Spadefoot Toads and Storm Sewers, Creative Nonfiction by Susan J. Tweit

Our second year in the Chihuahuan Desert was wet. Seven-point two inches of rain fell between December and June, over double the average, washing the landscape from river valley to mesa to mountain slope with a thin coat of green, as if wiped with a transparent stain that tinted the earth's bony skeleton. The evergreen leaves of the creosote bush turned from dusty olive to chartreuse. Cacti stems plumped out, filled with stored water. Delicate green annual grasses sprouted from the formerly bare soil, along with wildflowers in every hue. Baby lizards scurried everywhere, quail and cactus wrens raised huge broods, and the mosquitoes hatched early. The desert was flush.

One evening in May, towering blue-purple cumulonimbus clouds built overhead, obscuring the sky but for snippets of turquoise glinting through the cracks between the clouds, and bringing an early dusk. Thunder rumbled. Soon the storm broke. It was a real rattle-and-banger. Rain poured down, pounding an insistent rhythm on the roof, brilliant stabs of lightning flashed, and booming crashes of thunder shook our house. A bolt of lightning hit nearby with a loud "Crack!" knocking out the power. When the storm finally died down, the night was soggy. Puddles spilled over lawns and sidewalks; water flooded the roads hubcap deep in places.

My husband Richard and I walked into the dark backyard and listened to the storm recede. The air was cool and wet. Gradually, a distant sound insinuated itself through the nearby car traffic, a steady chorus that sounded like sheep bleating, interspersed with low, trilling rattles—spadefoot toads. The storm had summoned the small toads up from the soil to feed and call for mates.

Spadefoot toads seem miraculous. Indeed, they are among the desert's most paradoxical residents. Like all amphibians, spadefoots possess spongelike, porous skin that absorbs—or loses—water constantly. Small—the largest species grows to 3.5 inches long, no longer than most people's thumbs—they literally mummify when exposed to dry air, dehydrating completely in a matter of hours. They require water in which to breed; their gilled tadpoles cannot survive outside water's liquid world. Yet spadefoot toads live in sere country, including some of the continent's driest places. One species, Couch's spadefoot—the bleating sheep—manages to thrive in the hottest, driest parts of the Mojave and Sonoran Deserts where annual precipitation averages 2.5 inches per year, two years can pass without rain, and summer air temperatures often exceed 120°F. How can a water-loving amphibian survive in such a hostile environment?

Like many desert residents, spadefoots take refuge in the earth when times are too dry and the temperatures too extreme. Unlike most, spadefoots spend the majority of their lives—years, in extreme droughts—dug deep in the soil, barely respiring, dormant. They adjust to the scarcity and unpredictable occurrence of water by coming to the surface only on a handful of nights each year during the summer monsoon season when environmental cues tell them that water will be abundant. Their sudden appearance and equally sudden disappearance is one of the desert's summer miracles.

When they emerge, the small toads do so in torrents, like the water that they need. Cued by the sound waves of thunder, thousands of spadefoot toads dig out of their solitary burrows. Three needs drive the bug-eyed, bumpy-skinned amphibians: sex, food, and water. The toads hop to the

nearest water-puddles, temporary ponds, roadside ditches and rehydrate their parched bodies: they squat and absorb water directly through their porous skin. The male floats and bellows his desire. The curious trills and bleats of thousands of tiny swelling throats carry for miles. The female, drawn to the din, is besieged by suitors. While she sculls in the water, he clambers atop her, holding her with special toe pads, and squirts sperm over the floating mass of eggs that she exudes. After their mating frenzy, the small toads turn to the business of replenishing their stores of body fat for another long stay underground.

Spadefoot toads are not the only desert dwellers to emerge en masse following the summer rains. The brief profusion of water triggers the hatching or emergence of a variety of insects, including termites, one of the spadefoot toad's major foods. Several species of termites—the most common insects in the desert besides ants—stage mating flights at dusk during the summer rains, inadvertently providing a feast for the toads. But the toads' window of opportunity is small. The termite alates—the winged, reproductive stage—are vulnerable to the sticky tongues of spadefoot toads only in the few minutes that they are on the ground surface after they have mated and shed their wings. Further, just a few flights occur each year in any one place.

