Travel in alpine terrain: energy expenditures for locomotion by mountain goats and bighorn sheep

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Abstract
We used indirect calorimetry to measure energy expenditure for locomotion by three mountain goats (Oreamnos americanus) and five bighorn sheep (Ovis canadensis) in response to variation in level of activity, slope of travel, and snow conditions. The energetic increment of standing over lying averaged 26% for the two species. We could detect no difference between species in the net cost of travel on level surfaces (.hivin.x = 0.26 mL O2/(g.cntdot.kg)). Energy expended by bighorn sheep and mountain goats for lifting 1 kg of body weight one vertical metre on a 21.5.degree. slope (ca. 37 J/(kg .cntdot. m)) exceeded the highest cost documented for quadrupeds. Energy expended walking down a 21.5.degree. slope exceeded energy expenditure for horizontal locomotion, or was recovered ineffeciently (ca. 25% recovery of potential energy). The relative increase in the net cost of locomotion in snow achieved an asymptote when sinking depth exceeded 1.2-2.0 times brisket height. The slope of the relative increase in the net cost of locomotion as a function of sinking depth/brisket height was lower for mountain goats than for any North American ungulate studied to date. Consequently, mountain goats were less efficient than other species when snow was shallow, but were more efficient when it was deep. We explain this result on the basis of interspecific differences in locomotory behavior and foot loading.