Anuran Size at Metamorphosis: Experimental Test of a Model Based on Intraspecific Competition

Abstract

Size at metamorphosis and length of larval period are often, but not invariably, correlated within cohorts of larval anurans. Collins (1979) proposed intraspecific competition as one causative agent for such a correlation in nature, and this hypothesis was supported by observational data. This study reports the results of an experimental test of that hypothesis. Isolated larvae were reared at one of two food levels (high or low) and, during the period in which metamorphosis occurred, were maintained at either constant per capita food levels, or at increasing per capita levels designed to mimic effects of continual release from scramble—type, exploitative competition. Low food levels decreased average larval growth and average size at metamorphosis and increased average length of larval period. A reduced level of food had no effect on the relative variation in length of larval period. Only competitive release affected the relative variation in size at metamorphosis. All four treatments displayed a positive correlation between size at metamorphosis and length of larval period, but only in the combination of low food levels and competitive release was the correlation more than barely significant. Size at metamorphosis displayed a positive allometric relationship to length of larval period in the low food—release treatment, and a negative allometric relationship in the other three treatments. These results support Collins' (1979) model for intraspecific competition as one causative mechanism for such relationships. Certain aspects of these results support each of two distinct models for the timing of amphibian metamorphosis, but the data as a whole do not fit well with either model.