Hiding from the moonlight: luminosity and temperature affect activity of Asian nocturnal primates in a highly seasonal forest

Abstract

The effect of moonlight and temperature on activity of slow lorises was previously little known and this knowledge might be useful for understanding many aspects of their behavioural ecology, and developing strategies to monitor and protect populations. In this study we aimed to determine if the activity of the pygmy loris (Nycticebus pygmaeus) is affected by ambient temperature and/or moonlight in a mixed deciduous forest. We radio-collared five females and five males in the Seima Protection Forest, Cambodia, in February to May, 2008 and January to March, 2009 and recorded their behaviour at 5 minutes intervals, totalling 2736 observations. We classified each observation as either inactive (sleeping or alert) or active behaviour (travel, feeding, grooming, or others). Moon luminosity (bright/dark) and ambient temperature were recorded for each observation. The response variable, activity, was binary (active or inactive), and a logit link function was used. Ambient temperature alone did not significantly affect mean activity. Although mean activity was significantly affected by moonlight, the interaction between moonlight and temperature was also significant: on bright nights, studied animals were increasingly more active with higher temperature; and on dark nights they were consistently active regardless of temperature. The most plausible explanation is that on bright cold nights the combined risk of being seen and attacked by predators and heat loss outweigh the benefit of active behaviours.

Figure 1. Proportion of active behaviours across individuals in the study.

The standard error of the logit values were used to construct 95% confidence intervals indicated by the error bar.

Figure 2. Proportion of active behaviours across the night excluding periods of astronomical twilight.

The standard error of the logit values were used to construct 95% confidence intervals indicated by the error bar. Means are adjusted for other fixed effects.

Figure 3. Interaction between moonlight and temperature on activity of the pygmy loris as predicted by the model.

Figure 4. Location of Seima Protection Forest, Cambodia.

The study site is indicated on the map.

Figure 5. Frequency distribution of observations across moon phases.

Moon phase is expressed as the illuminated percentage of the portion of moon. When the moon was not above the horizon the moon phase was given a value of 0.

Figure 6. Mean, minimum and maximum nightly temperatures in the Seima Protection Forest during the study period.