[read from the beginning]
template—many unique to these four species. These traits have amazed and bedeviled researchers and naturalists for decades.
Taxonomy and Distribution. The Latin name for the genus of spadefoot found in the American West is Spea, which refers to the toad’s affinity for cavern-like environments (the word is related to spelunker). Spadefoot species are also found elsewhere in North America, in Europe, and even in arid North Africa. Some taxonomists think the North American spadefoot species should remain divided into two genera, but all seem to agree the species are very closely related. Successful interbreeding between spadefoot species actually sometimes occurs under natural conditions in the West where native habitat overlaps.
As adults, most spadefoot species are not much bigger in diameter than a silver dollar and spend most of their lives underground. They emerge only under cover of darkness and feed on insects and other invertebrates. A spadefoot can spend many months dormant underground and can take a pass on annual breeding if conditions are unfavorable or nonexistent. The spadefoot toad knows the value of waiting until the time is just right.
The species of this toad found throughout the Great Plains from Mexico well into Canada is aptly known as the Plains Spadefoot. It is distinguished, along with its closely related cousins—the Western Spadefoot, the Great Basin Spadefoot, and the New Mexico Spadefoot—by numerous ingenious survival strategies. Most notable of these evolution-induced behaviors is the spadefoot’s ability to burrow up to 15-feet or more into the ground and stay there for extended periods of time. How long? One researcher working in the middle of the last century relates a remarkable story of hard clay balls being discovered deep in the ground of a building excavation site. When the clay balls were broken open, living spadefoot toads emerged.
That story and many others perpetuate the legend that these diminutive creatures can live for years underground. But the researcher, Charles N. Bragg out of the University of Oklahoma at Norman, dispelled that notion in a definitive 1965 book, Gnomes of the
Night—The Spadefoot Toads. But Bragg also makes clear that the toads can live for many months dormant underground—perhaps approaching a full year. More recent research indicates one of the desert loving species can provision itself in a single night feeding on termite larvae to stay underground for a ten month stretch. During these long dormant periods, the spadefoot conserves fluids by casting itself inside a bubble of secretions and by recycling urine.
The spadefoot’s remarkable attributes both as an adult and in the larval phase oftentimes seem just plain perplexing. What exactly is the ecological niche of a creature that spends almost every living hour asleep, recycling urine in a dirt tomb formed of its own secretions?
Desert Genesis. The common name, spadefoot, derives from a black, hardened nail-like appendage on the hind foot of all of the species found in Mexico, the U.S. and Canada. This little spur allows these toads to literally screw themselves backward into the ground to escape daytime predators, the desiccation of the desert sun, as well as the droughts and deep frosts of the Canadian prairies.
Most researchers agree that this little toad originated in the Sonoran desert region of the American Southwest many thousands of years ago. Although spadefoot species are today found throughout much of North America, the primordial desert habitat is particularly pertinent to its development. Species of spadefoot toads are still found in the sand dunes of New Mexico, the low and high California deserts and the Great Basin of Utah, Idaho, and Nevada.
Glacial periods clearly influenced the distribution and development of spadefoot toads in North America. The Great Plains species found in Alberta and Montana probably colonized these reaches as the last glaciers receded. Other amphibian species living in the north have more southerly origins as well. Taxonomists refer to “speciation events” as colonies become isolated from ancestral populations. In the American West are found the Great Plains species from southern Alberta south all the way into Mexico. The Great Basin Spadefoot possibly better exemplifies speciation as the glaciers receded; this species occupies the dry basins of Nevada, Idaho, Utah, Colorado, and eastern Washington and Oregon. Researchers find characteristics of populations in Colorado differ markedly from those in the western basins, probably due to isolation. The New Mexico and Western species of spadefoot occupy the true desert lands of the American Southwest and Mexico and may harbor many of the characteristics of the ancestral species.
