Nocturnal

Nocturnal organisms are those that are active mainly at night and sleep during the day. Their activity pattern occurs in twenty-four hour cycles, known as a circadian rhythm. Nocturnal animals include bats, cats, owls, mice, scorpions, opossums, raccoons, coyotes, cockroaches, and moths.

Nocturnal animals occupy a niche that is complementary to that of diurnal animals. For example, owls have a nighttime role similar to that of hawks during the day. Moths fill the same niche at night as butterflies do during the day.

Many nocturnal animals have specialized adaptations for their nighttime activities. The eyes of most nocturnal animals are larger than those of diurnal animals, helping them to function well in low light. Many nocturnal animals have large ears that are exceptionally sensitive. An acute sense of hearing is helpful when sight cannot be relied on in the darkness. Similarly acute senses of smell, taste, and touch also aid in overcoming the disadvantages of low light conditions.

Most bats have developed a special sonar system called echolocation. They make high frequency calls either out of their mouths or noses and then listen for echoes to bounce from the objects in front of them. This is an effective means of finding their way around in low light and catch their food (generally insects). Fruit bats, one of the few diurnal bats, lack the ability to echolocate. This demonstrates the evolution of characteristics favoring their particular niche, in this case daylight activity versus nighttime activity.

Cats' eyes are well adapted to nocturnal activity. The eyes are relatively large, with pupils that can open wide in the dark and narrow down to slits in the sunlight. The size and position of the eyes on the head allow as much light as possible to enter them and ensure a wide field of vision. These are important factors in hunting and nocturnal prowling. A cat cannot see in total darkness, but it can see better in dim light than can most other animals. Also, cats have large, erect ears that help in the detection of prey in the darkness.

see also Diurnal.

Denise Prendergast

Bibliography

Towle, Albert. Modern Biology. Austin, TX: Holt, Rinehart and Winston, 1993.

Internet Resources

Forest Preserve District of Du Page County. <http://www.dupageforest.com/CONSERVATIONIST/FALL98/night.html>.

nocturnal, an early instrument designed essentially for measuring the time of night by means of Polaris and either the ‘Pointers’ of the Plough in the constellation Ursa Major, or the Guards of the constellation Ursa Minor. It was first described in print by Michel Coignet in 1581. The earliest nocturnals consisted of two concentric circular plates of brass or wood. The circumference of the larger was divided into twelve equal parts corresponding to the months of the year and that of the smaller into 24 equal parts corresponding to the hours of the day. The larger plate carried a handle and the inner plate was fitted with a long index, one end of which was pivoted to the centre of the plate. To ascertain the time by the nocturnal a projecting tooth at the position corresponding to 12 o'clock on the smaller plate was turned to coincide with the date on the larger plate. The instrument was then held at arm's length and Polaris viewed through a small hole at the centre of the instrument. The index arm was then turned until the bevelled edge coincided with the line joining the ‘Pointers’ of Ursa Major, or the Guards of Ursa Minor as the case might be, and the time of night was then read off the scale of hours on the smaller plate.

Nocturnals were also used for working out the time of high water for which the establishment of the port was known. It was not too difficult a matter to relate the time of the moon's southing and the age of the moon by means of circular scales attached to the nocturnal. By applying the establishment of the port to the time of the moon's southing, the approximate time of high water, or ‘full sea’ as it was called, could readily be found.

nocturnal A historical hand-held device for telling the time at night using the orientation of northern circumpolar constellations. It consisted of a dial marked with the hours, with indicators to be aligned with specific stars, notably the Pointers of Ursa Major. When oriented with the stars and with Polaris in the centre, the time was shown against the appropriate date.

noc·tur·nal / näkˈtərnl/ • adj. done, occurring, or active at night: most owls are nocturnal. DERIVATIVES: noc·tur·nal·ly adv.

nocturnal During the night-time; applied to a type of circadian rhythm in which the organism performs its main activities at night.

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