The energetic cost of bipedal hopping in small mammals

Abstract

Recent studies of locomotion in red kangaroos (Megaleia rufa, 18–28 kg)¹ and Australian hopping mice (Notomys cervinus, 36.6 g)² have indicated that over a certain range of speeds the energetic cost of bipedal hopping locomotion may become independent of speed. As much as 70% of the decrement in kinetic energy which takes place when the kangaroo lands may be stored in elastic elements and used to propel the kangaroo upwards and forwards as it takes off (leaving only 30% to be added by the muscles)^3,4. Presumably, this same phenomenon could be exploited by all bipedal hopping mammals and it seems possible that at high speeds this specialized mode of locomotion could result in large energetic savings when compared with the costs incurred by a similarly sized quadruped running at the same speed. This hypothetical lower cost of locomotion has been invoked as an explanation for the rather common occurrence of bipedal hoppers among desert and plains dwelling mammals, as it might substantially reduce the energetic cost of foraging, or travelling, for animals utilizing widely dispersed resources^5–7. The purpose of the present study was to extend the assessment of the metabolic expense of bipedal hopping to a broader range of body sizes and mammalian taxa. Using indirect calorimetry we measured the cost of locomotion for three bipedal hopping eutherians (0.03–3.0 kg) and one bipedal hopping marsupial (1.1 kg). When properly trained for sustained running, none of the three species deviated from the expected quadrupedal pattern relating cost of locomotion to running speed, even while running fully bipedally. Bipedality must therefore be associated with benefits other than those related to the energetic costs of locomotion.