Thermoenergetic challenges and daytime behavioural patterns of a wild cathemeral mammal

Abstract

The circadian thermoenergetics hypothesis (CTEH) suggests that endotherms benefit by being active during the warmer period of a 24 h cycle and by resting when temperatures drop, since this lowers energetic investment in thermoregulation. In accordance with the CTEH, cathemeral (i.e. active during both daytime and night-time) Azara's owl monkeys, Aotus azarae, of the Argentinean Chaco increase their daytime activity during the relatively cold winter compared to the summer. Still, it remains unclear whether these behavioural changes are explained by major shifts in energy balance. Thus, we sought to understand how thermal challenges influence the allocation of behaviours during the daytime that tend to conserve energy (resting), increase its availability (foraging) or expend it (travelling). We constructed a priori a set of linear models to evaluate the relationships between daytime behavioural patterns and photoperiod, moonlight, age, reproductive status and hourly, daily and weekly fluctuations in ambient temperatures, as well as some of their interactions. We analysed 4985 20-minute focal samples collected during 0600–2100 hours from 140 recognizable individuals from 15 groups across 13 years. Our results indicate that, from the warmest to the coldest months, daytime resting frequencies decreased by 31%, whereas daytime foraging frequencies increased by 131%, while travelling frequencies remained unchanged. Daytime activity patterns were explained by the interaction between weekly average temperatures and either lunar activity, age class or reproductive status. Generally, daytime activity increased as temperatures decreased seasonally, but this is effect was masked during full moons; and infants, juveniles, subadults and solitary adults used the daytime differently. Finally, travelling and foraging increased during longer days or at times of daily maximum temperatures but decreased when these two factors interacted (i.e. long summer days at daily maximum temperatures). In summary, daytime behaviours of A. azarae may be structured by thermoenergetic investment, as predicted by the CTEH.