Environmental correlates of emergence in spadefoot toads (Scaphiopus).

In arid habitats surface activity of spadefoot toads (Scaphiopus) is restricted to short periods following rains; the rest of the year is spent inactive underground. Therefore the perception of favorable conditions for emergence is critical to their survival. The environmental factors that may influence the emergence of Scaphiopus couchi and S. multiplicatus were studied. Low frequency sound (probably below 100 Hz) was the only primary emergence cue we identified for S. couchi. This stimulus alone was sufficient to elicit emergence from dormancy in deep burrows as well as from shallow summer burrows. These toads were extremely sensitive: less than 0.5 mm (0.02 in) of rain elicited 95% emergence from shallow summer burrows in roofed enclosures where no water reached the soil. Vibration from an electric motor consistently induced nearly 100% emergence from dormancy under very arid conditions-15% relative humidity, 30?C air and soil temperatures, and -15 atmospheres soil moisture tension. The only other factor that had a discernible effect on emergence was soil temperature below about 20?C, which inhibited the response to sound. Increasing soil temperature and wetting the soil to less than saturation failed to break dormancy in the absence of a sound stimulus.