Effect of variation in form on the cost of terrestrial locomotion

The mass-specific minimum cost of terrestrial locomotion (Cmin) decreases with an increase in body mass. This generalization spans nearly eight orders of magnitude in body mass and includes two phyla. The general relationship between metabolic cost and mass is striking. However, a significant amount of unexplained interspecific variation in Cmin exists at any given body mass. To determine how variation in morphology and physiology affects metabolic energy cost, we measured the oxygen consumption of three comparably sized insects running on a miniature treadmill; the American cockroach Periplaneta americana, the caterpillar hunting beetle Calosoma affine and the Australian field cricket Teleogryllus commodus. Steady-state oxygen consumption (VO2ss) increased linearly with speed. Cmin was similar for crickets and cockroaches (8.0 and 8.5 ml O2 g-1km-1, respectively), but was substantially lower for beetles (4.6 ml O2 g-1km-1). The predicted value of Cmin for all three insects was within the 95% confidence intervals of the Cmin versus body mass function. However, the 95% confidence intervals extend approximately 2.5-fold above and 40% below the regression line, making the variation at any given body mass nearly sixfold. Normalizing for the rate of muscle force production by determining the metabolic cost per stride failed to account for the interspecific variation in the cost of locomotion observed in the three insects. Ground contact costs (i.e. VO2ss multiplied by leg contact time during a stride) in insects were similar to those measured in mammals (1.5-3.1 J kg-1) and were independent of speed, but did not explain the interspecific variation in the cost of locomotion. Muscles of the caterpillar hunting beetle may have a greater mechanical advantage than muscles of the Australian field cricket and American cockroach. Variation in musculo-skeletal arrangement, apart from variation in body mass, could translate into significant differences in the minimum cost of terrestrial locomotion.