Crepuscular and cathemeral species may have intermediate adaptations ( 15), and behavior may be flexible to vary within species and among individuals according to factors such as time of year, habitat structure, food availability, age, temperature, and the presence or absence of predators ( 16 – 18). As properties of organisms that strongly influence performance within a particular environment, these strategies can be considered functional traits in themselves ( 14), but are also associated with suites of adaptations, with implications for the ecological roles of species and individuals. Strict nocturnality and diurnality are hence two ends of a continuum of possible strategies for partitioning time over the 24-h cycle. Thus, some species are apparently obligately diurnal in their peak activity patterns, some obligately nocturnal, obligately crepuscular (active primarily during twilight), or obligately cathemeral (significant activity both during daylight and night), and others make facultative use of both daylight and night ( 13), or show seasonal and/or geographical variation in their strategy. Behavioral traits are associated with a range of specialized adaptations, particularly in visual systems and eye morphology ( 11) and energetics and resource use ( 6, 12). Where energetic and thermal costs are not prohibitive, temporal niche partitioning may occur as species specialize and avoid competition by concentrating their activity within a particular section along the light gradient ( 9, 10). Furthermore, energetic constraints may force some species to be active throughout hours of both light and darkness ( 8). Thermal constraints may limit nocturnal activity when nighttime temperatures are low, and diurnal activity when temperatures are high. Activity during both daylight and semidarkness may be further constrained by covariance between the natural cycles of light and temperature the metabolic costs of thermoregulation place constraints on the time available for activity ( 7). During the hours when the sun is below the horizon, there is seasonal and latitudinal variation in the duration of “biologically useful semidarkness” in the form of twilight and moonlight ( 1), modified by both the lunar cycle and variable cloud cover, providing changing opportunities for animals able to use visual cues for key behaviors including foraging, predator avoidance, and reproduction ( 2 – 6). The rotation of the Earth partitions time into regular cycles of day and night, and although all points on the Earth’s surface receive roughly equal durations of light and darkness over the course of a year, at mid to high latitudes seasonal variation in day length imposes an uneven distribution throughout the annual cycle. Natural cycles of light and darkness structure the environment of the majority of eukaryotic organisms. 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. 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. 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 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. 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. The biogeography of diel time partitioning is, however, poorly understood. 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. 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.
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