Evolution of the Couch's Spadefoot Toad

New genetic strain of Couch's Spadefoot Toad in Midland?

Published Jul 16th 2000 in Wildlife

A number of parks in Midland are built in old playas. During a rainstorm, the 15- to 30-foot-deep swales are filled up quickly. In the 24 hours following a storm, these flood retention reservoirs will be pumped as dry as possible.

When I was a child, the land that is now known as Grafa Park was almost 2 miles out of town. The watershed for that playa came from then unused farm and ranch land that developers had obtained in anticipation of a predicted oil boom in the early 1960s. What is now Rusk Elementary was a maize field, while only a dozen or so houses had been built north of Golf Course Road.

In those days, there were no pumps in the playa and water would stand for a month or more after a good rain. I spent many hours there, wading around, catching toads the first evening after a storm. During the ensuing weeks before the playa dried out completely, I spent parts of almost every day catching tadpoles, fairy shrimp, tadpole shrimp, dragonfly larvae, and water beetles of several species. (We did not have a television until I was 13 years old, so I pursued entertainment instead of receiving it).

What little wildlife was present in those days would visit the playa for a drink. Raccoons, deer, mountain lions, javelina and ringtails had not yet come to Midland County. About the only animals here were the coyotes and foxes that left their scat after hunting the ducks that visited in the fall.

Although the water in the playa was muddy for a day or two right after a rain, it would eventually clear so that every blade of grass -- even 2 feet below the surface -- could be seen. The lowest end of the playa was perpetually damp, even in the 7 years of drought during the 1950's. Wildflowers, such as Annual Coreopsis, Bluebells, and Frogfruit, bloomed there most years. (Annual Coreopsis has not been found in Midland County for 30 years.)

We never worried about the health risks associated with exposure to standing water. As good little country kids, we knew not to drink the water. Drunks and cretins did not throw trash into the playa, so we did not think about busted glass and rusting beer cans and lockjaw. We all got "swimmer's ear" a time or two, though. On some summer days, kids would be present from "can" to "can't." Mudpies by the dozens were "cooked" by the girls, and mudball wars lasted for hours among the boys. Each time the playa was filled, we were guaranteed up to a month of fun.

A few mosquitoes would survive to adulthood, but an ecologically balanced rain-filled playa has plenty of predators to keep things in check. But as soon as the developers began building houses near the playa, people began to complain about the "nasty" water and the health threat from mosquito-born diseases. Now, every playa in town has a pump to quickly drain it.

"Civilization" separates people from the outdoors. From ignorance, people learn to see wild places as unhealthy. "All bugs are bad and covered with germs," a child is told the first time he or she bends over to examine the wondrous "otherness" of living creatures. Today’s kids rarely have the chance to play in an ecologically-balanced playa.

Part of our mission at the Sibley Nature Center is to reacquaint urban folks with the natural world. We help people learn the difference between folktales, urban legends and facts. For example, thousands of people have told us that tarantulas jump, because a brother’s friend’s cousin told them so. A tarantula’s anatomical structure has no mechanism to launch them even as much as 4 inches into the air, much less the 3 feet "that Betty Jo and Roberto saw."

People bring us critters and plants to identify nearly every day. Most of the specimens are easily recognized and we can immediately provide a fair amount of natural history information. When something new or unusual is brought in we love it. During the week of the June 19th, the moms of two summer camp kindergartners told us about toads they had discovered on an early morning walk at Hill Park.

The rains of the previous weekend had filled the park playa. A number of toads appeared, sang all night and laid dozens of strings of eggs before the pumps were turned on to drain the "reservoir.” But once most of the water was gone, the moms noticed that the toads appeared to be trapped against the grate covering the pump.

We asked them to bring one in so that we could find out if they were Gulf Coast Toads. For years a small population of this imported species had managed to survive at Hill Park. We were very surprised to find that the large toads were an almost uniform green. There is no such toad. A Green Toad is very small, and is dark green with tiny black speckles. "Well, some were mating with differently colored toads," we were told.

"What? We want to know more --"

"Okay, we will go take some pictures." Mrs. Banschbach brought us the photographs accompanying this story. While she was off photographing the toads, my assistant Brandon and I looked through our reference material and determined that the all-green toad that was brought in had to be Couch's Spadefoot Toad. Our determination was based on the shape of the foot, and the fact that it had no boss (raised area) between the eyes.

When we received the photographs from Hill Park, we noticed that most of the toads were of the solid green color, while a few had the classic Couch's Spadefoot coloration. "We may be seeing the development of an isolated population that is beginning to take on different characteristics from the rest of the species. Some day, this might be called the “Hill Park race” -- or if they continue to spread throughout town, the 'Midland subspecies.' And, given enough time, the solid green toads might not be able to breed with normally patterned Couch's Spadefoots, as they are doing in some of the photographs. They may become a truly separate species."

A week later the Midland Reporter Telegram published photographs from Cowden Park featuring children and "frogs." The "frogs" were Couch's Spadefoot Toads of both forms. Toads are not frogs - toads have bumpy skin, aestivate underground most of the year, can only hop (not leap), and have round noses instead of pointed noses.

"What would cause them not to be able to mate?" Kyle Engstrom, a teenaged summer camp volunteer, had been listening to our discussion with the moms.

"Each species of toad sings at a different frequency." I explained. "Because their ears can only distinguish one frequency, they do not hear other species in the same pond. When a playa has 10,000 toads of 5 different species singing all at once, each toad needs to be able to find the members of their own species. If our "Green Spadefoot" breeds in isolation for several generations, it is feasible that other genetic characteristics could become slightly changed, as is already happening with the color genes. The size of the throat sac may change, or the ear structure may change after several generations."

Brandon added to the explanation. "These types of tiny genetic adjustments occur continually. In Hawaii, there are completely blind insect species in two-hundred-year-old lava tubes. The blind insects developed from sighted species that crawled in as soon as the tubes cooled. It took many generations, of course."

I kept the explanation going. "Plants are another example of what genetic variability can produce. The Gone Native Arboretum has 10 different colors of Autumn Sage (Salvia greggii), for example, including one that showed up as a seedling. All are naturally-occurring "sports" that demonstrate the plant's genetic variability."

Jessica Sweeney, another summer camp volunteer and critter caregiver deluxe, noticed the photograph that showed toads against the drain pump’s grate. "Are they trapped? It looks like one pair is swimming away, but the toad that was brought in has a badly broken leg -- how did it break its leg?" Mrs. Banschbach told Jessica that the leg was already broken when she caught it.

"I wish the City would not pump the water down so quickly. I know it is the "right" thing to do, but it seems a shame to hurt the toads, and have thousands of their eggs sucked into the drains." Jessica shook her head in resignation. "Maybe some of them survive the ride and go swirling down The Ditch and end up surviving out in the country."

How many people have heard the incredible chorus of 10,000 toads in a natural playa? How many people have returned to that playa 3 weeks later to see the 750,000 baby toadlets hopping away en masse? Nature's fecundity is humbling to mere humans.