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{{wikt}}
{{for|the atmospheric optical effect|crepuscular rays}}
[[Image:Photuris lucicrescens.jpg|thumb|250px|An adult [[Firefly]] (''Photuris lucicrescens'') or "Lightning Bug" – a crepuscular [[beetle]]]]
'''Crepuscular''' [[animal]]s are those that are active primarily during [[twilight]], that is during [[dawn]] and [[dusk]].<ref name="Winn">
{{Cite book
| title=Dictionary of Biological Psychology
| first=Philip | last=Winn
| publisher=Taylor & Francis
| page=194
| isbn=0-415-13606-7
| year=2001
| postscript={{inconsistent citations}}}}</ref>

==Etymology and usage==
The word ''crepuscular'' is derived from the [[Latin]] ''crepusculum'', meaning "twilight".<ref name="Winn">{{Cite book | title=Dictionary of Biological Psychology | first=Philip | last=Winn | publisher=Taylor & Francis | isbn=0-415-13606-7 | year=2001 | postscript={{inconsistent citations}}}}</ref> Its sense accordingly differs from [[diurnality|diurnal]] and [[nocturnality|nocturnal]] behavior, which respectively peak during hours of daylight and dark. The distinction is not absolute however, because crepuscular animals may also be active on a bright moonlit night or on a dull day. The use of the terms often is vague, for example, many animals that are casually described as nocturnal are in fact crepuscular.<ref>{{cite web|url=http://www.bookrags.com/research/crepuscular-ansc-01/ |title=Crepuscular Summary |publisher=BookRags.com |date=2010-11-02 |accessdate=2011-07-11}}</ref>

Special classes of crepuscular behaviour include '''[[matutinal]]''' (or "matinal") and '''[[vespertine (biology)|vespertine]]''', denoting species active only in the dawn or only in the dusk, respectively. Those that are active mainly during both morning and evening twilight are said to have a [[Bimodal distribution|bimodal activity pattern]].

==Adaptive relevance==
The various patterns of activity are thought to be [[antipredator adaptation]]s in the main, though some could equally well be predatory [[adaptation]]s. Many predators [[Foraging|forage]] most intensively at night, whereas others are active at midday and see best in full sun. Thus, the crepuscular habit may both reduce predation pressure, thereby increasing the crepuscular populations, and in consequence offer better foraging opportunities to predators that increasingly focus their attention on crepuscular prey until a new balance is struck. Such shifting equilibria are ubiquitous in ecology.

Some predatory species adjust their habits in in response to competition from other predators. For example, the subspecies of [[Short-eared Owl]] that lives on the [[Galápagos Islands]] is normally diurnal, but on islands like [[Santa Cruz Island (Galápagos)|Santa Cruz]] that are home to the [[Galapagos Hawk]], it is crepuscular.<ref>{{cite web|last=Frederick|first=Prince|title=Night herons in the day!|url=http://www.hindu.com/mp/2006/04/15/stories/2006041502340100.htm|work=Metro Plus Chennai|publisher=The Hindu|accessdate=15 January 2012}}</ref><ref>{{cite web|last=Merck|first=John|title=The community of terrestrial animals|url=http://www.geol.umd.edu/~jmerck/GEOL388/lectures/07.html|work=Field Studies II: The Natural History of the Galápagos Islands|publisher=University of Maryland Department of Geology|accessdate=15 January 2012}}</ref>

Apart from the relevance to predation, crepuscular activity in hot regions also may be the most effective way of avoiding thermal stress while capitalizing on available light.

==Occurrence of crepuscular behaviour==

A number of familiar [[mammal]] species are crepuscular, including [[hamster]]s, [[cat|housecats]], [[dog]]s,<ref>{{cite web|url=http://books.google.com/books?id=9k11of3lHJUC&pg=PA12&lpg=PA12&dq=dogs+crepuscular&source=bl&ots=p-JUEcvvCN&sig=ZtrkQN9VplAHUKPPyxMVBnXGv1E&hl=en&ei=mTgdTM_kBNWS4gb22dGWDg&sa=X&oi=book_result&ct=result&resnum=11&ved=0CEUQ6AEwCg#v=onepage&q=dogs%20crepuscular&f=false |title=The Ecology of Stray Dogs: A Study of Free-Ranging Urban Animals - Alan M. Beck - Google Books |publisher=Books.google.com |date= |accessdate=2012-04-13}}</ref> [[rabbit]]s, [[ferret]]s, [[guinea pig]]s, and [[rat]]s. Other crepuscular mammals include [[prosimian]]s, [[red panda]]s, [[deer]], [[moose]], [[chinchilla]]s, the common [[mouse]], [[skunk]]s, [[wombat]]s, [[quoll]]s, [[spotted hyena]]s, [[bobcat]]s, [[tenrecidae]], [[capybara]]s, [[African wild dog]]s, and the extinct [[Thylacine|Tasmanian tiger]]. Crepuscular birds include the [[Common Nighthawk]], [[Chimney Swift]], [[American Woodcock]], and [[Spotted Crake]].

Many [[moth]]s, [[beetle]]s, [[fly|flies]], and other [[insect]]s are crepuscular and in particular, vespertine.

==See also==
{{Light Ethology}}
* [[Crypsis]]
* [[Crepuscular rays]]

==References==
{{reflist}}

[[Category:Ethology]]

{{ecology-stub}}

Crepuscular - Revision history

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