Our Window on Nature

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Desert Rip Van Winkle

Filed under: Critters — Lowell Christie -- July 17, 2015 @ 11:52 am

SpadefootA desert in early July is no place for tourists. The sun parches the soil with 110-degree temperatures – desiccating the plants and bleaching the mountains to a drab and uniform beige. You’d like to keep driving until you find someplace cooler, but it’s been a long day; reluctantly, you stop for the night.

In no more time than it takes to settle into your campsite, puffy white clouds grow from mere wisps into a black, towering mass. There’s a thunderstorm in the making, and it’s headed your way. You welcome the relief of a good, hard rain washing over your motorhome, pounding on your roof, and massaging the heat out of the air. Then, a couple of hours later, after dark, you hear the most ungodly racket. An entire herd of sheep? Right there in the campground? Heat-induced madness or minor miracle?

Not sheep; toads – Couch’s spadefoot toads. Surprisingly, this parched land of lizards and rattlesnakes also houses amphibians. Desert toads aren’t any different from their cousins that live in moister climates, in that they require puddles and ponds during their breeding seasons, but a toad that lives in a land where there is little moisture must time its feeding and breeding cycle to coincide with the short period of summer rains. The Couch’s spadefoot toad does this by hiding out below the ground for 10 to 11 months, and emerging only after the first substantial summer storm.

Some species of spadefoot are found across much of the nation, but only the Couch’s spadefoot displays this highly specialized desert adaptation. Its internal time clock tells the Couch’s when to start scratching toward the surface. By late June the toad may be only a few inches down, waiting for a soaking rain to draw it out into the fresh air. One could logically assume that the penetration of water triggers the toads’ emergence; but, in fact, spadefoots actually respond to the sound and vibration of water droplets hitting the earth. At that point, thousands of spadefoots scramble to the surface and the race is on – to breed, to lay eggs, and for the young to mature to the point that they can burrow underground once those small, life-giving pools of water disappear.

Adult Couch’s spadefoots don’t burrow near their breeding ponds. They dig down into the dry, desert dirt, and from there they emerge. And then, although both sexes of spadefoots come out together, only the males immediately seek breeding pools. Once the males locate a place with enough water (all it takes is a puddle a foot deep or less) they begin calling in voices that sound much like that of bleating sheep. The females ignore the call at first, stopping to feed for a while. But within a few hours they join the males at the pool and begin the annual nightlong orgy that insures the continuance of the species.

Since each breeding pair may lay and fertilize a thousand eggs in a single night, and since there may be a good many toads of several different species in any one pond, the potential for toads is enormous. This needs to be the case, because the toad’s mortality rate is high. The eggs must quickly develop into tadpoles, and the tadpoles into adults that are capable of surviving outside the water. In the case of the Couch’s spadefoot, this growth cycle is condensed into as few as nine days. But this species grows at a price. A Couch’s toadlet is but the size of a dime when it first creeps onto solid ground; a tender, bite-sized morsel for every snake and larger toad that comes along.

Under ideal conditions of heat and moisture, Couch’s spadefoot eggs hatch within 24 hours. If you were to linger for a few days and wander in search of a good-sized puddle, you might find countless tiny tadpoles swimming around in it. The growing tadpoles feed on algae and on plant and animal detritus, eating constantly in this life-and-death race with the steadily dropping water level. Tadpoles must have water; otherwise, they’re doomed to suffocation, for at this stage they breathe through gills. Only when they metamorphose into toadlets will they have lungs for breathing air.

As the moisture evaporates and the water level decreases, the tadpoles display an interesting behavior. Rather than swimming randomly around the pool, they group together in a mass. Most scientists speculate that this is a method of keeping cool by creating mutual shade, but zoologist Dr. Mark Dimmitt suggests that the tadpoles are actually conserving heat, as it is the high temperatures that permit the Couch’s rapid rate of maturation.
Frequently, the water evaporates before the tadpoles are able to leave the pool, so they’re left flopping in mud. To gain extra hours, or perhaps a day, the creatures shimmy back and forth, scraping out hollows to catch the last few tablespoons of water – enough to keep their gills moist and still able to function. Packed together in the last rain puddle, their tadpole-scraped hollows drying out, they either die or turn into toads.

Once their lungs develop, the toadlets leave the pond immediately and seek shelter under a bush or a piece of wood. It will be a few days yet before they are strong enough to burrow down into the earth; they still have some growing to do. Up until this point, day or night made little difference to the Couch’s, but now that they’re out on terra firma, they adopt nocturnal feeding habits. Toads have water-permeable skins, and were they out in midday, they would dehydrate in the dry desert air.

Whether they are simply burrowing down a few inches to wait for the next rain, or are beginning another 11 months of estivation, Couch’s spadefoots dig their own burrows. On their hind feet they have tiny black “spades” that help with the shovel work. The toads plant themselves firmly, get a good grip with their forelegs, and simply wiggle their way underground with a back-and-forth movement. It takes only minutes for a Couch’s spadefoot to disappear below the surface, and from there it digs as deep as is necessary to get down to damp soil. Gradually, the toads’ body processes slow to the lowest level necessary to keep them alive, and they sleep through until another summer.

What a miracle of nature they are – terrestrial animals that see daylight for only eight to 10 percent of their lives. But that’s what it takes for toads to survive in the desert.

FMC786

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