Noisy clocks and silent sunrises: Measurement methods of daily activity pattern

From insects to mammals, many animals engage in behaviours known to follow cyclic patterns over days (e.g. singing, diving or foraging behaviours). Many of them are regulated by external factors, such as light intensity, and are thus associated with sunrise, sunset or zenith. However, these astronomical events do not occur at the same time everyday: they vary with both the time of the year and the latitude. Logically, therefore, behaviour timing should be recorded relative to these events. Yet, in the field, recording the timing of behaviour is much less difficult with a clock, which is often deemed a suitable common proxy. In this paper, we assess the potential methodological problems associated with analyzing behaviours on the basis of clock time rather than with the actual position of the sun. To demonstrate the important difference between these methods of analysis, we first simulated a behaviour set at sunrise and compared the time of occurrence with the two methods. We then used a dataset, based on a long-term monitoring of hunting behaviour of African wild dogs, Lycaon pictus, to reveal how using clock time can result in erroneous assumptions about behaviour. Finally, we investigated the occurrence of sun time records in published field studies. As a majority of them did not take into account the relevance of astronomical events, it is probable that many result in faulty behavioural timings. The model presented can change clock-recorded time into actual deviation from astronomical events to assist current protocols as well as correct the already recorded datasets. © 2011 The Authors. Journal of Zoology © 2011 The Zoological Society of London.