Day length variation and seasonal analysis of behaviour : research article

Annual cycles in day length are an important consideration in seasonal analyses of behaviour. Seasonal variation in day length not only represents an ecological constraint on activity, but also imposes methodological restrictions on analyses. This paper examines the implications of monthly variation in day length using data from a troop of chacma baboons (Papio cynocephalus ursinus) at De Hoop Nature Reserve, South Africa. Time spent feeding, moving, grooming and resting each month were all significant positive functions of day length, confirming its importance as an ecological constraint. More importantly, the results highlighted the necessity of including day length as an independent variable in this form of analysis. Where day length is excluded, there are problems interpreting relationships, since it is impossible to separate out the independent effects of temperature and day length. The analyses also confirmed that percentage time budgets, which have sometimes been used in this form of analysis, are subject to significant biases where data are from populations experiencing substantial degrees of day length variation. Future research must be aware of the methodological constraints imposed by seasonal variation in day length, and further work is clearly required to fully determine its importance.