This desert type of spadefoot habitat is termed “xeric,” which refers to the characteristic dry environment. Bragg used the term to apply to the desert spadefoot’s breeding habits as well. Over the millennia, the spadefoot group expanded its range and came to occupy “mesic” habitats, too—those having moderate levels of moisture. Spadefoot toads have successfully colonized the East Coast from Connecticut to Florida and most of the American southland. The genus of spadefoot occupying these more moist environs still exhibit most of the traits that define the desert dwellers, though: an affinity for temporary pools in which to breed; breeding when conditions are optimal rather than during a set season; and a remarkably short duration of the larval—or tadpole—phase.
The spadefoot occupied this entire habitat long before primitive horses, giant sloths, and wooly mammoths roamed the North American continent. And the little toad remains on (or in) its landscape long after these and so many other species have gone extinct. Clearly, its survival strategies have worked well so far.
Holding an adult spadefoot toad in the palm of your hand can tickle as it tries to gain a little traction to excavate a getaway. If you disturb one too much, expect a large drop of pee to materialize in your palm as the toad relieves its bladder. A spadefoot may also bleat and emit an unpleasant skin secretion to further deter predators. They aren’t very palatable either—at least to humans—according to researcher Charles Bragg, who tasted these secretions. The experience left his “scientific curiosity well satisfied,” he wrote in Gnomes of the Night. Adapted survival strategies, such as explosive breeding and malodorous secretions certainly contribute to the success of the species, but once again are characteristics hardly unique to any single genus of amphibians.
In fact all these devices pale in comparison to the survival strategies exhibited during the spadefoot’s first half of life in water. As an adult, the spadefoot toad exhibits almost vampire-like abilities to secret itself away underground for long periods, if necessary, emerging only on special nights. But as larvae, under certain conditions, certain individual spadefoot tadpoles receive mysterious messages— possibly from the dead—to turn into predatory cannibals.
Forms of Feeding.
Life for a spadefoot tadpole is predictably uncertain, and the drying of its temporary pool is a main source of stress. Another is the availability of food within such a confined area. Spadefoot tadpoles usually have plenty of company, since a single female can lay perhaps three thousand eggs in the course of a breeding session, and after a seasonal rain a breeding pool can be choked with coupled toads going about their procreative business.
Under ordinary conditions, spadefoot larvae are vegetarians, peacefully cruising the spawning pool for tiny bits of moss and plant detritus. The mouthparts of the tadpoles are suited to this type of feeding, having several rows of tiny projections called papillae that encircle the orifice. Bragg and other early researchers observed that some species of spadefoot toads also naturally feed on tiny shrimp in waters that support this food source. These tadpoles develop mouth parts slightly more adapted to capturing this type of food.
Bragg and others noted another form of feeding often develops in some of the temporary—or ephemeral—pools: The tadpoles feed on the bottom mud, sometimes in a cooperative schooling manner. Since spadefoots typically use the same breeding pools from year to year, Bragg speculated that previous generations of spadefoot tadpoles that died as the pool dried up enrich the soil with nutrients that were once their own bodies. “Even in death, they serve their species by building organic food for the next lot of tadpoles,” Bragg wrote in a professional paper. The cracked, baked mud of the dried up natal pool contains a sort of desiccated meal that is reconstituted by seasonal rains. Then the soup is on to sustain the next generation of spadefoot tadpoles as they grovel in the muddy remains of their predecessors.
More recent research seems to indicate that the muddy remains of their predecessors also contain hormonal messages to succeeding generations. The overriding message seems to be “hurry up, or become tadpole food of the future.” But there may be more to the message in the mud.
Rise of Cannibalism. Early observers of spadefoot larvae noted that dead and dying tadpoles in any pool were routinely poked at and fed upon by their healthier brethren. Bragg and others also noted the presence in certain pools of a few giant tadpoles with outsized heads and grotesque mouthparts feeding aggressively on healthy and dying spadefoot tadpoles as well as the tadpoles of other species. Bragg, in the early years of a professional life largely devoted to the study of spadefoot toads, at first confused these predatory cannibalistic tadpoles as a distinct species. He and his colleagues soon confirmed they were not a discrete species—these select few giant cannibals emerge somehow from the hoi polloi of spadefoot tadpoles. This phenomenon is also observed in certain populations of salamander larvae.
