Intrasubject and intersubject variabilities in the daily rhythm of total locomotor activity in horses

The observation of locomotor activity and resting behavior is used to determine time budgets, which may give some indication for the assessment of horses' welfare concerning management and housing. The aim of this study was to investigate total locomotor activity (TLA) daily rhythm in athletic horses (Thoroughbred, Standardbred, and Italian Saddle), with special attention to the intrasubject and intersubject variabilities. Ten subjects for each breed were divided into two subgroups, one housed in box and one housed in paddock. TLA was recorded for 10 consecutive days by mean of an activity data logger, Actiwatch Mini, applied at each horse by mean of a headstall. The obtained data were analyzed using the method of single cosinor. All horses, in all experimental conditions, showed a daily rhythm of TLA. On the obtained circadian parameters (mesor, amplitude, acrophase, and robustness), a multivariate analysis of variance was applied to investigate the effect of breed, housing conditions, and days of monitoring. It showed a statistically significant effect of breed on all circadian parameters; an effect of housing condition on mesor, amplitude, and robustness; and an effect of days of monitoring on acrophase and robustness. The analyses of intrasubject and intersubject variabilities indicate that organization of TLA daily rhythm in Standardbred and Jumper is characterized by greater accuracy of control within and between the individuals of a breed. In contrast, Thoroughbreds showed a high intrasubject and intersubject variability for all parameters studied, except for the intravariability of acrophase, denoting a different response to noxious stimuli that was observed not only among the subjects but also within the subject.