they did

Female mountain lions choose protected places to give birth, but little was known about these den sites. The researchers, studying mountain lion ecology in eastern California, noticed from radio telemetry that several adult females had restricted their daily movements and kept returning to the same places, a sign that they had natal dens.

• Finding the dens. They searched these three areas, listened for the calls of very young kittens, and then used a trained hound to locate the dens and kittens. They estimated the kittens’ ages from published criteria.
• Describing the dens. They estimated tree height, canopy closure and horizontal cover by eye at each den.
• Measuring temperature. At two dens they placed a recording thermograph on the den floor and an identical one on the ground less than 100 m away, on a shaded north-facing slope, recording temperatures every hour for about a month at each den.
• Comparing. They compared den and outside (ambient) temperatures by day and by hour of the day.

2. What they found

The dens

• All three dens were along the Owens River, in dense groves of willows up to about 4 m tall with plenty of wild rose, each within 50 m of the river.
• Kittens at dens 1 and 3 were estimated at 20 days old or less, and at den 2 at 10 days old or less.
• Canopy closure was nearly 100%, and direct sunlight never reached the ground on any midday visit.
• Cover was so dense that standing researchers could not see each other at 3 m or less. The researchers could reach the kittens only by crawling in.
• The den floors were littered with fallen leaves, tree trunks, branches, twigs and bark.

Temperatures

• Dens were cooler than the outside air in all but 2 of the paired hourly readings (less than 0.2%).
• Mean ambient and den temperatures: den 2 (September to October 1994), 13.18°C outside and 6.01°C in the den; den 3 (August to September 1995), 20.47°C outside and 15.22°C in the den.
• The cooling effect was strongest in hot weather. Above 31°C, the outside air was on average 21.92°C warmer than den 2 and 13.56°C warmer than den 3. That is over 3 times (den 2) and nearly 5 times (den 3) the difference at moderate temperatures (31°C and below).
• Temperatures inside the dens varied less. At den 2 the mean daily range outside (28.96°C) was nearly double that in the den (15.79°C); at den 3 the outside range (32.54°C) was more than 1.5 times the den range (20.89°C).
• Temperatures varied with the hour of the day at both dens. At den 2 they also changed from day to day and fell as the season went on; at den 3 there was no day-to-day effect, and the seasonal declines were smaller.
• At the lowest outside temperatures recorded, the dens were not warmer than the outside air.

3. What they concluded

• These mountain lion dens moderated high outside temperatures, supporting Shaw’s (1989) idea that dens protect young, helpless kittens from heat.
• The dens gave no warmth advantage at the lowest temperatures recorded, although they might protect kittens in colder conditions than the study encountered.
• Dens in thick, woody vegetation may hide kittens from predators and also shield them from direct sun. Because the den keeps temperatures more stable, kittens may need to move less to keep cool or warm, which could make them less likely to be found by predators.
• The authors suggest that thermal conditions differ between types of dens (caves, rocky areas, dense thickets), and that mountain lions may choose den sites partly for the thermal advantage they give.

4. How tracking was or was not used

No tracking by people is reported. The dens were found by radio telemetry of the mothers, by listening for kitten calls, and by a trained hound that located the dens and kittens. The paper does not describe how the hound worked.

6. Other technologies, skills or innovations

• Radio telemetry of adult females, used to spot restricted movements around natal dens.
• Recording thermographs (model RTM, Ryan Instruments) placed in pairs, one on the den floor and one outside nearby, logging every hour.
• A trained hound to find dens and kittens.

7. Implications for conservation (from the paper)

• Dense riparian willow and rose thickets provided important thermal benefits and concealment for young kittens in this area.
• Den sites that reduce kitten movement may lower the chance of discovery by predators and so improve the survival of young mountain lions.