Biogeography of time partitioning in mammals

Abstract

Many animals regulate their activity over a 24-h sleep-wake cycle, concentrating their peak periods of activity to coincide with the hours of daylight, darkness, or twilight, or using different periods of light and darkness in more complex ways. These behavioral differences, which are in themselves functional traits, are associated with suites of physiological and morphological adaptations with implications for the ecological roles of species. The biogeography of diel time partitioning is, however, poorly understood. Here, we document basic biogeographic patterns of time partitioning by mammals and ecologically relevant large-scale patterns of natural variation in "illuminated activity time" constrained by temperature, and we determine how well the first of these are predicted by the second. Although the majority of mammals are nocturnal, the distributions of diurnal and crepuscular species richness are strongly associated with the availability of biologically useful daylight and twilight, respectively. Cathemerality is associated with relatively long hours of daylight and twilight in the northern Holarctic region, whereas the proportion of nocturnal species is highest in arid regions and lowest at extreme high altitudes. Although thermal constraints on activity have been identified as key to the distributions of organisms, constraints due to functional adaptation to the light environment are less well studied. Global patterns in diversity are constrained by the availability of the temporal niche; disruption of these constraints by the spread of artificial lighting and anthropogenic climate change, and the potential effects on time partitioning, are likely to be critical influences on species' future distributions.

Keywords: cathemeral; night.

Fig. 1.

Examples of recorded diel activity patterns illustrating the four main time-partitioning strategies used to classify terrestrial mammals in this study (–67).

Fig. 2.

Phylogenetic tree showing 3,510 mammal species allocated to one of four time-partitioning strategies. The colored radial bars represent the dominant time-partitioning strategy for the species from an extensive literature search; the internal radial tree shows a species-level mammal phylogeny (55, 56). Selected major clades are shown.

Fig. 3.

Global distribution of mammal species richness, broken down by temporal niche.

Fig. 4.

Global annual totals of biologically useful daylight, twilight, and moonlight, and global distribution of artificial light used in this analysis. Biologically useful light is expressed as the ratio of the illuminated duration when the estimated temperature is between 0 and 35 °C, and the total duration between these thermal limits (daylight, twilight, moonlight, and darkness). Twilight is defined as the duration when the sun is less than 12° below the horizon; moonlight is the duration when the lunar disk is more than 75% illuminated, unobscured by cloud cover and the sun is more than 12° below the horizon. Artificial light derived from DMSP satellite data for the year 2009 from National Oceanic and Atmospheric Administration National Geophysical Data Center.

Fig. 5.

Global distribution of observed (Left) and modeled (Right) proportions of mammal species with different time-partitioning strategies. Pseudo-R² values, calculated following ref. as the squared Pearson correlation coefficient between observed and predicted values, are as follows: nocturnal R² = 0.701; diurnal R² = 0.213; crepuscular R² = 0.668; cathemeral R² = 0.736.

Fig. 6.

Effect sizes and relative importance of predictor variables in spatial error models. The bars represent the effect size, calculated as the magnitude and direction of the Akaike-weighted coefficients of terms in the set of models within 5 AIC units of the best model. Figures to the Right of bars represent the relative importance, defined as the proportion of models within the set in which the term is included.