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Creatures of the Night (Fall, 1991)  Printer friendly version Printer friendly version
Working the Night Shift

by Mary Taylor Gray (Young)

Creatures of the Night

Working the Night Shift
A Little Night Music
DOW–Working for Wildlife
Colorado Taxpayers Help Nongame and Endangered Wildlife

Cover of the fall, 1991 issue of CWC, "Creatures of the Night".Nocturnal—Active at night.
Diurnal—Active primarily in daylight.
Crepuscular—Active at dawn and dusk.

Each evening the visual world changes–light fades, images become indistinct, color disappears. The sun retreats, and humans–along with other sight-dependent creatures of daylight–do the same. As people close their doors against the darkness, Colorado's meadows and woodlands come alive with creatures of the night.

Many animals work the night shift because darkness offers them protection from the heat of the day (see Colorado's Wildlife Company, 1990 summer compendium). Others seek the darkness to hide from predation. While some prey animals come out only at night, others are primarily active after dark, but sometimes work during the daylight hours. As prey behavior dictates predator behavior, a large number of predators hunt during the night. Nocturnal predators also fill an ecological niche distinct from diurnal predators. By hunting at night, for example, most owls do not compete with diurnal raptors. And since most hunters are, in turn, hunted by something else, the predators also find protection in darkness. Prey and predator species that successfully work the night shift have adapted in various ways to their light-reduced environment.

Night Eyes


grasshopper mouseSome animals have successfully adapted to the night shift with an enhanced ability to see in reduces light. A variety of modifications enable "night eyes" to gather as much light as possible. Nocturnal species, like owls and ringtails, have large eyes with pupils that dilate widely. Enlarging the pupil four times its original size increases the amount of light the eye can gather by a factor of sixteen. Light-colored areas of fur or feathers (sometimes called "eye disks") surrounding the eyes also increase the light reflected into the eyes. The back portion of the night eye is oversized, producing a larger image. Owl eyes are so large there is no room for muscles to move the eyes; they move their head instead. An owl's flexible neck can be turned more than 180 degrees to either side, so an owl can look directly backwards.

If your headlights have ever caught the luminescent glow of a deer's eyes you've seen another night-vision mechanism at work. The tapetum, a mirror-like membrane behind the retina, produces this eye shine. Light that passes the retina without being absorbed is reflected back by the tapetum, increasing light received by the retina.

Hearing and Silence


Despite their large eyes, owls do not rely on sight to hunt; an owl can locate and nab prey unerringly using only its finely tuned sense of hearing. Its ears are asymmetrical–one is set higher on the head than the other–allowing the bird to orient in a vertical as well as horizontal plane relative to which ear the sound reaches first. In fact, research has verified that a barn owl sitting 10 feet away from a mouse in an unlit barn can locate the mouse accurately to within 3 inches.

Because prey animals often rely on sound to alert them, the owl has evolved as a silent hunter. With wide, rounded wings and soft feathers, it flies soundlessly. A serrated edge of the first primary feather breaks up and silences the flow of air across the wing. Owls also hunt the same territory night after night, relying on knowledge of the terrain to aid their hunt.

Echolocation


Spring through fall, bats patrol the night skies and gobble up thousands of insects in one shift. True specialists of the night, these amazing winged mammals use echolocation to navigate and to locate prey in the darkness. They emit ten to two hundred high-pitched pulses per minute, and the frequency increases as the hunter closes in on its victim.

Produced by the larynx, the high-pitched sounds of echolocation pass through the bat's nostrils or open mouth (which is why bats appear to be ferociously baring their teeth as they fly). Some bats have horseshoe or leaf-like appendages on their noses to help direct these sounds. The bat's huge ears pick up the echoes as the sounds bursts bounce off objects. They judge position and distance to object or prey by the time elapsed between emitting a signal and reading its echo. Imagine pinpointing insects as small as mosquitoes strictly from reflected sound waves when both bat and insect are moving!

The emitted pulses are so loud they would be comparable to the deafening roar of a jet engine if we could hear them. To avoid being deafened by their own calls, bats have sound-dampening structures in their ears. With navigational and data gathering skills like that, who needs daylight?

Smell and Heat Sensors


raccoonNeeding to avoid summer heat, most snakes are nocturnal. At the same time by human standards, snakes have poor vision and are nearly deaf. So how does a limbless, blind, and deaf creature hunt at night?

Although snakes lack both eardrum and external ear, they can sense vibrations through the ground. And by flicking out its tongue, a snake "tastes" the air, bringing scent molecules to receptors called a Jacobson's organ in the roof of the mouth. With this highly sensitive equipment, a snake can track a prey animal through grass like a hound on the scent.

Rattlesnakes can detect infrared radiation emitted from warm-blooded animals. Specialized pores (loreal pores) along the snakes' upper jaw sense external sources of body heat–especially useful for finding mice after dark on a cool Colorado evening. Pit vipers, as these heat-sensing snakes are called, can sense the presence of a fist-sized mouse one foot away. They also can sense temperature changes of one thousandth of a degree centigrade.

Collective Senses


Many nocturnal animals lack the high-profile adaptations of real night specialists like bats and owls; instead they call on their collective senses of sight, smell, and hearing to make nighttime living work for them.

Mammals: Some nocturnal mammals are familiar to people, while others are rarely seen. The bandit-masked raccoon is a well-known nocturnal hunter. The raccoon's shy cousin, the ringtail, hunts at night and is almost never seen by people. One of the ringtail's primary prey animals is the nocturnal woodrat or pack rat.

Kangaroo rats are classic nocturnal rodents. You won't see them by day because they shelter in their burrows, but shine a light across their grassland or desert habitat at night, and you'll see a world alive with jumping 'roo rats. Kit foxes, too, are almost entirely nocturnal, perhaps because kangaroo rats are a primary food source.

Other mammals are sometimes seen during the day, but they do most of their food gathering and hunting at night. The badger–a virtual digging machine–sniffs out mice, gophers and other rodents in burrows below the ground and digs rapidly to uncover them. The carnivorous grasshopper mouse patrols rodent runs at night, preying on other nocturnal mice (including other grasshopper mice!) as well as grasshoppers. Through equipped with their own unique defenses, both the skunk and the porcupine prefer nighttime for their foraging.

North America's only marsupial, the opossum, wakes at twilight to begin its search for fruit, roots, insects, small mammals or anything it can find. With few defenses beyond its habit of playing dead when threatened, the opossum derives protective advantage from the dark. (P.S. For you opossum fans: This animal also gains survival advantage from its high reproductive rate. And–a little known fact–the opossum has the greatest number of teeth of any North American mammal.)

Herptiles (Amphibians and Reptiles): As with many other nocturnal animals, darkness affords salamanders, frogs and toads better protection from predators. It also increases their hunting success because their insect prey is more active and plentiful at night. But most herptiles are nocturnally active because it helps their temperature regulation. With moist, porous skin, the tiger salamander must avoid the drying effect of the hot sun. Salamanders emerge at dusk from their daytime hiding places under leaf litter or at a pond's edge.

Fish: Some fish are more active at night then during the day. Brown trout increase their hunting success by feeding on aquatic insects that drift downstream at night. Lake trout move higher in the water as the sun goes down.

Next: A Little Night Music

(The information contained in these issues of Colorado's Wildlife Company was accurate at the time of original publication. Situations and circumstances described, staff positions, contact information, and dates of some events may have changed in the interim. Present knowledge and understanding of biological and behavioral facts and information may also be different, now, than presented here.)


        Last Updated: 5/12/2009 2:29 PM