2nd World Congress of Herpetology

The anuran amphibian Scaphiopus couchii occurs in the southwestern deserts of North America where its seasonal activity cycle is strictly regulated by summer rainfall. Breeding populations that assemble after torrential storms have been sampled from the same area of the Sonoran Desert in Arizona from 1981 to 1992. Analysis of growth rings from the bones of S. couchii collected during seven consecutive seasons (1986–92) has enabled a comprehensive reconstruction of population age structure. Females may first breed at 3 years of age and males at 2 years, with a maximum longevity of around 13 and 11 years, respectively. The majority (65%) of breeding animals are at least 5 years old, 33% are 7 years and older and 5% may live for more than 10 years. Studies on the age structure of breeding populations have revealed a series of dominant cohorts which originate in wet summers particularly favourable for reproduction; each cohort can be followed for up to 4 successive years at which time it is replaced by the next dominant age‐group. The annual ring record has shown that individual S. couchii growth rates are highly variable and the width of bone produced each year can be correlated with the amount of summer rainfall (and hence feeding opportunities). During the 12 years of fieldwork, there has been a virtual alternation between wet (1981, 1983, 1984, 1986, 1988, 1990, 1991) and dry (1982, 1985, 1987, 1989, 1992) summers, and 49% of animals show corresponding alternation of thick and thin growth bands. Despite a pre‐metamorphic development period of only 10 days, tadpole mortality is high in ephemeral ponds, and post‐metamorphs are vulnerable during the very restricted period in which they can accumulate reserves for hibernation; however, evidence from the age‐distribution of breeding populations indicates good survival after maturity. Analysis of individual growth rates shows that, in general, faster‐growing animals are not often represented in the older cohorts and that slower‐growing animals tend to live the longest. These studies have shown that the population dynamics of S. couchii in the desert are governed by the duration and intensity of summer rainfall, the effects of which are recognizable in the growth records of individuals and in the age‐structure of breeding populations. The succession of dominant cohorts that results from this environmental variation precludes construction of a general life‐table for S. couchii .