Night Creatures

At twilight's last gleaming, the desert literally comes alive with ... Night Creatures

When the desert sun at last descends below the horizon, the land seems to emit a sigh of relief, the air becomes perceptibly cooler, and hues of the arid terrain appear more vivid, washed with a gentler light. This is the "golden hour," when day begins to blend into night—and when a seemingly deserted landscape starts to reveal its shy inhabitants.

At rare desert ponds and streams, animals cautiously approach for a drink. The larger and more familiar of the desert mammals (coyotes, mule deer, javelinas, bobcats, and jackrabbits) are all dependent on sources of water. During hot, dry periods of the year, they are

largely nocturnal, for positive moisture balance is more easily maintained at night, thanks to lower temperatures and higher humidity. Smaller mammals, such as ground squirrels and kangaroo rats, are independent of pools of water. These seemingly thirstless creatures obtain all the moisture they need from the food they consume. But they, too, prefer the night, for reasons both of water conservation and of safety.

It is more than coincidence that only small mammals can survive without drinking. A principal cause of water loss in animals is the expelling of waste products from the body. The main components of mammals' urine are water and urea, a nitrogenous compound, and the percentage of water in the urine is dependent on kidney efficiency. Smaller animals have more efficient kidneys and lose relatively less water than larger animals. In the absence of this characteristic, small mammals would be far more rare in arid lands, since they would be restricted to the immediate vicinity of a permanent water supply.

After evaporation from the surface of the body and mouth, breathing is probably the most important cause of moisture loss in desert conditions. In humans, for example, in order to exchange carbon dioxide For oxygen inside the lungs, lung tissue must be moist. After inhaling the dry desert air, we exhale moist air. Over the course of a day, this can result in a significant loss of water.

Desert animals have evolved various ways to limit the moisture lost in breathing. Kangaroo rats, for instance, have a relatively long and narrow nasal passage, as compared to larger mammals, which creates more surface area for passing air to contact. As air leaves the lungs, much of its moisture is recaptured before it is expelled from the body. The rats also spend the day in deep burrows where the surrounding air is cooler and moister.

Once the last red and yellow hues of sunset have faded and are replaced by the cool white light of the moon and stars, the desert is a world almost devoid of color. But even the sunset is just shades of gray to most of the creatures of the night, since good night vision comes at the expense of color vision. And for many of these animals, the world is sensed more through degrees of seductive or warning odors and alluring or alarming sounds than sight.

One of the more curious investigators is the banner-tail kangaroo rat. When danger threatens, its agile evasive actions deny predators more than one shot at making a meal of it. Each night the kangaroo rat sets out on trails formed by generations of animals in search of food. The rat is fairly easy to see because of a conspicuous white patch on the tufted tip of its long tail. For a relatively helpless creature, this seems an inappropriate advertisement declaring, "Here I am!"

But the patch serves a purpose, often a lifesaving one-particularly when the rat's nemesis, the great horned owl, soars overhead. When that happens, the scenario may go something like this: Unaware of the silent wingbeats of the bird, the kangaroo rat scurries across an open field. With a final burst of speed, the owl makes a strike. But the owl hits the rat, not on its head, but On the dancing white tail spot! At the same moment, the rat shoots into the air and off to one side. And, as the rat disappears into one of several holes in a bare mound, probably an ancient mansion of its ancestors, the owl is left to contemplate a few white hairs in its talons.

Obviously the white tail-tip is an advertisement, but one that draws attention away from the rat's body. This trick of attracting a predator's attention to the relatively expendable tail is very common in nature, and in the desert, it is best developed in the lizards. Many of these reptiles, including the nocturnal gecko, are able to let the tail drop off, seal the wound by constricting muscles at the break point, and eventually grow a new tail. Once the tail is detached, it continues to wriggle and squirm by reflex action; with luck, this will distract the predator long enough for the lizard to escape. In such cases, the predator has not been completely cheated since the tail can be eaten, and the gecko's tail, which is used to store excess fat, is particularly nutritious.

One of the best-adapted creatures of the night is the rattlesnake. Though deaf, as are all snakes, it has an excellent sense of smell and an ability to locate warm objects. Most snakes and many lizards smell with their tongues. All those with a keen sense of smell have forked tongues which they flick into the air to catch small bits of the chemicals that convey odors. After sampling the air, the tongue is placed into a groove on the roof of the mouth (the Jacobson organ) where receptors identify the odor.

The heat-sensing organs of rattlesnakes have given this group the name "pit vipers." On either side of its head, between nostril and eye, a rattlesnake has a hole or pit. Inside of these pits are receptors that can sense the infrared radiation given off by warm-blooded animals. Even in absolute darkness, without the use of its acute

(BELOW) Providing life-sustaining water, a thunderbead bursts over the desert, helping to create breeding habitats for (RIGHT) the buge Colorado River toad and (FAR RIGHT) Couch's spadefoot toad. A rainstorm of sufficient intensity and volume triggers a breeding frenzy among the spadefoots, but the Colorado River toads' breeding signal is still a secret. Both species lay their eggs in short-lived desert pools; to survive, the young must batch before the water and mud dry and disappear.

