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Nocturnality is an animal behavior characterized by activity during the night and sleeping during the day. The common adjective is "nocturnal" (e.g., Bats are nocturnal.).
Nocturnal animals generally have highly developed senses of hearing and smell, and specially adapted eyesight. Some animals, such as cats & ferrets, have eyes that can adapt to both low-level and bright day levels of illumination (see metaturnal). Others, e.g. bushbabies and (some) bats, can function only at night. Many nocturnal creatures have eyes that seem to be "too big" compared to the rest of their head and body like tarsiers and some owls.
Diurnal animals, e.g. squirrels, songbirds, etc., are active during the daytime. Crepuscular species, i.e. rabbits, skunks, and hyena, are often erroneously referred to as nocturnal. Cathemeral species, like fossas and lions, are active both day and night.
Being active at night is a form of niche differentiation, where a species' niche is partitioned not by the amount of resources but by the amount of time (i.e. temporal division of the ecological niche). Hawks and owls can hunt the same field or meadow for the same rodents without conflict because hawks are diurnal and owls are nocturnal.
Nocturnality is a form of crypsis, an adaptation to avoid or enhance predation. One of the reasons that (cathemeral) lions prefer to hunt at night is that many of their prey species (zebra, antelope, impala, wildebeest, etc.) have poor night vision. Many species of small rodents are active at night because most of the dozen or so birds of prey that hunt them are diurnal. There are many diurnal species that exhibit some nocturnal behaviors. For example, many seabirds and sea turtles only gather at breeding sites or colonies at night to reduce the risk of predation to themselves and/or their offspring.
Another reason for nocturnality is avoiding the heat of the day. This is especially true in arid biomes like deserts, where nocturnal behavior prevents creatures from losing precious water during the hot, dry daytime. This is an adaptation that enhances osmoregulation. One of the reasons that (cathemeral) lions prefer to hunt at night to conserve water.
Many plant species native to arid biomes have adapted so that their flowers only open at night when the sun's intense heat can't wither and destroy their delicate, moist blossoms. These flowers are pollinated by bats, another creature of the night.
In zoos, nocturnal animals are usually kept in special night-illumination enclosures to reverse their normal sleep-wake cycle and to keep them active during the hours when visitors will be there to see them.
Hedgehogs and sugar gliders are just two of the many nocturnal species kept as (exotic) pets. Cats have adapted to domestication so that each individual, whether stray alley cat or pampered housecat, can change their activity level at will, becoming nocturnal or diurnal, to suit their environment and/or the routines of their owners.
A person who is nocturnal is referred to as a night owl; he or she is of the "eveningness" chronotype. A nocturnal lifestyle can be the result of certain jobs or careers, a busy social life, or symptoms of certain diseases.
- Niche (ecology)
- Niche differentiation
- Competitive exclusion principle
- Antipredator adaptation
- List of nocturnal birds
- List of nocturnal animals
- N.A.Campbell (1996) Biology (4th edition) Benjamin Cummings NY. ISBN 0-8053-1957-3
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