But when they appear, termite alates constitute the toad equivalent to Dove bars. They contain more fat and calories than some 200 other species of insects, and they provide much more energy per unit weight than other toad foods such as beetles or crickets. The toads gorge themselves on the rich insects, eating as much as half their body weight in one night's feeding and storing so much fat that in just a few nights of feasting a spadefoot toad can provision itself to spend a year or more dormant in its solitary underground burrow.

After a night of feeding and sex, spadefoot toads retreat underground. Sometime after dawn, the adults dig themselves into the moist soil with the digging tools for which they are named—"spades," black, sharp-edged scrapers on the underside of each lower hind leg. They seem to swim backwards into the soil, wiggling from side to side as each powerful hind foot ultimately pushes dirt outwards. The small toads dig for ten to fifteen minutes, rest for the same length of time, then resume digging.

Below ground—away from the thirsty desert air—a spadefoot toad's porous skin can be an asset. Even during the relatively humid rainy season spadefoots' skin leaks some water to the air when they surface at night to forage, but when the toads return to their shallow burrows they absorb water from the moist soil. After the rainy season, the toads dig winter burrows as deep as three feet into the soil. As long as the soil stays moist, spadefoots can pull water from the soil. As the soil dries out, however, their spongelike skin becomes a liability. Although spadefoot toads can survive loss of a surprising amount of body water—up to sixty percent—the parched soil can eventually suck their tissues dry, slowly mummifying the toad.

One of spadefoots' anti-dehydration strategies is to let several outer layers of skin dry out and slough off around their body, forming a desiccation-resistant cocoon. Another is to store water in their bladder—which can hold an amount equal to thirty percent of their body weight—as an emergency "canteen." But spadefoot toads' most powerful strategy takes advantage of a physical property of water: if the water on one side of a permeable membrane (like a toad's skin) has a higher concentration of stuff dissolved in it—salt, for instance, or urea—than is dissolved in the water on the other side of the membrane, water will flow through the membrane to the more concentrated side. In other words, if the toads can raise the concentration of dissolved substances in their body water, they can prevent the water from

slowly leaking away. Spadefoots control their body water concentration by storing the urea from urine in their tissues. As the soil dries out and grips the remaining water more tightly, the toads quit urinating, instead storing urea in gradually increasing concentrations. (Spadefoot toads can tolerate levels of urea that would kill a human.) While dormant, their metabolic activity reduced to just a flicker, the little toads somehow manage to maintain an equilibrium between the concentration of their body water and the water pressure of the drying soil.

When the parents dig themselves into the ground after a hurried night of spadefoot sex, they leave behind egg masses floating in rainwater ponds and puddles. For this new generation of spadefoot toads, the race has just begun. The eggs must hatch, and the gilled, aquatic tadpoles must metamorphose into air-breathing toads before their ephemeral aquatic environment vanishes. In order to beat the drying puddles, spadefoot toads race to adulthood more quickly than most amphibians. Couch's spadefoot eggs hatch within twenty-four hours and sprint to air-breathing maturity in just over a week! (By comparison, bullfrog tadpoles take two leisurely years.) As space and resources grow scarce in their shrinking world, the scores of tiny, black spadefoot tadpoles feed frenziedly on plants and small aquatic animals, and sometimes on each other. Surviving tads emerge from their puddle, gulping air and still dragging their tail.

The spadefoot toads whose bellowing chorus attracted our attention that May night were surfacing in response to the booming thunder. Although spadefoots do dig upwards as soil temperatures warm in the spring, and occasionally emerge to briefly forage for food after light rainstorms, only low-frequency sound waves—simulating those of thunder—elicit the dramatic appearance of the hordes that fill the night with sound. Thunder is apparently the environmental cue most consistently associated with the kind of downpours that produce the puddles and ponds that spadefoots need for

mating and egglaying. Unfortunately, off-road vehicles used for travel in the desert—especially motorcycles—produce sound waves similar to those that stimulate the small toads to dig to the surface, expending precious calories and moisture. If roused during the dry months—the majority of the year in much of spadefoots' habitat—the little toads may die.

