Adaptations of the Amphibian, Scaphiopus couchi, to Desert Conditions

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Abstract
Spade-foot toads (Scaphiopus couchi) have been discovered recently in the Colorado Desert of California (Sonoran Desert), a region averaging approximately 2 1/2 inches of rainfall per year and having very high summer temperatures (to 50 C). This is one of the driest and hottest areas in North America. The species was apparently more widely distributed along the California side of the Colorado River in an earlier pluvial period. The animals now found there appear to be relict populations that have survived in a few isolated situations (in dry washes and at the edge of sand dunes), due to special environmental conditions. Adults remain in burrows beneath dense vegetation during dry periods. Toads living at the sand dunes presumably bury themselves in a permanently wet layer in the sand. Adults appear to retain several layers of partially shed skin, which presumably form semi-impermeable membranes that reduce moisture loss. Body fluids of field animals just after emergence contain very high osmotic concentrations (approximately 20% compared to hydrated animals (7 to 8%). This also presumably tends to reduce water loss, perhaps even to aid in extracting moisture from a damp environment. In the laboratory, eggs require less than 48 hours for hatching at room temperature, and tadpoles produce legs within ten days. Observations indicate they develop even more rapidly under field conditions. Young toads kept in the laboratory grew to approximately one-half adult size within three months. An extreme local drought is currently in its 20th year. However, this drought has probably been exceeded in intensity in the not-too-distant past. Consequently, these conditions have probably been experienced by members of this species previously. Therefore, there is little reason to believe the present drought necessarily will eliminate the California populations of S. couchi.