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Text continued from page 17 sense of smell, a rattlesnake can accurately strike at and kill a small mammal.

In general, for every improvement in a predator's ability to hunt, there tends to emerge a subsequent improvement in the prey's ability to escape. Biologists call this the "Red Queen paradox" or hypothesis, from the Red Queen's conversation with Alice in Lewis Carroll's Through the Look-ing-Glass: "...it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!"

For the most part, moths are deaf, but some moths have evolved the ability to sense sound in a very narrow range of frequencies-the same frequencies a bat uses for its sonar. When a flying moth detects the bat's sonar field, it can suddenly drop out of the air like a dead leaf. Even so, of course, there are no guarantees of survival, especially if the moth stays on the ground. There are many hunters out at night scouring the ground, inspecting cracks, even searching under rocks. The earthbound moth could encounter a gecko, scorpion, tarantula, or perhaps a grasshopper mouse or desert shrew.

One of the more interesting moths of the desert night is the small white yucca moth, which is a faithful pollinator of the yucca plant. The yucca and the yucca moth have developed a mutually advantageous relationship. The most efficient way for the flowers of this plant to be pollinated is for the yucca moth to scoop up a sticky yellow ball of pollen, take the ball to another flower, and physically stuff it down a tube at the top of this flower's female organ. The moth's motivation and reward are the opportunity to lay its eggs on the soon-to-befertilized ovules of the yucca flower. As the yucca seeds begin to develop and grow, the moth eggs hatch, and the larvae feed on the seeds. Some of the seeds survive, are dispersed, grow into new plants which eventually flower-and the cycle continues.

The cactus known as the night-blooming cereus, the flowers of which open in early evening and wilt shortly after sunrise, also has a special relationship with a particular insect: the hawkmoth.

Some species of hawkmoths are quite large, and a three-inch wingspan is not unusual. They beat their wings very rapidly and, like the hummingbird, they hover at the mouth of a flower while they reach deep within it for the nectar at its base. The hawkmoth uses its proboscis (mouth parts elongated to form a drinking straw) to probe the flower. When not using it for sucking nectar, the moth carries its proboscis under its chin, tightly wound like a watch spring.

Moths are also very sensitive to odors. The males of most species pursue and locate mates by following a subtle odor trail through the air. No insect has a nose. All breathe through tiny pores, called spiracles, which pass through various parts of their bodies. The antennae are the organs that detect odors. When the female moth is ready to mate, she releases a tiny quantity of an extremely potent chemical called a pheromone. The male is able to follow this scent to the female, and he discriminates between this odor and those produced by females of another species.

Ever since the darkness of night has become complete, giant toads (about the size of guinea pigs) have been leaving their underground homes and traveling to the beckoning moisture of a pond. Through some unspoken agreement, the toads have decided to begin breeding this night.

Colorado River toads do not need a violent summer rainstorm to initiate breeding, unlike the spadefoot toads which also live in this desert. Virtually all breeding by spadefoot toads, at any one pond, takes place on one frenzied night. Indeed, we know much more about the spadefoot toads than these familiar giants, the Colorado River toads.

West of Tucson, Clayton May of Pima Community College and I have marked and followed more than 300 Colorado River toads over the last six years. Yet we still do not understand what causes them suddenly to decide to travel a mile or more (FAR LEFT AND CENTER) The yucca cactus and the yucca moth have a symbiotic relationship: the moth pollinates the plant, and its offspring feed on the yucca seeds. (ABOVE) The night-blooming cereus, as its name indicates, opens its dramatic flowers only at night. While appearing as weathered sticks above ground, the plant grows from a 50-pound turnip-like edible root. (RIGHT) The ring-tailed cat, often found in canyons, can rotate its hind feet 180 degrees a characteristic that helps to make it an excellent climber.

to a pond where they will meet other toads, choose mates, and breed. We do know that they have an amazing sense of direction. We have transported toads as much as a mile from their burrows, to which they often return within 24 hours.

Tonight early arrivals at the pond have already taken up a chorus, a tiny strangled scream that can hardly be heard across the water. Perhaps this puny squeak is some form of satire performed by these behemoths. To this riddle we do not yet have an answer. But it is not this tiny voice that finds the male a mate; rather it is his relentless pursuit of any object that moves. If a male grasps another male by mistake, the compromised toad will produce a loud chuckle which gains him his freedom.

Even with daylight, the toads' breeding orgy continues, lasting through the day and sometimes the next night. But most of the other nocturnal creatures have returned to their daytime homes by sunrise; and as the morning advances, the desert seems once again almost unpopulated.