1. Body postures adopted by an animal can serve behavioural functions, homeostasis, or energy balance. 2. We investigated the function of holding the gaster curled forward under the thorax in acacia ants, Pseudomyrmex spinicola Emery, by testing whether ants adopted this posture for defence, thermoregulation, or for efficient locomotion. 3. For the defence hypothesis, we expected an increase in the proportion of ants with curled gasters after a visual threat, a vibrational disturbance of a branch, or the release of nestmate's alarm pheromones. Our data did not support these predictions. 4. For the thermoregulation hypothesis, we found a positive correlation between temperature and proportion of curled-gaster ants. However, we did not find a reduction in the proportion of curled-gaster ants after shading them, as predicted by this hypothesis. 5. Our data supported the locomotion hypothesis: curled-gaster ants walked 1 cm s -1 faster than ants with the gaster held straight. Straight-gaster ants walked with the thorax closer to the surface, a posture that likely shifts the centre of gravity closer to the surface in a manner similar to gaster curling. 6. Studying the role of the body posture in acacia ants and other insects will provide a better understanding of the kinematics of walking in challenging angles with respect to gravity. © 2011 The Authors. Ecological Entomology © 2011 The Royal Entomological Society.