Using a Low-Cost Trackball System to Assess Repeatability of Movement in Small Animals

Individual differences in movement patterns are increasingly recognised as important within ecology. In the laboratory, they are, however, often quantified through relatively coarse measures. We describe a low-cost trackball system incorporating a 3D-printed holder, commercially available polystyrene ball and implemented with open-source software. We used this system to record six parameters of walking behaviour (e.g., translational velocity, total rotation) and tested the hypothesis that these walking parameters are repeatable across individuals. We tested 30 ground beetle individuals, Carabus problematicus, each in two trackball trials 1 week apart. Individuals were repeatable and differed consistently in several movement parameters (including distance walked, translational velocity and path straightness) but not in others (e.g., total rotation and sinuosity). Trackballs allow quantification of walking parameters for a wide range of animals, enabling identification of individual differences in specific aspects of walking, both fine-scale and long-distance aspects of movement. In doing so, trackballs offer novel insights into behavioural ecology, including consistent animal personality differences.