Diel activity patterns—the distribution of activity throughout the daily cycle—are fundamental in animal ecology5. These patterns reflect when organisms seek food, socialize, and perform other necessary tasks while also accounting for risks1,2. Activity patterns vary among species. Some organisms may maintain activity over extended periods while others exhibit brief peaks6. They may be predominantly active during the night (nocturnal), day (diurnal), twilight (crepuscular), or may lack pronounced peaks with relation to day and night (cathemeral). Furthermore, there can be substantial variation within species and between populations6. Mammals illustrate a broad range of such behaviours.
While mammals today occupy all temporal niches (day, night, twilight), early mammal species are thought to have been primarily nocturnal to avoid the predation risk imposed by diurnal dinosaurs—an idea known as the “nocturnal bottleneck” hypothesis7. Following the extinction of the non-avian dinosaurs (66 Ma)8, mammals diversified and adapted to fill the available temporal niches7,9. Physiologic, morphological, and behavioural adaptations9 including endothermy, eye forms10, and enhanced sensorial systems allowed mammals to thrive under the different illumination and temperatures associated with day and night.
Endothermy permits mammals to exploit multiple temporal niches11,12. Nonetheless, species-specific physiological characteristics, in interaction with morphology (e.g., body size), may still favour activity schedules that moderate thermal stress13. For instance, in the absence of other factors, large species in warm regions may be forced to avoid overheating by avoiding activity in the hottest periods14. By contrast, small species that can lose heat rapidly may avoid cold and focus activity in warmer periods15,16. Small mammals such as mice and rats avoid diurnal predation by favouring nocturnal activity but may nonetheless be active during the daytime due to food scarcity, low nighttime temperatures, or low risks from diurnal predation1,17.
Species interactions may influence and control diel activity patterns within communities3,4. For instance, predators may favour periods where their prey are active, whereas prey species may avoid periods when their predators are active5,18. Potentially, this can involve both top-down or bottom-up processes19–21. Bottom-up and top-down are key classifiers for the regulation of food web dynamics19–21 and have the potential to influence how species within an assemblage may behave22. In a top-down process, the temporal activities of certain species (e.g., prey) seek to avoid the time use of others (e.g., predators)23. For example, small carnivores may avoid activity in periods when they are more likely to encounter larger predators, with similar avoidance expected for prey species to avoid their predators18,23. Alternatively, this can be a bottom-up process in which predator species match their activities to that of their prey or competitors22. For instance, mesopredators in south-western Europe were found to match their activity to that of their prey24. There is evidence for both bottom-up and top-down determination of activity patterns in a few sympatric species22–24. Yet, we do not know the degree to which bottom-up and top-down processes operate in nature and whether the resulting patterns are consistent across regions.
Humid tropical forests provide a useful context for exploring these questions as the influence of seasonality is low and similar environmental conditions are found in biogeographically distinct regions13. These forests encompass many of the most diverse and rich terrestrial biomes on earth and the maintenance of such diversity likely involves biotic interactions25. Trophic composition of tropical forest mammal communities appears relatively consistent across tropical regions26 and has been attributed to convergent evolution, likely due to similarities in environment and adaptations across distant forests27. We expect that the processes that shape trophic interactions and composition may also influence diel activity patterns.
We studied the daily activity patterns of ground-dwelling and scansorial mammals inhabiting protected tropical forests across the Neotropics, Afrotropics, and Indo-Malayan tropics (Fig. 1). We used time-stamped images from standardized large-scale camera-trap surveys implemented by the Tropical Ecology Assessment and Monitoring (TEAM) Network in 16 protected areas (Table S1)28. Using multinomial analysis, we investigated how diurnal, nocturnal, and crepuscular activity was related to trophic guild and body size and whether any such patterns were consistent among regions.
We tested three hypotheses (Fig. 2). First, if top-down processes regulate the diel activity of animals in a community (H1), we predicted (1a) that prey species (e.g., herbivores) should exhibit diel activity patterns that avoid those of predators (e.g., carnivores and omnivores) of a similar size (interguild avoidance), and (1b) smaller members of a trophic guild (especially carnivores and omnivores) should exhibit diel activity patterns that avoid that of larger members of the same guild (intraguild avoidance). If bottom-up processes regulate the diel activity of animals in a community (H2), then (2) diel activity patterns of predators should match that of prey species (herbivores, insectivores, and small omnivores). Finally, if the energetic cost of thermoregulation constrains diel activity of tropical mammals (H3), then (3) large mammals should be more active during the night when it is colder and small mammals more active during the day when it is warmer.
We extracted the probability for the activity (0–1) during day, night, and twilight, and the correspondent upper (UCI) and lower (LCI) 95% confidence intervals for the given range of body mass and trophic guild derived from the multinomial model in every region with the lowest AIC. Diel activity was best modelled when including body mass, trophic guild, and their interaction for the three regions (Fig. 3, Table S2).
Fig. 3. Distribution of daily activity in relation to body size and trophic guilds of tropical ground-dwelling and scansorial mammals in three regions. Estimates correspond to the probability of activity during the day, night, and twilight extracted from the model fitted to TEAM camera-trap data. Tick marks above the x-axis indicate the typical body mass of species analysed. Colour hue indicates where the model interpolates among observations of the sizes presented (darker) versus extrapolates beyond values in the data for that trophic guild and region (lighter).