The observation resolution -- a neglected aspect with critical influence on movement-based foraging indices

Abstract: Movement-based indices such as moves per minute (MPM) and proportion time moving (PTM) are common methodologies to quantify foraging behaviour. Hundreds of studies have reported these indices without specifying the temporal resolution of their original data, despite the likelihood that the minimal stop and move durations can affect MPM and PTM estimates. Our goal was to empirically determine the sensitivity of these foraging indices to changes in the temporal resolution of the observation. We used a high speed camera to record movement sequences of 20 Acanthodactylus boskianus lizards. We gradually decreased the data resolution by ignoring short stops and either ignoring, elongating or leaving short moves unchanged. We then used the manipulated data to calculate the foraging indices at different temporal resolutions. We found that movement-based indices are very sensitive to the observation resolution, so that realistic variation in the minimal duration of stops and moves could lead to 68 percent and 48 percent difference in MPM and PTM estimates, respectively. When using the highest resolution, our estimate of MPM was an order of magnitude higher than all prior reported values for lizards. Also, the distribution of stop durations was well described by a single heavy tailed distribution above 0.35 seconds. This suggests that for A. boskianus there is no reason to ignore short stops above this threshold. Our results raise major concerns regarding the use of already published movement based indices, and enable us to recommend how new foraging data should be collected.