A number of factors affect the time of day an animal is active.
Predators hunt when their prey is available, and prey try to avoid the times when their principal predators are at large. The temperature may be too high at midday or too low at night.[2] Some creatures may adjust their activities depending on local competition.
Etymology and usage
The word crepuscular derives from the
Latincrepusculum ("twilight").[3] Its sense accordingly differs from diurnal and nocturnal behavior, which respectively peak during hours of daylight and darkness. The distinction is not absolute, because crepuscular animals may also be active on a bright moonlit night or on a dull day. Some animals casually described as nocturnal are in fact crepuscular.[2]
Special classes of crepuscular behaviour include matutinal, or "matinal", animals active only in the dawn, and vespertine, only in the dusk. Those active during both times are said to have a bimodal activity pattern.
Adaptive relevance
The various patterns of activity are thought to be mainly
antipredator adaptations, though some could equally well be predatory
adaptations.[4] Many predators
forage most intensively at night, whereas others are active at midday and see best in full sun. The crepuscular habit may both reduce predation pressure, increasing the crepuscular populations, and offer better foraging opportunities to predators that increasingly focus their attention on crepuscular prey until a new balance is struck. Such shifting states of balance are often found in ecology.
Some predatory species adjust their habits in response to competition from other predators. For example, the subspecies of
short-eared owl that lives on the
Galápagos Islands is normally active during the day, but on islands like
Santa Cruz that are home to the
Galapagos hawk, the owl is crepuscular.[5][6]
Apart from the relevance to predation, crepuscular activity in hot regions also may be the most effective way of avoiding heat stress while capitalizing on available light.
Crepuscular flight activity is preferred by some animals, such as the
walnut twig beetle, due to warmer temperatures, moderate wind speeds, and low barometric pressure.[7]
Crepuscular activity can be influenced by the
lunar cycle due to the change in nocturnal light. This creates changes in animal
sleep,
reproduction, and
foraging behaviours, often becoming less active during periods of low light.[8]
Migration
Animal patterns of activity sometimes change during
migration due to changes in environmental conditions.
Mule deer are crepuscular, but they are only active at
sunset before and during migration. In the
spring they are only active at
sunrise because the
snow is at its
hardest, so it is easier for the deer to move without sinking in the snow.[9]
During migration, some types of
swallow are active primarily during daytime hours with some activity during twilight hours.[10]
Human impact on crepuscular behaviour
Crepuscular animal activity is affected by
human activity, because humans are diurnal. Crepuscular animals are less likely to participate in typical foraging or reproductive behaviors and deal with increased
stress and
mortality rates when humans are present.[11] Animals may change their usual
activity patterns in response to the presence of humans. For example,
Asian black bears may avoid areas with high human activity during the day, but go to these locations during twilight or nighttime hours.[12]
Light pollution impacts crepuscular behaviour because it mimics natural light conditions, leading crepuscular animals to behave as they would on nights with more moonlight.[8]
^Merck, John.
"The community of terrestrial animals". Field Studies II: The Natural History of the Galápagos Islands. University of Maryland Department of Geology. Archived from
the original on 23 June 2012. Retrieved 15 January 2012.
^"White-Tailed Deer". Animals. National Geographic Partners, LLC. 12 March 2010. Archived from
the original on October 23, 2016. Retrieved July 15, 2017.