We used the example of a simulated behaviour set at sunrise for ease of demonstration. To highlight the importance of this, we first used a simple mathematical model to investigate the potential error of recording behaviours based on ‘clock time’, according to both the location and the duration of the study. The main goal of this work is to provide a simple tool for correcting the time at which behaviours are recorded when using a clock in order to make it corresponds to solar time. We thus aimed at characterizing the potential error in recording behaviours with a clock, according to study duration and geographical location. While the use of such a proxy for sun time may be justified for short-term studies close to the equator, where the difference is small, the increase of this difference with increasing duration and latitude has never been quantified. Consequently, clock time is often used in analysis rather than being converted to solar events. Consequently, the difference between clock time and sun time will be greater at high latitudes.Īlthough the difference between clock time and sun time is known, clock time is much easier to record when logging behaviours in the field. Moreover, the tilt of the earth on its axis generates a difference in annual variation of sun time according to latitude. In fact, patterns of behaviour may appear to differ if analyzed by clock time rather than by the deviation from sun time. Consequently, observations of behaviours lasting months should take into account the variation of daylight length. The ‘clock time’ of sunrises (zenith or sunsets, hereafter referred to as ‘sun time’) differs according to the latitude, longitude and date of the year. Lunar events are also of biological importance. Numerous animal activities are likely to be a function of either light intensity or ambient temperature and thus of the sun's position in the sky: time of sunrise, zenith or sunset, or more generally ‘sun time’ rather than ‘clock time’. Many studies of diel activities highlight the importance of the moment of the day in regulating animals’ daily behavioural cycles (Aschoff, 1966 Daan & Aschoff, 1974 Boulos, Macchi & Terman, 1996 Semenov, Ramousse & Le Berre, 2000 Metcalfe & Steele, 2001). This change is plotted on what is known as an analemma. These differences are due to the earth's tilt on its axis (23.5°) and its elliptical orbit around the sun. The sun's position in the celestial sphere, recorded at the same ‘time of day’ (hereafter referred to as ‘clock time’), changes on successive days throughout the year.
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.ĭaily events are classically positioned in time with a clock on a 24-h period. 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. Finally, we investigated the occurrence of sun time records in published field studies. 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. 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. 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.
Yet, in the field, recording the timing of behaviour is much less difficult with a clock, which is often deemed a suitable common proxy. Logically, therefore, behaviour timing should be recorded relative to these events. However, these astronomical events do not occur at the same time everyday: they vary with both the time of the year and the latitude. Many of them are regulated by external factors, such as light intensity, and are thus associated with sunrise, sunset or zenith. From insects to mammals, many animals engage in behaviours known to follow cyclic patterns over days (e.g.
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