Lizards with rhythm? Multi-day patterns in total daily movement

Irregular and unpredictable day-to-day patterns in locomotor activity are often observed in animal movement patterns. While ecological processes may drive some variability in activity, underlying rhythms may modulate activity patterns independently of exogenous factors. We investigated multiple day (infradian) locomotor activity rhythms, in long-term continuous records of individual daily step counts, in a natural population of the Australian sleepy lizard Tiliqua rugosa. We examined temporal periodicity in locomotor activity using spectral analysis of sequential, overlapping 12-day windows of activity data. Lizards were inactive on average I day in 6, independent of ambient temperature, and showed infradian locomotor activity rhythms. In a drought year, 2002/2003, rhythms did not exceed randomized activity patterns. In an average rainfall year, 2003/2004, 6 and 2-day cycles occurred significantly more often than by chance. Individuals normally moved independently of conspecifics, although there was some short-term synchrony of activity rhythms. Rainfall did not consistently produce a synchronized pulse of activity in the population. We propose individually based rhythms, associated with differing optimal thermoregulatory requirements for foraging and digestion. In studies of population ecology, individual activity rhythms may explain day-to-day variation in encounter rates.