Why do certain individuals become cannibals and not others? What triggers the cannibalistic response? Why does it occur in some pools and populations and not others?
Some researchers building on Bragg’s work believe the cannibal tadpoles emerge in response to hormonal messages, one perhaps left to them in the muddy nutrients of failed generations before them. Researcher Robert Denver is not among them. The hormones are “very labile and do not survive very harsh environmental conditions,” he wrote in an email recently. Still, something’s at work here.
During prolonged droughts, a simple message from preceding generations to “hurry up” may not be enough to sustain the species over the long term. The explosive breeding strategy works well enough, but a large number of undersized toadlets might require a bit of an insurance policy to hedge the evolutionary bet. A handful of cannibalistic tadpoles that emerge from the general army can attain all the nutrients needed to metamorphose into larger, healthier spadefoot toadlets. These select few tadpoles become creatures with outsized heads, jaw muscles, and beak-like mouth parts to tear and devour the flesh of their fellows in the pool.
Another surprising recent study indicates that a spadefoot cannibal tadpole somehow recognizes tadpoles of its own species and avoids attacking and feeding upon them when other species are present. Some breeding pools will be shared by toad species that are many times larger than spadefoots as adults. But the tadpole phase will be smaller than those of the spadefoot species—fair prey for the predatory cannibal. As the pool dries, though, this honor among tadpoles is lost and the cannibal will feed upon its own species as well. The schooling activities observed by Bragg and others in certain pools are now believed to be a collective defense against predatory cannibals. As a rule, the predators do not attack individuals in schools, preferring to work isolated individuals found around the perimeter.
All evidence points to these cannibals developing into the exact same spadefoot as produced by any of the other tadpoles. This emergence of cannibals from the larger body of tadpoles also carries an evolutionary compromise: Fewer individuals will succeed, but those that do emerge larger and healthier, perhaps able to enter into their first breeding season in top condition.
More than a decade ago I came to know one of these cannibalistic monsters and grew to love and admire the toad he became.
Moby the Toad. During the turn of this century, my small family and I were living near the Miramar Air Base just east of La Jolla in San Diego County, California. We were to learn the area is host to the Western Spadefoot—one of the desert species perhaps resembling the prototypical spadefoot of the dim past. Most evenings included a walk with our dogs in Rose Canyon, a narrow dry gulch with a rail line and a washed-out two-track jeep trail. These walks up the canyon often extending onto the undeveloped property of the air base, with its sandy bluffs and scrub desert landscape. My son, Dominic, then aged 9, and my daughter, Lauren, at 6 were perfect junior naturalists and we always carried a specimen jar, a butterfly net, and a magnifying lens on these explorations of the desert.
The low desert of California can see many months without rain, particularly through the late summer and fall months that so often feature the famous Santa Ana Winds and southern California wildfires. But in early winter, the skies can just open up with rain. After one of these deluges we noticed the night-time mating songs of tree frogs and other distinctive calls coming from temporary pools on the bluffs surrounding the head of Rose Canyon. We started to carry a flashlight on these evening walks to capture the nocturnal mating activities at the ponds. We rarely saw much, since the frogs and toads would silence as we approached these shallow waters. But daytime revealed lots of spawn—the gelatinous substrate that harbors amphibian eggs. Under a magnifying lens, we could see movement within the eggs. Visits over the following days revealed the emerging tadpoles. Of course the kids wanted to take some home.
And so our tiny three-room apartment came to accommodate a plastic tray filled with generic tadpoles, joining an aquarium of Pacific Tree Frogs, and temporary visitors that might include common fence lizards and the occasional spider or mantis that th e kids collected. I insisted that Dominic keep a log of the tadpoles’ development. I felt the tadpoles could survive on the scummy growth that quickly accumulated in the eutrophied pond water, but the kids regularly sprinkled in fish food as well. As the low winter California sun warmed and evaporated the water, I noticed after about ten days that something had to be done. Tadpoles were dead and dying, of course, but about three or four had emerged as giants, feeding upon the remains of their captive colleagues. One particularly aggressive giant tadpole was white with pink eyes. The container smelled pretty bad.
