Differential scaling of locomotor performance in small and large terrestrial mammals

It has been observed that the relationship between locomotor performance and body mass in terrestrial mammals does not follow a single linear trend when the entire range of body mass is considered. Large taxa tend to show different scaling exponents compared to those of small taxa, suggesting that there would be a differential scaling between small and large mammals. This pattern, noted previously for several morphological traits in mammals, has been explained to occur as a result of mechanical constraints over bones due to the differential effect of gravity on small and large-sized forms. The relationship between maximum relative running speed (body length s(-1)) and body mass was analysed in 142 species of terrestrial mammals, in order to evaluate whether the relative locomotor performance shows a differential scaling depending on the range of mass analysed, and whether the scaling pattern is consistent with the idea of mechanical constraints on locomotor performance. The scaling of relative locomotor performance proved to be non-linear when the entire range of body masses was considered and showed a differential scaling between small and large mammals. Among the small species, a negative, although nearly independent, relationship with body mass was noted. In contrast, maximum relative running speed in large mammals showed a strong negative relationship with body mass. This reduction in locomotor performance was correlated with a decrease in the ability to withstand the forces applied on bones and may be understood as a necessary stress reduction mechanism for assuring the structural integrity of the limb skeleton in large species.