When the rain faded to a gentle patter, we took a flashlight and walked through our subdivision in the darkness. Across the busy road, past wet fields of cotton, the sound swelled around us. It seemed to emanate from a normally dry storm sewer retention basin, a several-block area dredged out so that it slopes toward one grassy comer where gapes an eight-foot high grate covering a huge drain culvert. After one of our intense summer thunderstorms, water pours into the grass-covered basin from the parking lots and asphalt streets uphill, collecting here rather than flooding the adjacent low ground.

At the edge of the basin, we shone a flashlight into dark water. Hundreds of small toads floated nearby, their throats swelling and shrinking, swelling and shrinking. We heard both the low trill of southern spadefoots and Couch's sheep-like bleat. I located a trilling toad in the beam of the flashlight and we watched it as it floated near the edge of the basin. Behind it the main stream of black water rushed by, carrying plastic milk jugs, discarded motor oil containers, disposable diapers, and other debris. The small toad called with all of its might: first it sucked in air, its belly swelling as if it had swallowed a hockey puck, then it blew up the pale skin of its throat like a glistening balloon; its mouth opened, producing the high, nasal trill. After a pause, the floating toad gulped in another mighty breath of air, pushed out its pale throat, and opened its mouth into trilling song again.

The city views this place where toads call as just a piece of plumbing, part of the city flood-prevention system; its only purpose is to collect the torrents of

water that rush off of city streets and parking lots after thunderstorms. But the toads know it differently. Indeed, located at the eastern edge of the floodplain of the Rio Grande, this has always been toad habitat. Originally part of the bosque, valley bottom woods, this basin was once shaded by tangles of low, spreading honey mesquite and tornillo, screwbean mesquite, hackberry, and Mexican elder trees. Wavering lines of stately, tall cottonwoods formed a higher canopy here and there, marking the outer edges of the river's successive floods. Orioles wove their intricate hanging pouch nests under the cottonwood canopy, Lucy's warblers sang from the mesquites, tiger swallowtails fluttered through the sunny spots between the trees, deer slipped through the bosque to drink at the river in the evening, river otters slid down slippery mud banks to frolic in the water. And after the summer rains, spadefoot toads collected to sing and mate in the temporary puddles and ponds.

In 1840, this part of the Mesilla Valley was deeded to Mexican settlers by the government of Chihuahua in the Dona Ana Bend land grant. This quiet part of the bosque probably changed little until nine years later, after the United States had won this area in the war with Mexico, and the new town of Las Cruces was platted nearby to accommodate American settlers. What is now a storm sewer retention basin was then part of the Perfilio Jemente tract, a parcel of grazing land allotted to the Jemente family as part of the immense Dona Ana Bend grant. The Rio Grande flowed along the edge of the tract until after a big flood in 1885, when it relocated its channel three miles farther west. In 1892, the Jemente family lost their tract of bosque—including the current storm sewer basin—to a certain John D. Bryan, in judgment for a debt of $199.75. From then on, the tract remained in Anglo hands.

The completion of Elephant Butte Dam upriver in 1916, with its promise to control floods and irrigate some 175,000 acres of valley bottom land, fired a

speculative land boom in the Mesilla Valley. The Jemente tract—as it was still called changed hands four times in the next fourteen years, and during that time was probably cleared of its bosque overstory and planted in field crops—alfalfa, cotton, or perhaps vegetables. World War II and the testing of the atomic bomb at nearby White Sands Missile Range brought a boom of another sort to the former bosque—housing developments. The Jemente tract was sold again, this time to a speculator assembling land for a subdivision. In 1959, the city bought a piece of the former Jemente tract-turned-subdivision. On part of the purchase were built tennis courts and a swimming pool; the remainder, a two-block-long by one-block-wide rectangle, sprouted softball fields. But after each big rain, the softball fields flooded several feet deep. Eventually, the former bosque was dredged and turned into a storm sewer retention basin. For much of each year the grassy basin stands seemingly empty, waiting for rain.

Now the old bosque is coming full circle. A community group has "adopted" the storm sewer retention basin, vowing to replant the bosque and build nature trails to wind through the small area. The group began by planting fifty trees—cottonwoods, desert willows, hackberries, and others—during our wet spring. But the toads always knew. Through all of the past hundred fifty years of changes—grazing, bosque clearing, plowing, subdividing, dredging, and re-planting of bosque—the toads have reappeared each year to bellow their lust, mate, and gorge on insects.