I insisted to the kids that we had collected a mix of species and that we had to let the tadpoles go before the giant cannibals devoured the whole container of tadpoles and—quite possibly—each other. They convinced me to keep just one, selecting the grotesque white giant tadpole, which had already developed hind legs and feet.
As we released the remaining tadpoles up on the bluffs, we noticed that most of the diminishing pools were almost completely dried up. The mosaic of cracked mud revealed just a few tiny sludge pots that were literally alive with tadpoles. In a seeming struggle to survive desiccation, the writhing mass of tadpoles fought a race against time and death. Slightly disgusted by the sight, we let our captive tadpoles go in one of the few pools that still contained standing water.
Over the next several days we visited the dried out natal pools on the bluff and noticed a huge number of extremely small toadlets all along the banks—we actually thought they were Pacific Tree froglets they were so tiny. Upon closer inspection we could see they were some type of toad. A decade later as I discovered the research papers of Charles Bragg (Adaptations of Survival Value in Spadefoot Toads and, of course, Gnomes of the Night), my memories of the Rose Canyon bluffs swept back to me. These miniature toads were almost insect-like, they were so small. But there were a lot of them.
Meanwhile the lone white captive tadpole in its plastic tray soon sprouted front legs and would sip air from the surface of the water. It was clearly entering metamorphosis. We began to refer to the giant white tadpole as “Moby” in homage to Melville’s great white whale. Dominic and Lauren put a rock in the container so the creature could climb onto something dry to complete the process. Since we believed that we had some type of frog tadpole, what emerged surprised and delighted us.
The Eye of the Toad. On close inspection, the most striking feature of a spadefoot toad is its eyes. Like all toads, the eyeballs seem glued to the top of the head. But in the spadefoot, the pupil is vertical rather than horizontal as found in other North American frogs and toads. And the color of the iris is absolutely vibrant, often copper or gold, and seems lit from within. Almost every lay account of the spadefoot begins with its distinctive jewel-like eyes. The skin is also distinctive, lacking the warts of the large garden toads, yet not so smooth and slick as that of frogs. The color of the skin can range from jade green to mottled grey in the desert species.
The toadlet that emerged in our apartment that winter also had distinctive eyes: deep pink—almost ruby, really. And the skin was white with tiny red spots on raised tubercles. The toadlet was albino. Of course upon confirmation that it was indeed a spadefoot, the little toadlet was immediately dubbed Moby the Toad.
Albinism among amphibians is not terribly uncommon. Certain spadefoot tadpole populations have exhibited 20 percent albinism, although that proportion probably does not survive into adulthood. At least one early observation indicated high predation rates on albino adult spadefoot toads by raccoons. Albinism may be a marked card in the evolutionary poker game.
The little white toadlet seemed to have shrunk from the giant tadpole it had been only a few days earlier. It now seemed diminutive—hardly bigger than my little fingernail. In nature, this is an extremely vulnerable period and is one of the reasons for explosive breeding. Perhaps only one percent survive metamorphosis to enter the sub-adult phase. A much larger success ratio might overrun the West with spadefoot toads. It seemed time to find a suitable container and to look into nursing this creature into adulthood.
The local pet store offered a large plastic cylinder-shaped aquarium which I filled near to the top with native sands and a little topsoil and litter. We also picked up some live mealie worms. The toadlet acclimated quickly to its home, at first squinching itself into the upper layer of dirt with just its pink eyes showing. Later, Moby would disappear for months, occasionally appearing up against the clear plastic near the bottom of the container, eyes closed and surrounded by a bubble of mucous-like material. A toad, if you have never met one, wears a perpetual, benign smile and Moby seemed quite happy cloistered in his pot. My daughter, Lauren, remarked that owning Moby was like having a combination pet and houseplant.
The Care and Feeding of a Toad. In nature, the adult spadefoot is mostly an immobile hunter, waiting patiently for an ant, moth, spider or worm to move within striking range. The toad will sidle into the direction of movement and when the moment is right, flicks its sticky tongue perhaps its own length or longer, collecting the prey into its mouth. After many moments of near immobility, that motion is lightening quick.
