Mediterranean Spur-Thighed Tortoises (Testudo graeca) Have Optimal Speeds at Which They Can Minimise the Metabolic Cost of Transport, on a Treadmill

SIMPLE SUMMARY: Understanding the energy that animals use to move around is important, as it can shed light on how they make decisions about where and how to locomote. Tortoises are unique among vertebrates in having a shell, which influences almost all aspects of their biology. Here, we experimentally quantified the metabolic cost of transport in Mediterranean spur-thighed tortoises walking on a treadmill while also quantifying the kinematics of their movement. We found, in line with previous studies, that tortoises move more efficiently than predicted and present the first data demonstrating a curvilinear cost of transport over their speed range. We conclude that tortoises have an optimum speed at which they move to minimise their metabolic cost of locomotion. ABSTRACT: Tortoises are famed for their slow locomotion, which is in part related to their herbivorous diet and the constraints imposed by their protective shells. For most animals, the metabolic cost of transport (CoT) is close to the value predicted for their body mass. Testudines appear to be an exception to this rule, as previous studies indicate that, for their body mass, they are economical walkers. The metabolic efficiency of their terrestrial locomotion is explainable by their walking gait biomechanics and the specialisation of their limb muscle physiology, which embodies a predominance of energy-efficient slow-twitch type I muscle fibres. However, there are only two published experimental reports of the energetics of locomotion in tortoises, and these data show high variability. Here, Mediterranean spur-thighed tortoises (Testudo graeca) were trained to walk on a treadmill. Open-flow respirometry and high-speed filming were simultaneously used to measure the metabolic cost of transport and to quantify limb kinematics, respectively. Our data support the low cost of transport previously reported and demonstrate a novel curvilinear relationship to speed in Testudines, suggesting tortoises have an energetically optimal speed range over which they can move in order to minimise the metabolic cost of transport.