Frog and Toad—In the Desert

By Ron Harton

Our small group of Desert Survivors, led by Craig Deutsche, climbed into the Panamint Mountains bordering Death Valley National Park through Surprise Canyon. A stream cascaded through the canyon, and we stepped in and out of the water as we pulled ourselves over rocks heading higher into the mountains. It was a wonderful Memorial Day weekend. A canyon wren’s trill welcomed us, stream orchids (Epipactis gigantea) bloomed beside us, hedgehog cacti thrust their purple blooms from the canyon walls, willows arched overhead, and at our feet—frogs and toads.

"Frog!" someone exclaimed up ahead. Sure enough, it was still there when we at the back of the group arrived—a little Pacific treefrog, green with a black eyestripe, nestled into a rock cleft by the stream. In spite of its name, it prefers rocky crevices to trees. A few minutes later, we climbed through a narrow section of canyon and emerged in a flat open area. Water filled a small depression on top of a flat rock. The pool was only about a foot wide and no deeper than three inches, yet there were eight tadpoles swimming around. They looked healthy—plump round bodies and bulging eyes that seemed to look up quizzically at us as we peered in on their aquarium. They would be exposed to the sun all day, and we thought that they had better hurry if they were going to make it to adulthood before their pool dried.

Finding frogs and tadpoles in the desert surprises some of us who associate those amphibians more with wetlands. I don’t know if the famous Mr. Frog and Mr. Toad from the children’s storybook series ever visited the desert, but if they did, they would find that quite a few of their relatives live there. Great Basin toads, Southern Spadefoot toads, Couch’s Spadefoot toads, Barking toads, Woodhouse toads, Great Plains toads, Sonoran green toads, Sonoran Desert toads, Red-spotted toads, and the Pacific, Canyon, and California treefrogs are some of the species found in the American deserts. Like other frogs and toads, these desert amphibians have a two-stage life cycle. In the first stage the eggs hatch into aquatic creatures with gills. These tadpoles metamorphose into air-breathing terrestrial adults. Since they don’t have permanent wetlands, desert frogs and toads have developed ways of adapting to the dry, hot climate.

The Couch’s Spadefoot toad (Scaphiopus couchii) is a desert miner. It constructs burrows three feet deep or more and spends up to ten months of the year underground. The toad uses a sharp-edged, spade-like tubercle on the bottom of each hind foot. It does an amphibian version of the twist, the toad twist, rotating its body and feet first one direction and then the other to dig backwards into the ground. Not picky about digging its own burrow, though, it will also use the burrows of kangaroo rats or other animals. Couch’s Spadefoot likes to live along washes. It is found from the eastern part of California across the deserts into Central Texas. In California they can be found along washes in the creosote bush zone. When the rains come, and the water flows in the washes, they leave their burrows, lay eggs, and then return to their burrows. Their calls, which sound like the bleating of sheep, can be heard shortly after the rain. The eggs may hatch in only nine hours after being laid, and the tadpoles develop to adults in as few as ten days. Other types of frogs and toads in areas with permanent water may take months or even years to develop from tadpoles to adults. The life cycle of Couch’s Spadefoot toad is adapted to using the temporary water of the washes. If the rains do not arrive, the toad may behave like the desert tortoise and wait in its burrow until the next year. During these extended waiting periods, the toad may develop a coating similar to extra layers of skin, which protect it from desiccation.

In southern Australian deserts, the water-holding frog (Cyclorana platycephala) performs a survival feat similar to Couch’s Spadefoot. It, too, lives in a deep burrow and only emerges after a rain. Most of its life is spent underground. In its burrow, it secretes a mucus lining on the inside of its chamber and over itself. This hardens forming a membrane sack or shell that keeps the frog from desiccation. After a rain, the frog emerges by tearing apart the membrane.

How do the Spadefoot toads, three feet underground and encased in extra skin sacks, know when it is raining? Perhaps they hear the rain, like the baby tortoises we learned about at the Desert Tortoise Natural Area. A biologist there told us during a service trip that she has watched baby tortoises she was raising pour out of their shelter at the first sound of raindrops on the roof. Frogs and toads do have a well-developed sense of hearing. High-pitched sounds are picked up by their "ears"—a membrane, the tympanum, which is stretched across a ring of cartilage behind the eye. Low-pitched sounds are transmitted through the skin and body.

Frogs and toads also have well-developed senses of smell and sight. They have separate olfactory nasal and vomeronasal organs. These organs have similar functions to those of snakes—detecting chemical clues related to breeding and the identification of prey. Their large protruding eyes provide a wide field of view in order to detect predators and prey. Frogs can retract their eyes into their sockets. When retracted, they bulge against the roof of the mouth and aid the frog in swallowing food. Some frogs have a pineal organ similar to the third eye of reptiles. It is sometimes visible as a pale patch on top of the head. This "third eye" reacts to light and may aid the frog in orientation and navigation.

