The fast and the fractalous: speed and tortuosity trade off in running ants

The thermal sensitivity of locomotor performance has often been described in terms of speed, but the trajectory of locomotion may play an equally important role in capturing prey or escaping predators. Hypotheses based on physical constraints or behavioural plasticity predict relationships between the speed and the tortuosity of running, which should affect the thermal sensitivity of locomotion. We measured the speed and tortuosity of running by leaf-cutter ants over a range of temperatures from 10 degrees C to 40 degrees C. Tortuosity was estimated by the fractal dimension of each path. As we expected, ants ran faster at higher temperatures, but they also followed straighter (less tortuous) paths. A negative relationship between speed and tortuosity was observed both within and among thermal environments. Both biomechanical and behavioural mechanisms might have caused the observed relationship. Turning at high speeds should be more difficult because of the force needed to overcome inertia, and turning at low speeds could help ants evade a predator. Staged encounters with predators should help to define the ecological significance of the trade-off between speed and tortuosity.