Modelling coordinated nocturnal rhythms in ungulates

Studying animal behavior is an important aspect of ethology and behavioral biology and a prerequisite to improving animal management in zoos. As the nocturnal behavior of many ungulate species is, in contrast to the behavior during daylight, poorly studied, a better understanding of nocturnal behavior is necessary to improve animal welfare. We analyse the nocturnal behavior of ungulates recorded in a large number of German and Dutch zoos. These animals show a switching between standing and lying phases, which can be associated with a certain degree of regularity. Interestingly, this regularity is not always captured in the simple length distributions of behavioral phases but shows in the autocorrelation and in the coordination of standing and lying across animals. Particularly, this phenomenon often occurs in younger animals. We provide an explanation to this phenomenon by proposing a stochastic model that can describe these processes. For individual behavior, regular standing cycles are assumed to be potentially interrupted by short lying phases, such that a regular background rhythm appears in the autocorrelation but not necessarily in the raw length distribution of the activity phases. For coordinated behavior, crosscorrelation functions allow to analyse the degree to which pairs of animals that are sharing the same stable box show a synchronization of their standing-lying rhythms. In the data set, our analyses suggest that indeed, baseline regularity does not seem to be reduced in younger animals. Instead, younger animals showed increased probabilities for interruption of standing phases by short lying phases. In addition, the coordination of the standing-lying rhythm between animals in the same box ranged up to 100% and decreased with the distance between boxes. We also found systematic delays between the standing activity of young and adult animals.