Estimating the costs of locomotion in snow for coyotes

1. What they did

Coyotes evolved in the open, mostly snow-free landscapes of central and southwestern North America, but over the last 50 to 100 years they have spread north and east, into places like southeastern Quebec where snow is often more than a metre deep. The authors asked how much extra energy coyotes spend moving through snow, and whether they behave in ways that cut that cost.

They worked in two parts:

• Measuring the cost with dogs. Three coyote-sized domestic dogs were fitted with heart-rate transmitters, since heart rate closely tracks energy use. After training to walk on a slack leash, each dog walked about 300 m per trial: first on a plowed road at different speeds, then in many different snow conditions. The researchers measured the dogs’ sinking depth, travel speed and heart rate, and built models predicting heart rate from sinking depth and speed.
• Following wild coyotes. In two winters, they searched the study area by truck or snowmobile in the early morning for fresh coyote tracks (less than 12 hours old), then back-tracked them on snowshoes, measuring distance with a hip-chain. Every 50 m they measured the coyote’s track sinking depth, and snow depth and hardness both on the trail and 1 m off it. Hardness was measured with a weighted copper pipe (a penetrometer) dropped from 10 cm, designed to sink like a coyote’s foot. They also recorded how far coyotes travelled on snow packed by snowmobiles, other human activity, animals, or frozen waterways.

Finally, they used the dog models to estimate how much energy coyotes spent, and saved, in the snow conditions they actually used.

2. What they found

Dogs: speed and snow

• Resting heart rate was about 72 beats per minute and rose slightly in colder weather (about 0.6% for each degree drop).
• On a hard surface, heart rate rose in a straight line with speed: about 6 beats per minute (about 8% of resting heart rate) for every 1 km per hour faster.
• In snow, sinking depth (1 to 35 cm) affected heart rate much more than speed did, and the cost climbed ever more steeply as the dogs sank deeper.
• At 1 km per hour, heart rate was 136 on a hard surface, 143 on a coyote’s track in the snow, and 149 off the track. At 5 km per hour it was 165, 173 and 180.
• Going from 1 to 5 km per hour raised heart rate by about 20%, the same as sinking 19 cm in snow at 1 km per hour. The model predicted heart rate would double at a sinking depth of 33 cm.

Wild coyotes

• In the harsh winter, coyotes travelled much more on packed snow than in the mild winter: 48% versus 28% of their distance in forest, and 63% versus 17% in rural areas.
• Use of snowmobile trails and other coyotes’ tracks rose in the harsh winter. Rural coyotes used hare tracks more often than forest coyotes, though not often.
• Coyote tracks sank about 10 to 11 cm on average, in snow about 56 to 60 cm deep.
• Even off packed trails, coyotes chose their route closely: snow was about 3 cm shallower on their trail than 1 m away, and harder, with the penetrometer sinking 2.3 cm deeper off the track.
• The penetrometer matched coyote sinking depth well in the rural landscape and less well in forest.

Energy

• Moving through snow raised the predicted heart rate by about 4% to 6% compared with a hard surface, and the coyotes’ choice of harder snow saved a similar amount.
• Assuming 10.9 km travelled a day at 1 km per hour, moving about would make up 27% of a coyote’s daily energy use on hard surfaces and 29% in average snow.

3. What they concluded

• Snow raises the cost of moving for coyotes, and sinking deeper raises it ever more steeply, as in deer, elk and caribou. The same body mechanics may apply to dogs and deer.
• Coyotes in deep-snow country reduce this cost by their behaviour: using trails packed by people and other animals, especially in harsh winters, and choosing shallower, harder snow.
• Speeding up is costly, which may explain why coyotes in many studies travel at about 1 km per hour or less. The authors propose that slow travel suits predators that must cover large areas to find prey.
• Fast chases through soft snow are very costly for coyotes, which would explain why they ambush hares in snow and make shorter chases than the large-footed Canada lynx.
• The authors hypothesize that snow packed by people, especially snowmobile trails, helps coyotes move in deep snow, may let them live in marginal forest habitat, and may have helped their spread into northeastern North America.

4. How tracking was or was not used

Tracking was central to the field study. The researchers searched for fresh coyote tracks in snow, judged them to be less than 12 hours old, and back-tracked them on snowshoes. Along the trails they measured the coyotes’ track sinking depth every 50 m, compared snow on and off the trail, and recorded how far each coyote travelled on snowmobile trails, roads, frozen water, and the tracks of hares, other coyotes and other species.

5. Tracker or observer certification

Not reported. The paper does not say who did the snow-tracking or what experience they had. N. Bergeron and D. Murray are thanked for their work in the field.

6. Other technologies, skills or innovations

• Heart-rate transmitters implanted under the skin of the dogs (Telonics HR-300), with heart beats counted from a tape recorder connected to a telemetry receiver.
• A home-made penetrometer (a copper pipe partly filled with lead, 60.3 cm long, 2811 g) dropped from 10 cm to mimic the pressure of a coyote’s foot in snow.
• Hip-chain to measure distances along trails.

7. Implications for conservation (from the paper)

• Snow packed by human activity, especially snowmobile trails and plowed roads, may make it easier for coyotes to live in forested areas and may have helped their range expansion in northeastern North America.
• Differences in how well canids and deer cope with deep snow may help explain why white-tailed deer are more vulnerable to coyote predation in deep-snow winters.