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Speed and Time‐Energy Budget for Locomotion in Golden‐Mantled Ground Squirrels

Kenagy, G. J., Hoyt, Donald F.
Ecology 1989 v.70 no.6 pp. 1834-1839
Spermophilus, allometry, basal metabolic rate, digestible energy, energy costs, energy efficiency, fungi, gait, home range, locomotion, nitrogen content, predation, prediction, small mammals, squirrels, walking
To determine the impact of locomotion on the total energy budget of a small mammal, we measured the frequency distribution of speeds and the time budget for locomotion in a population of Spermophilus saturatus, golden—mantled ground squirrels. Using these data together with laboratory—measured rates of energy expenditure for the walking and running gaits, we computed energy costs of locomotion in the field and compared these to field measurements of total daily energy expenditure. The animals moved in two distinct modes: walking, with mean speed 0.21 m/s (all values: <0.7 m/s), and running, with mean speed 3.63 m/s (range: 2—6 m/s). Of total daily time above ground mean: 7.37 h), 26.9% (°2 h) was spent walking and 3.6% (δ1 h) was running. The average daily movement distance (DMD) of 5 km/d consisted of 1.50 km walking and 3.53 km running. Walking required more time and net energy than running, yet walking resulted in less than half the distance covered by running. The net added energy cost of all moving around was 28.75 kJ/d. This net locomotion cost represents a 29% increase above basal metabolic rate (BMR) and comprises 13% of total daily energy expenditure (DEE). Our observation that this "ecological cost of transport" (ECT), being 13% of DEE, is so substanital suggests that energetic efficiencies should play an important role in determining speeds of locomotion. Our measurements suggest that the squirrels run when "commuting" around the home range rather than walking because it conserves both time and energy. We also suggest that the squirrels run at their maximum aerobic speed (3.6 m/s) rather than at their less physiologically demanding minimum speed for the running gait (2 m/s) to reduce exposure to predation. Our determination of ECT for golden—mantled ground squirrels is 15 times as great as an allometric prediction of ECT, due largely to a DMD that is 8 times as great as predicted. Daily movement distance should vary among species in relation to behavior. In S. saturatus the great magnitude of DMD is apparently a reflection of the dietary dependence of this species on hypogeous fungus. This food is distributed in patches and has low digestible energy and nitrogen content, necessitating that a large volume of the food be obtained, over a large area.