Locomotor behavior of nocturnal ghost crabs on the beach: focal animal sampling and instantaneous velocity from three-dimensional motion analysis.

Previous laboratory measurements of the energetics and biomechanics of locomotion have defined performance limits for the ghost crab Ocypode quadrata. To discover whether these animals naturally operate within these limits, a novel infrared videotaping system was used to observe nocturnally active ghost crabs in the field for three-dimensional motion analysis (N = 27) and focal animal sampling (N = 24). Instantaneous movement velocity, movement duration, pause duration and stride frequency were determined from video tapes. Voluntarily active crabs moved at a mean instantaneous velocity of 8 cm s-1. Stressed crabs (i.e. those captured and released into the field site) moved at a mean velocity of 83 cm s-1. The mean movement and pause period durations of voluntarily active animals moving along the beach were 11.2 and 23.4 s, respectively. Stressed crabs had much shorter movement (1.4 s) and pause (7.6 s) durations. Despite the differences in mean movement and pause duration, both voluntarily active and stressed crabs moved for an average of approximately 30% of the observation period. These data indicate that voluntarily active ghost crabs primarily move at velocities that can be sustained aerobically and that their performance is not likely to be altered by moving intermittently. By contrast, stressed crabs move at faster speeds that are closer to the limits of their continuous locomotor performance (e.g. escape behavior and aggressive encounters). In the laboratory, the endurance capacity of crabs moving continuously at these rapid speeds is only a few seconds. However, in the field, the stressed crabs are able to move intermittently for more than a few seconds, yet they do not fatigue. These observations suggest that the performance limits of the stressed crabs are increased by moving intermittently.