Motion-triggered video cameras reveal spatial and temporal patterns of red fox foraging on carrion provided by mountain lions

1. What they did

Carrion (the meat left on dead animals) is a rich but short-lived food. Where snow and cold make small prey and seeds hard to reach, carrion may be a limiting food for red foxes, which are too small to kill deer or elk themselves. The authors asked how red foxes use the kills of mountain lions in the southern Greater Yellowstone Ecosystem (GYE), Wyoming, and how they balance the need for food against the risk of being killed by a larger carnivore.

• Finding kills. From 2012 to 2015 they captured mountain lions with baited cage traps and scent-trailing hounds and fitted them with Global Positioning System (GPS) collars. CyberTracker-certified researchers visited clusters of GPS locations (two or more locations within 150 m, lasting at least 4 hours) to find prey.
• Filming the kills. Where there was enough meat left to draw scavengers, they set paired motion-triggered video cameras recording 60-second clips. They filmed 232 kills of prey over 25 kg: 138 in winter (November to April) and 94 in summer (May to October). Cameras stayed on average about 6 days.
• Reading the videos. For each kill they recorded whether foxes came and fed, the most foxes seen together in one frame, total fox feeding time, whether foxes fed before or after the mountain lion left, and vigilance (looking, listening, moving to investigate).
• Where foxes scavenged. They compared kills with and without foxes by elevation, aspect, terrain ruggedness, habitat type and distance to forest edge, with statistical models for each season.

They predicted that foxes would use more kills in winter, wait until the mountain lion had left, be more watchful while it was nearby, and prefer kills near forest edges or in cover.

2. What they found

Season

• Foxes scavenged at 70.3% of winter kills versus 48.9% of summer kills.
• They fed longer in winter: on average 102.7 minutes per kill (standard deviation 138.3) versus 39.7 minutes (74.0) in summer.
• When foxes were present, there were about 1.5 times as many in winter (1.83 versus 1.16 foxes per kill).
• Red foxes were the most common scavenger at mountain lion kills in this study area.

The mountain lion’s presence

• In summer, foxes mostly waited: they fed at 20 kills while the mountain lion was still feeding, and at 36 kills after it had left.
• In winter, they fed whether or not the mountain lion was still there (61 kills versus 65).
• Foxes that fed while the mountain lion was present were no more watchful than after it had left.

Where they scavenged

• Winter kills were closer together than summer kills (on average about 457 m apart versus about 1008 m).
• In winter, the best model linked foxes with distance to forest edge, and the second model showed foxes more likely at kills at higher elevations. In summer, foxes were more likely at kills at lower elevations.
• All these effects were small, and in both seasons a model with no landscape features performed nearly as well. Foxes used kills in all habitats and showed little preference for any landscape feature.

3. What they concluded

• Red foxes benefit from mountain lion kills all year, and especially in winter, when they come to more kills, in greater numbers, and feed for longer.
• The authors suspect foxes lean on carrion in winter to cope with food shortage. Winter kills were also more clumped (where elk gather), and bears, which compete for carcasses, were hibernating.
• In winter, foxes fed close to the mountain lion as often as after it left, without extra vigilance. The authors suggest foxes may be hungrier and take more risks in winter, or find kills faster, perhaps even by following mountain lions (they had no fox movement data to test this).
• Food availability may influence fox foraging decisions more strongly than the risk of being killed by another predator.
• Mountain lions tolerate heavy scavenging, so foxes may not harm them; instead foxes help spread the carrion that this apex predator provides.

4. How tracking was or was not used

Tracking was used to find the kills, not to study the foxes. CyberTracker-certified researchers investigated GPS clusters on the ground to locate and assess prey killed by mountain lions, and judged whether enough meat remained to attract scavengers. Mountain lions were captured with scent-trailing hounds, in winter when snow helped researchers detect them. The fox data came from video, not from tracks and signs.

5. Tracker or observer certification

The site investigations were carried out by CyberTracker-certified researchers, citing Evans et al. (2009) and Elbroch et al. (2011). The paper does not give the level of certification or which researchers held it. Video was used to record fox behaviour; no test of video-reading accuracy is reported, and the authors note they could not tell individual foxes apart, especially in coarse black and white night footage.

6. Other technologies, skills or innovations

• GPS collars (Lotek Globalstar S or Iridium M; Vectronics Globalstar GPS Plus) and GPS cluster investigations to find kills.
• Paired motion-triggered video cameras (Bushnell) recording 60-second videos with 30-second delays, catalogued in an Access database.
• The minimum number of foxes per kill was the most seen together in one frame, since individuals could not be told apart.
• Geographic Information System (GIS) mapping in ArcGIS, with a 30 m elevation model and Gap Analysis Program land cover.

7. Implications for conservation (from the paper)

• Large carnivores, and mid-sized cats like mountain lions in particular, are essential producers of carrion that supports biodiversity and ecosystem health.
• Scavengers such as red foxes spread these resources across the landscape, for example by carrying off and caching meat, which supports ecosystem structure and ecological resilience.
• Future research should test whether mountain lions shield foxes from coyotes and allow foxes to live at higher densities alongside them.