A spadefoot’s large eyes on the top of its head actually assist the swallowing reflex by retracting downward, putting pressure on the roof of the mouth and forcing the bolus of food down the hatch. You can actually observe a toad as it squeezes close its eyes and struggles to swallow a large insect. An adult spadefoot can collect very large insects and actually seems to prefer big meals. Professor Bragg and others have suggested that females feed more aggressively than males to build reserves for egg laying.
After a couple of evenings, our albino toadlet was coaxed into flicking its tiny tongue at a wounded mealie worm dangling from a toothpick. We cheered loudly at this milestone accomplishment and the kids argued over who would get to feed him next. As the toad grew, progressively larger insects were introduced to the container. Oftentimes we would leave a dozen pet store-bought crickets in the container where they would remain for days on end. And then one morning we would discover every one missing—our only evidence that Moby had even emerged in the night.
When rain would come, we simply set the container outdoors on the patio and let rainwater fall onto the soil. Researchers have discovered that some species of spadefoot toads are reactive to hard rain falling on the ground overhead—more so than the response garnered by simply injecting water into the soil. This pounding of hard rain on the desert surface apparently triggers something in the spadefoot, and it will at the very least emerge to feed. If the amount of precipitation is high enough, the male spadefoot toad will make its way to the closest ephemeral pool and commence attracting females with its distinctive song. [link to songs of the spadefoot] The following year’s January rains flooded the spadefoot’s container when we left it outdoors and we found Moby floating on the water. It was about this time that we heard Moby sound its mating call in the night, indicating the male sex if not full maturity. Indeed, the natal pools on the Rose Canyon bluffs were once again vibrating with amphibian songs. Dominic captured an adult spadefoot which we commingled with the albino toad for a few days, but no response was forthcoming. Dominic even toyed with the idea of letting Moby swim in one of the breeding pools for a night tethered to land by a bit of fish line. He reconsidered after witnessing several of the pools laid waste by big, four-wheel drive rigs with jacked up suspension kits whose drivers were looking to scatter a little mud. They actually scattered quite a lot. This devastation of the pools was disturbing and difficult to observe, as was the huge housing complex going in on some of the last open ground on the Miramar Mesa.
Interspecies Breeding. After a suitable rain, the male spadefoot toads arrive in the temporary spawning pools first and issue a distinctive song to attract the females. Researchers now believe the female spadefoot develops eggs year-round as feeding conditions provide for development. The eggs can be reabsorbed as necessary by the female during long periods of dormancy. The eggs further develop, perhaps in response to the pounding rains or the calls of the males. In either case, when conditions are optimal, the females will join the males in the sexually-charged hot tub that is the natal pool.
Some researchers believe the females move toward the calls of the fittest males— the constant singing takes energy after all and perfect pitch apparently does matter. The male responds and clasps the female low from behind in what is known as inquinal amplexus. Water from the pool is believed to hydrate both the jelly-like material that encompasses the eggs issued by the female and the sperm from the male that fertilizes the eggs as they are issued by the female. The fertilized strings of spawn may stick to vegetation in the pool or remain in loose clumps near the bottom. The several spadefoot species exhibit slight, but important differences in these mating behaviors. The females move toward the calls of the fittest males—the constant singing takes energy after all and perfect pitch apparently does matter.Spadefoot toads “are notorious for trying to mate with any other frog of comparable size that moves near them,” one biologist observed in a professional paper. The situation in the breeding pools is complicated in some localities because several species of spa defoot can be present. In a few areas of the Southwest, two or even three species of spadefoot toads will naturally interface where habitat overlaps. The species can even share the same breeding pools at the same time. In the past, biologists felt the species remained discreet through evolved habits, such as distinct mating calls and other mating behaviors.
Karin S. Pfennig at the University of North Carolina at Chapel Hill has conducted studies that indicate a female spadefoot toad in a pool containing only her own species will more successfully select larger, more robust males based on their calls. In pools with mixed species of spadefoot toads, however, she found that a female would not aggressively seek out a robust male of her own species. Rather, the female would simply try to locate a male of her own species, settling for any available male.
