The moon's influence on the activity of tropical forest mammals

Abstract

Changes in lunar illumination alter the balance of risks and opportunities for animals, influencing activity patterns and species interactions. We examined if and how terrestrial mammals respond to the lunar cycle in some of the darkest places: the floors of tropical forests. We analysed long-term camera trapping data on 86 mammal species from 17 protected forests on three continents. Conservative categorization of activity during the night revealed pronounced avoidance of moonlight (lunar phobia) in 12 species, compared with pronounced attraction to moonlight (lunar philia) in only three species. However, half of all species in our study responded to lunar phases, either changing how nocturnal they were, altering their overall level of activity, or both. Avoidance of full moon was more common, exhibited by 30% of all species compared with 20% of species that exhibited attraction. Nocturnal species, especially rodents, were over-represented among species that avoided full moon. Artiodactyla were more prominent among species attracted to full moon. Our findings indicate that lunar phases influence animal behaviour even beneath the forest canopy. Such impacts may be exacerbated in degraded and fragmented forests. Our study offers a baseline representing relatively intact and well-protected contexts together with an intuitive approach for detecting activity shifts in response to environmental change.

Keywords: camera trapping; diel activity; lunar phases; lunar philia; lunar phobia; temporal niche.

Figure 1.

Illustration of the study design. (a) Time-stamped camera trap images are aggregated into 15 min intervals and mapped onto available site-specific diel and lunar periods. (b) Red dots in the example belong to the nine-banded armadillo (Dasypus novemcinctus), an apparently lunar phobic species. Multinomial logistic regression models are used to quantify the probability of a species using a given diel or lunar period. (c–h) Three analyses explore (1) diel activity patterns (c) and the prevalence of lunar phobia and lunar philia during nocturnal activity (d), (2) the effect that the level of nocturnality of a species (e) has on its propensity to exhibit lunar phobia (f), and (3) changes in diel activity patterns and total activity levels during periods with full moon (g) versus other lunar phases ( h ). The blue boxes delineate the part of the diel region involved in each assessment or comparison. Photos: TEAM Network.

Figure 2.

Posterior samples (dots) of multinomial probabilities mapped onto ternary diagrams. Shown are example posterior samples for diel activity (left; activity during day, night and twilight periods) and lunar activity (right; nocturnal activity during full moon, new moon and transitional phases). The ternary diagrams are divided into seven and four regions for diel and lunar activity delineation, respectively. Designation to activity categories (grey text) is made according to the region into which the majority (coloured dots) of posterior samples are mapped. The examples show a diurnal-crepuscular (left) and a lunar phobic species (right).

Figure 3.

Diel and lunar categorization charted across phylogenetic trees of tropical forest mammals in three realms. The distribution of tropical moist broadleaf forest (green regions) and location of study areas (black dots) included in this analysis are shown on the map. Lunar responses are shown for species that exhibited altered activity patterns suggesting avoidance of or attraction to the moon. Species exhibiting lunar phobia or lunar philia based on strict categorization of nocturnal activity are marked with an asterisk. Lunar phobia, manifested as reduced activity during moonlit nights, was more common than lunar philia, manifested as increased activity during moonlit nights. Rodents, particularly nocturnal species, were overrepresented among lunar phobic species, followed by members of the Cingulata (including armadillos) and Didelphimorphia (opossums). Phylopic silhouettes credits: Cuniculus paca, Dasypus novemcinctus, Silvilagus brasiliensis and Tayassu pecari by Gabriela Palomo-Munoz; Philander opossum by Milena Cavalcanti, Patricia Pilatti and Diego Astúa; Hoplomys gymurus, Atherurus africanus, Cricetomys emini, Hystrix brachyura and Leopoldamys sabanus provided by Anonymous; Petrodromus tetradactylus under universal public domain license.

Figure 4.

Overview of ternary classifications for diel and lunar activity (columns 1 and 2) and responses to lunar phases (columns 3 and 4) for four example species (rows). The species shown in the top row (common tapeti or forest cottontail) was classified as lunar philic, whereas the bottom three rows show lunar phobic species. Column 1: ternary plots of diel activity posteriors. Column 2: ternary plot of lunar activity posteriors. Column 3: ternary plots showing difference of diel activity (potential temporal niche shifting) between periods with full moon (green) and without moonlit nights (orange). Column 4: posterior distribution of the difference between overall activity (related to the number of photographic detection events) during periods with versus without full moon. Negative (red) and positive (blue) values indicate a reduction or an increase, respectively, in overall activity during periods (multiple 24 h periods; figure 1) with full moon. See electronic supplementary material, figures S1–S3, for results for all species classified as either lunar phobic or philic.