The tadpoles we saw in the pool in Surprise Canyon were not Spadefoot toads, though. They were probably "true toads." The popular distinction between frogs and toads seems to be European in origin. Both frogs and toads are in the Anura order of the class Amphibia. European frogs have smooth moist skin and live near water. European toads have warty-looking skin and spend more time living on land. However, in the rest of the world, some frogs look warty and stocky like toads and some toads have smooth skin. In short, all frogs and toads can be properly called frogs. The term "toad" may be used informally for any frog that resembles a warty, short-legged member of the genus Bufo, even though they are not in that genus. To be a "true toad," a frog has to be a member of the genus Bufo. Spadefoot toads, genus Scaphiopus, can be distinguished from the true toads of the genus Bufo by their eyes. The Spadefoot toads have cat-like, vertical pupils. Bufo toads have horizontal pupils.

The tadpoles of Surprise Canyon may have been Red-spotted toad tadpoles (Bufo punctatus). This three-inch toad gets its name from vermilion-tipped warts on its back. It likes to live in the rocky walls of canyons. In fact, if the Spadefoot toads are desert miners, the Red-spotted toads are desert rock climbers. They climb up rocks in canyons that have streams and seepages to find shelter in crevices during the day. They come out at night to hunt insects. The toad survives dry spells by storing water in its bladder. It can store as much as thirty percent of its body weight as water in its urinary bladder. It reabsorbs the water as needed to stay hydrated.

Another true toad of the desert, the Sonoran Green toad (Bufo retiformis), lives in southern Arizona areas such as Organ Pipe National Monument. It is only one to two inches long but has a colorful pattern of green spots on its black back. Famous as a ventriloquist, it can project its voice twenty feet or more from its actual location.

The Sonoran Desert toad (Bufo alvarius), also known as the Colorado River toad, is the largest toad in the West at over seven inches long. Only the Bullfrog (Rana catesbeiana) rivals it in size, but the Bullfrog is not a native species west of the Rockies. Although it prefers to stay near permanent water, such as the lower Colorado River, it ranges throughout the desert, sometimes miles from permanent water, hiding in holes and burrows dug by other animals. Its large parotoid glands on the side of its head secrete a sticky white chemical that can paralyze or even kill small predators who try to eat it. The poison may irritate human eyes and skin, too, but it does not cause warts.

The spelunker of desert frogs is also one with an unusual voice. The Barking frog (Hylactophryne augusti) likes to live in limestone caves and will also make its home in wells, deep rock crevices and mine shafts,. It lays its eggs in rainwater pools in these locations. At night it comes out to hunt near clumps of cactus or yucca. The Barking frog’s call sounds like a small dog yapping several times.

Several types of treefrogs live along desert streams. The most common is the little Pacific treefrog (Hyla regilla). It is very common along the California coast, too, and its famous two-syllable call, kreck-ek, is easily identifiable. The last syllable is higher than the first. It’s often heard in spring and also in the night scenes of movies and television programs. In the desert, treefrogs, although good climbers, live under rocks in vegetation close to the water’s edge. Canyon and California treefrogs also live in desert canyons with intermittent or permanent streams. All three of these treefrogs can change color to better camouflage themselves. California treefrogs have spots when living at streams with granite rocks and are plain-skinned when living in canyons with sandstone rocks.

Although often overlooked, frogs and toads are an important part of desert fauna. Look for them in the canyons after the spring rains, and the next time you walk up a hot dry wash, remember the toads that may be patiently waiting deep underground. If you are interested in helping study and protect frogs and toads, check out the North American Amphibian Monitoring Program at http://www.im.nbs.gov/amphibs.html. Another informative online site is Froggy at http://frog.simplenet.com/froggy/index.shtml.

BIBLIOGRAPHY

Cogger, Harold G. and Zweifel, Richard G., Eds. Reptiles and Amphibians. New York: Smithmark, 1992.

Jaegar, Edmund C. The California Desert. Stanford: Stanford University, 1965.

Krutch, Joseph Wood. The Desert Year. New York: Viking Press, 1952.

Larson, Peggy. The Deserts of the Southwest. San Francisco: Sierra Club Books, 1977.

Stebbins, Robert C. California Amphibians and Reptiles. Berkeley: University of California Press, 1972.

Stebbins, Robert C. Western Reptiles and Amphibians. Boston: Houghton Mifflin Company, 1985.