In zoology, a crepuscular animal is one that is active primarily during the twilight period: being matutinal (active during dawn), vespertine/vespertinal (active during dusk), orboth. This is distinguished from diurnal and nocturnal behavior, where an animal is active during the hours of daytime andof night, respectively. Some crepuscular animals may also be active by moonlight or during an overcastday.
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 . The temperature may be toohigh at midday or toolow at night. Some creatures may adjust their activities depending on local competition.
The word crepuscular derives from Latin , . Its sense accordingly differs from diurnal and nocturnal behavior, which respectively peak during hours of daytime and night. The distinction is not absolute, because crepuscular animals may also be active on a bright moonlit night or on a dullday. Some animals casually described as nocturnal are in fact crepuscular.
Special classes of crepuscular behaviour include matutinal, or "matinal", animals active only in the dawn, and vespertine, only in thedusk. Those active during both times are said to have a activity pattern.
The various patterns of activity are thought to be mainly adaptations, though some could equally well be predatory adaptations. Many predators forage most intensively at night, whereas others are active at midday and see best in fullsun. 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 inecology.
Some predatory species adjust their habits in response to competition from other predators. For example, the of short-earedowl that lives on the is normally active during the day, but on islands like SantaCruz that are home to the , theowl is crepuscular.
Apart from the relevance to predation, crepuscular activity in 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 walnuttwig beetle, due to warmer temperatures, moderate wind speeds, and low barometric pressure.
Crepuscular activity can be influenced by the lunar cycle due to the change in nocturnal light levels. This creates changes in animal sleep, reproduction, and foraging behaviours; crepuscular animals often become less active during periods of lowlight.
Animal patterns of activity sometimes change during migration due to changes in environmental conditions. are crepuscular, but they are only active atdusk before and during migration. In the spring, they are only active atdawn because the snow is at its hardest, so it is easier for the deer to move without sinking in thesnow.
During migration, some types of swallow are active primarily during daytime hours with some activity during twilight hours.
Crepuscular animal activity is affected by human activity, because humans are diurnal (active duringday). Crepuscular animals are less likely to participate in typical foraging or reproductive behaviors and experience increased stress and mortality when humans are present.
Animals may change their usual activity patterns in response to the presence of humans. For example, Asian blackbears may avoid areas with high human activity during the day, but go to these locations during twilight or hours.
Light pollution impacts crepuscular behaviour because it mimics natural light conditions, leading crepuscular animals to behave as they would on nights with more moonlight.
Many familiar mammal species are crepuscular, including the endangered Amazon river dolphin, some species of bats, hamsters, housecats, , rabbits, ferrets, rats, jaguars, ocelots, bobcats, servals, strepsirrhines, , bears, deer, moose, sitatunga, capybaras, chinchillas, the common mouse, skunks, squirrels, foxes, wombats, wallabies, possums and marsupial gliders, tenrecs, and spotted hyenas.
Snakes, lizards, and frogs, especially those in desert environments, may be crepuscular.
Crepuscular birds include the common nighthawk, , , , American woodcock, , white-breasted waterhen, and European nightjar.
Many moths, beetles, flies, and other insects are crepuscular and vespertine. Forexample, , a SouthAmerican necrophagous beetle, is most active at dusk and dawn.