The influence of snow on lynx and coyote movements: does morphology affect behavior?

1. What they did

Lynx and coyotes are mid-sized carnivores of similar size and weight, and both live where snow covers the ground in winter. Lynx have large feet for their weight, while coyotes have small feet and are thought to struggle more in snow. The authors asked whether coyotes make up for their small feet through behaviour, and how this shapes where and how each species moves.

They worked in a valley in southwestern Yukon, Canada, through the winter of 1988-89:

• Body measurements. They weighed and measured skinned carcasses of 58 lynx and 10 coyotes from trappers, and 21 snowshoe hares. They traced the paws to get foot area, then worked out foot-load (body mass divided by foot area) and chest height.
• Snow conditions. At 16 sites at low and high elevations, they measured snow depth and snow hardness eight times over the winter. Hardness was measured by dropping a “penetrometer” (a soft-drink can filled with lead, 150 g) from 50 cm and measuring how deep it sank.
• Track counts. They recorded every lynx and coyote track crossing a network of snowmobile trails to compare how much each species used low and high elevations.
• Following tracks. They followed fresh tracks of lynx for 208 km and of coyotes for 188 km. Every 500 paces they measured snow depth and hardness in the track and 1 m to the side, and how deep a footprint had sunk. They noted how much of the time the animal travelled on trails made by other animals, every chase of prey (with the number of bounds and the outcome), and places where the animal scavenged.

2. What they found

Feet and bodies

• Coyotes carried 3.4 to 8.8 times the foot-load of lynx, and 5.8 to 8.1 times that of snowshoe hares. Hares had the lowest foot-loads of the three.
• Chest height did not differ between the two predators. Neither species ever sank as deep as their chest; footprints rarely sank more than 20 cm.

Sinking in snow

• On snow of the same hardness, coyotes sank deeper than lynx, but never by more than 4 cm. This was much less than the difference in foot-load would suggest.
• How deep lynx sank depended on snow hardness only; for coyotes, both snow depth and hardness mattered.

Where they travelled

• Coyotes used low elevations more than lynx: 64% of coyote track crossings were at low elevations, compared with 26% for lynx. Snow was shallower at low elevations (about 40 cm against 55 cm higher up).
• Coyote tracks were on shallower and harder snow than lynx tracks.
• Both species chose routes where the snow was shallower and harder than 1 m to the side, even when they were not on another animal’s trail.
• Coyotes travelled on animal trails for 52% of the distance tracked, lynx for 37%. Both used hare trails about equally, but coyotes used other trails (lynx, coyote, wolf, moose and human trails) more often.
• Even with all this, coyotes still sank deeper overall: 9.5 cm on average, compared with 7.7 cm for lynx.

Chases and scavenging

• All successful chases, and most unsuccessful ones, were of snowshoe hares.
• Coyote chases took place on shallower, harder snow than lynx chases.
• Coyote chases took fewer bounds than lynx chases, and successful chases took fewer bounds than unsuccessful ones in both species. Successful coyote chases averaged under one bound; successful lynx chases about two.
• Forty-five percent of coyote scavenging sites were found while coyotes were travelling on other predators’ trails. Only 25% of lynx scavenging was along trails, all of them lynx trails.

3. What they concluded

• Lower foot-load is a major advantage in snow. Lynx have this advantage through their body shape.
• Coyotes made up for much of their disadvantage through behaviour: using lower elevations, choosing shallow and hard snow, and travelling on trails made by other animals. On hard snow they sank about as little as lynx.
• Hard snow was limited, though, and in soft snow coyotes sank deeper. Overall, snow was more of a handicap for coyotes than for lynx.
• Travelling on predator trails also gave coyotes extra food through scavenging, so they may have used these trails partly for food and not only for easier travel. The amount of food scavenged was relatively small.
• Snow may have shaped hunting tactics. Because their high foot-load would slow them in snow, coyotes may have needed to start chases at shorter distances and ambush hares rather than run them down, unlike the long chases typical of canids.

4. How tracking was or was not used

Tracking was the core field method. The researchers followed fresh lynx and coyote tracks in snow after each snowfall until fresh tracks could no longer be told from older ones (usually 7 to 10 days), and measured distance by counting paces. Along the trail they measured footprint sinking depth and snow on and beside the track, recorded when the animal was walking on another animal’s trail (and whose), read chases from the tracks (number of bounds, success or failure, prey species), and noted scavenging sites. They also counted track crossings on snowmobile trails to compare use of elevations. Because coyote and red fox tracks looked similar there, they identified coyote tracks by track size together with the known home ranges of radio-collared animals, sightings, and knowledge that foxes avoid coyote areas.

5. Tracker or observer certification

Not reported. Five co-workers are thanked for “diligent assistance in the field”, but nothing is said about who did the tracking or their experience. Every result depends on reading the trails correctly (for example whose trail an animal was on, the number of bounds in a chase, and telling coyote from red fox tracks).

6. Other technologies, skills or innovations

• A simple home-made penetrometer (a lead-filled soft-drink can dropped from 50 cm) to index snow hardness.
• Paw tracings measured with a digital planimeter to work out foot-load.
• Radio-collared animals, used to help estimate how many lynx and coyotes lived in the area and to help tell coyote tracks from fox tracks.