Recent research indicates that female spadefoot toads will, under stressed circumstances, actually select male toads from an entirely separate spadefoot species. This constitutes a surprising discovery, since intraspecies preference —known as reinforcement—is considered something of a biological cornerstone. In an area where habitat naturally overlaps, Marie Simovich, a researcher at the University of San Diego, and Karin Pfennig from UNC discovered that female spadefoots in poor condition and faced with droughts that threatened to quickly dry up the breeding pool were attracted to the calls of male spadefoot toads of a separate, more desert–like species.
Pfennig and Simovich found that females would seek out and select males of the desert species based on their mating calls. The hypothesis is that these males can deliver to the progeny a shorter period to complete metamorphosis—an important survival consideration to the stressed females. The viability of the next generation might be compromised somewhat by the cross-breeding. The species observed by Pfennig and Simovich to be interbreeding are not the closest of relatives and the sexual viability of the adults might be affected— they may be less fecund. But the quicker time to metamorphosis for the tadpoles seems worth the risk by stressed females. This study may indicate an ability of the spadefoot to naturally breed back into the genetic matrix the essential survival ingredient that a short maturation period offers.
Death of a Toad. It’s been almost a decade now since our evening hikes in southern California’s Rose Canyon. Moby the Toad made the move back to west-central Montana with the rest of the family and seemed to do well for about a year or so. His rare appearances were something of a family event. But the non-evolutionary test of living in a log house with wood heat at a fifty five hundred-foot elevation eventually won out. Perhaps estivation—dormancy during the heat of summer—failed to convert to hibernation—dormancy during the cold of winter. At any rate, at some point Moby the Toad failed to emerge from the sandy soils of his aquarium, although it was months before we came to the realization. My son, Dominic, and I eventually confirmed his death as we sifted through the aquarium soil over the backyard compost pile. The white, desiccated remains of Moby the Toad still offered that trademark benign smile.
The spadefoot’s remarkable attributes both as an adult and in the larval phase oftentimes seem just plain perplexing. What exactly is the ecological niche of a creature that spends almost every living hour asleep, recycling urine in a dirt tomb formed of its own secretions? The sordid sexual preferences exhibited in the natal pools must have a design beyond pure impulse—but what? And what are we to make of a creature that so readily produces a few monstrous predatory cannibals to feed upon the young of its own kind? Really, the spadefoot’s gritty survival tactics in the face of climate changes and environmental challenges may be its true natural offering.
From my vantage, I see the lessons fairly regularly, however misguided. For example, a recent summary of scientific breakthroughs for the year 2010 notes verification that the genetic stock of northern Europeans and other races of modern humans features a significant contribution from Neanderthals. Some researchers had long believed that interbreeding among homo sapiens and Neanderthals almost certainly occurred, but the pairings were barren because of interspecies differences. And yet here I sit with smug knowledge, finally, of why the Irish are such heavy runners.
A friend reviewing the cannibal portions of this manuscript immediately mentioned the investment predator Bernie Madoff. That may have been her personal experience; I respond with my own favorite—psychopathic sexual predators and killers. When the environmental conditions get really rough, we may want to have a few on board the ark, just in case. Who knows?
Over these ensuing years I have learned that the Western Spadefoot—Moby’s species—is considered by many to be threatened over much of its habitat. Sadly, human development and destruction of breeding habitat may underlie the slip in numbers. Here in Montana, the Plains Spadefoot might be somewhat better off—it is currently listed as a “species of concern.” Clearly, more studies are needed, since frog and toad species seem to be in steep decline here, as they are elsewhere in the Northern Hemisphere. Can the remarkable adaptations of the spadefoot toad keep pace with the hard-edged environmental pressures of the 21st Century?
Copyright 2011 © by Paul J. Driscoll
Paul Driscoll is a public information specialist at the Montana Department of Environmental Quality. In his spare time he develops essays on natural history topics.