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The Remarkable Spadefoot Toads
(Scaphiopus and Spea)
by Anne M. Maglia
Division of Herpetology, Natural History Museum, The University of Kansas
Originally published in the Cold Blooded News, Vol.25, No.3, March 1998.
Almost everywhere we look lately, frogs seem to be in the spotlight. From concerns over populations of deformed frogs to Superbowl commercials, these amphibians have received a wealth of publicity. Because amphibians are water-dependent (their semipermeable skin dries easily), we usually equate frogs with warm, moist environments. This is evident by the fact that when we think of frogs, we usually picture typical pond-dwelling frogs (Bullfrogs, Cricket frogs) or brightly colored tree frogs (e.g., Red-eyed tree frogs, Poison-dart frogs). But some of the most extraordinary amphibians, those that live in very dry places, receive little attention. In particular, the spadefoot toads (Scaphiopus and Spea), found in dry plains and deserts throughout the US and northern Mexico, are some of the most behaviorally and physiologically interesting frogs around.
In part, it's their own behavior that keeps them from being noticed. Spadefoots are very secretive, spending most of their time hiding in burrows. In fact, those that live in strictly desert areas are only active on the surface for about two weeks during the year. As an adaptation to living in dry places, all spadefoots burrow down far enough so that the moisture content in the soil is the same as in their skin. This behavior insures that they can live for very long periods without losing any water to the environment. During the rainy season, they only burrow about two inches below the surface, but during draughts they can be found more than three feet underground. They burrow feet-first like most other digging frogs, but spadefoots have thick shovel-like spades (made of tissue similar to our fingernails) on their feet to help them move dirt quickly.
But they aren't always so elusive. During the night, when there is enough moisture in the air to keep them from drying out, they come out of their burrows to hunt for food. Because they are built for digging, and not jumping or climbing trees, they need to be very wary of predators. They, like most toads, rely on several antipredator mechanisms to ward off would-be attackers. If they detect motion, they will stay completely still and rely on their camouflaged skin to blend in with the environment. If threatened, they will inflate their lungs to make themselves appear bigger. They also will secrete distasteful skin secretions, which are often accompanied by a strong odor (some smell a lot like peanut butter).
Because spadefoots live where rains and available water are unpredictable, they do not have a breeding season. Instead, males will wait in their burrows for optimal weather conditions, and then will race to any available waters and let out a relatively loud call for their size (much like a deep "bleet" from a sheep). This call can be heard for about a mile, and other amorous males will join in and set up adjoining territories in the water. The females will then emerge from their burrows and join the males in their territories.
Because the temporary waters that they breed in may only last for a few weeks, most of the population will mate on the first night of heavy rains. Hundreds of thousands of eggs will be laid in large clumps in waters that may have already begun to evaporate. To survive to adulthood, these eggs must develop into toadlets that can leave the water in a matter of days. The developmental cycle of most spadefoots is fast -- eggs hatch in one day and tadpoles metamorphose in about four weeks (compared to a few years for the typical bullfrog). But in their brief life as a tadpole, these frogs show some of the most remarkable behaviors of any amphibians. Spadefoot tadpoles will sometimes group together in huge schools. Because they eat by filtering particles of plant material from the water column, schooling may help stir up settled plant material from the bottom of the pond. Schooling may also help to protect against predation by insect larvae. And depending on the density of tadpoles in the water, some of them may develop to have larger heads and mouths than others of the same species, and instead of eating vegetation will cannibalize their fellow tadpoles.
Although many researchers have described the behavior and physiology of spadefoots, still very little is known about much of their biology, including their internal anatomy, fossil record, and relationships with other frogs. Part of my research includes describing development of spadefoot skeletons and determining how anatomical differences among them may relate to changes they have gone through over time. This will help in a better understanding of the history of this group and how their current distribution may relate to such things as environmental changes. By paying close attention to spadefoot toads, we can gain many insights into the biological diversity present in all frogs, and acquire a better understanding of these remarkable animals.
Editor's note: This article is the result of our Grant Program. The CHS is proud to have been able to help with the funding of this research project. Thank you Anne M. Maglia for this wonderful insight to the Spadefoot toads.
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