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 that incorporates a 3D-printed holder, a commercially
available polystyrene ball, and is 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 one 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 of 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.