Does energy minimisation constrain behavioural plasticity? Long-term activity budgets of a model folivore-frugivore.

Optimal foraging theory predicts that time-minimising foragers should reduce activity when resources are abundant, yet empirical tests examining how dietary generalists modulate activity budgets in response to multiple environmental and intrinsic factors remain scarce. We analysed 6693 focal samples collected over 10 years from 21 mantled howler monkeys (Alouatta palliata) inhabiting a tropical rainforest in Los Tuxtlas, Mexico, to test predictions derived from energy minimisation theory. Using beta regression models with cyclical time controls, we examined how young leaf availability, fruit availability, ambient temperature, relative humidity, daily rainfall, age and reproductive state influenced the proportion of time allocated to resting, feeding, locomotion and social behaviour. Young leaf availability exerted effects nearly 40 times stronger than fruit availability on activity budgets, with increased leaf availability associated with reduced feeding and increased resting. Temperature positively predicted resting time and negatively predicted feeding time, with post hoc breakpoint analysis identifying a threshold at 31.6°C above which resting increased sharply, consistent with a transition from behaviourally flexible to thermally enforced inactivity. Relative humidity did not significantly predict any activity, suggesting temperature alone drove the thermoregulatory constraint on behaviour under the conditions observed. Lactating females rested more than cycling females but did not increase feeding time, suggesting energy conservation rather than intake maximisation during the most energetically demanding reproductive phase. Age had no detectable effect on any activity category across the 3-31-year age range, indicating that time allocation strategies are established early and maintained throughout adulthood. Variance attributable to year exceeded that attributable to individual identity or social group, highlighting the importance of interannual environmental fluctuation in shaping behavioural flexibility. Contrary to predictions that resting should remain invariant as an energetic baseline, resting was the most responsive activity to environmental variation, functioning as a behavioural buffer that absorbs fluctuations in resource availability and thermal conditions, although this flexibility is constrained above a critical temperature threshold with implications for population resilience under projected climate warming. These findings support time-minimisation strategies in folivore-frugivores and provide a framework for understanding behavioural responses to environmental change in energy-limited tropical primates.