Explaining the scaling of transport costs: the role of stride frequency and stride length

The mechanisms which enable large animals to transport a unit of body mass through a unit distance at a lower metabolic cost than smaller animals have been the subject of numerous studies. Recent investigations have concluded that stride frequency is a main determinant. We examine the role of both stride frequency and stride length in determining the scaling of the cost of transport. Slopes for regressions between stride frequency and speed and stride length and speed were determined in four species of rodents. These data were pooled with literature values for the slopes of stride frequency, stride length and cost of locomotion (all vs. speed) for a total of 17 species ranging in size from 30 g to 250 kg. Interspecific equations were calculated for each of these slopes versus body mass, and residuals from these allometric lines were calculated. Residuals were compared to see if variation in the rate of cost increase at a given size is related to variation in the rates of stride frequency and/or stride length increase. The residual analysis revealed that the variation in transport cost is explicable only in terms of the interaction of stride frequency and stride length slopes. The product of the scaling exponents for stride frequency slope and stride length slope is not significantly different from the scaling exponent for the cost of transport. A model seeking to explain the scaling of the cost of transport must therefore consider the influence of both stride length and stride frequency. We propose that absolutely longer limbs allow large animals to minimize the rate of increase of stride frequency and stride length with speed, and that this allows utilization of muscles with lower intrinsic rates of contraction, which in turn results in a lower mass‐specific cost of transport.