Minimum cost of transport in Asian elephants: do we really need a bigger elephant?

SUMMARY Body mass is the primary determinant of an animal’s energy requirements. At their optimum walking speed, large animals have lower mass-specific energy requirements for locomotion than small ones. In animals ranging in size from 0.8 g (roach) to 260 kg (zebu steer), the minimum cost of transport (COTmin) decreases with increasing body size roughly as COTmin∝body mass (Mb)–0.316±0.023 (95% CI). Typically, the variation of COTmin with body mass is weaker at the intraspecific level as a result of physiological and geometric similarity within closely related species. The interspecific relationship estimates that an adult elephant, with twice the body mass of a mid-sized elephant, should be able to move its body approximately 23% cheaper than the smaller elephant. We sought to determine whether adult Asian and sub-adult African elephants follow a single quasi-intraspecific relationship, and extend the interspecific relationship between COTmin and body mass to 12-fold larger animals. Physiological and possibly geometric similarity between adult Asian elephants and sub-adult African elephants caused body mass to have a no effect on COTmin (COTmin∝Mb0.007±0.455). The COTmin in elephants occurred at walking speeds between 1.3 and ∼1.5 m s–1, and at Froude numbers between 0.10 and 0.24. The addition of adult Asian elephants to the interspecific relationship resulted in COTmin∝M –0.277±0.046b. The quasi-intraspecific relationship between body mass and COTmin among elephants caused the interspecific relationship to underestimate COTmin in larger elephants.