Breeding activity, estimated age-structure, and growth in Sonoran Desert anurans

We investigated breeding activity of a community of desert anurans at two sites in north-central Phoenix, Maricopa County, Arizona, during 1990-1995. Four species used these sites for breeding during the summer monsoon season: Colorado River toad, Bufo alvarius, Great Plains toad, B. cognatus, red-spotted toad, B. punctatus, and Couch's spadefoot toad, Scaphiopus couchii. Breeding activity was restricted primarily to 1990 and 1992 when significant rainfall events (>25 mm of rainfall within 24 h) occurred repeatedly. No breeding (i.e., oviposition) occurred during 1991 or 1993, and only S. couchii bred on a single night in 1994 and B. alvarius on a single night in 1995. We undertook a skeletochronologic analysis of lines of arrested growth in digits and femurs with all four study species. Conservative estimates of age based on lines of arrested growth (LAG's) suggest that populations of all four taxa were relatively young (average age between two and four years) at both sites in 1994 and 1995. Growth, as indicated by the amount of bone deposited between LAG's, was greatest during the second year in B. alvarius and B. cognatus, but not B. punctatus or S. couchii. These results indicate rapid growth to maturity but do not support the notion that these anuran amphibians are long-lived organisms that exhibit an extended reproductive lifespan as adults in response to a highly variable, harsh desert environment.