Developmental Plasticity of Scaphiopus Couchii Tadpoles in an Unpredictable Environment

Couch's spadefoot toads (Scaphiopus couchii) breed in ephemeral desert ponds of variable duration. Pond duration depends on initial water depth and frequency of rainfall. In four years on Tornillo Flat (Big Bend National Park, Texas, USA), larvae metamorphosed only from low—density ponds, or ponds that refilled before drying. With sufficient food S. couchii larvae developed very rapidly (8—16 d). Rapid development maximized the probability of completing development before the pond dried, but resulted in small size at metamorphosis. I raised larvae in a factorial field experiment manipulating food level, larval density, and pond duration to test if timing of metamorphosis can be altered by pond drying and to test if the response to drying is affected by growth history. Both pond duration and growth history affected timing of metamorphosis, but pond duration explained most of the variation in timing of metamorphosis. For a given pond duration, larvae in high—food ponds metamorphosed earlier than did larvae in low—food, low—density ponds. Short pond duration accelerated metamorphosis at a smaller size and with relatively smaller hindlimbs. Larvae may actively avoid desiccation by metamorphosing earlier from short—duration ponds, but probably benefit by delaying metamorphosis in longer duration ponds. The similar relations between pond drying and metamorphosis in experimental ponds and natural ponds indicate that developmental plasticity may play an important role in the ecology of this population.