Temperature dependence, energetics, and speed of locomotion have important implications for the ecology and evolution of ants. Here, we report the results of a full-factorial study investigating the responses of active metabolic rate (AMR), voluntary locomotion speed, and cost of transport (COT) to three temperature acclimations (7-10 d; 20 degrees, 25 degrees, and 30 degrees C) and three test temperature treatments (2 h; 20 degrees, 25 degrees, and 30 degrees C) in a seed-harvester ant, Messor capensis. By using a strong-inference approach, we assessed these trait responses in the context of the beneficial acclimation hypothesis and its alternatives. Results showed that AMR was not affected by acclimation temperature, indicating limited phenotypic plasticity for this trait. By contrast, voluntary running speed was consistently higher when ants were acclimated at 25 degrees C, providing support for the optimal acclimation hypothesis. COT was not affected by acclimation or treatment temperature. In addition, while AMR was repeatable and similar across temperatures (r=0.371-0.683), the repeatabilities of running speed (r=0.191-0.826) and COT (298 J kg(-1)m (-1)) were highly variable, suggesting constraints on ant locomotion under certain thermal conditions. Minimum COT (298 J kg(-1) m(-1)) in M. capensis was well within the range of values for other terrestrial arthropods of a similar body size. This study emphasizes the need to investigate a variety of performance traits rather than a single one and to expand the limited body of work on plasticity of insect locomotion.