Prey Selectivity and Size-Specific Diet Changes in Bufo cognatus and B. woodhousii during Early Postmetamorphic Ontogeny

Abstract

Ontogenetic diet shifts have been documented in many anurans (Smith and Bragg, 1949; Oplinger, 1967; Labanick, 1976; Christian, 1982; Donnelly, 1991; Lima and Moreira, 1993). These studies compared the diets of juveniles as a whole to those of adults (e.g., Livezey, 1961; Clarke, 1974a; Gittins, 1987). Diet patterns among juvenile size classes, however, have yet to be investigated. Bufonids exhibit high postmetamorphic growth rates early in ontogeny, as do other anurans (Clarke, 1974b). These differences in body size may be a means of avoiding overlap in resource use (Schoener, 1974; Toft, 1985). Hence, diet shifts associated with rapid growth during this relatively short period may be important for survival to reproductive age and may affect interactions with heteroand conspecifics (Werner and Gilliam, 1984). Discrete, size-specific diet shifts may result in selectivity for different prey among toad size classes (Werner and Gilliam, 1984). Some authors have suggested that certain Bufo feed indiscriminately (e.g., Smith and Bragg, 1949; Clarke, 1974a; Duellman and Trueb, 1986; Larsen, 1992). However, these conclusions could be an artifact of pooling size classes consisting of selective feeders into a single class for dietary analysis. Indeed, recent studies indicate that toads are selective predators (e.g., Toft, 1980, 1981). We quantified ontogenetic differences in the diets of toads over a range of sizes that occur within the first month after metamorphosis, and compared prey consumption to prey availability for several prey types consumed by juvenile toads during early ontogeny. We collected postmetamorphic, juvenile toads by hand while walking a transect 1 m from the edge of Rose Lake, a prairie-pothole marsh located approximately 5 km NE of Vermillion, Clay Co., South Dakota. We collected 36 Bufo cognatus and 14 B. woodhousii on 15 July 1992, and 20 B. woodhousii on 27 July 1993. During 1992, all breeding occurred on 15-18 May and most tadpoles of Bufo transformed around 15 June (Graves et al., 1993). The average SVL of B. woodhousii at metamorphosis in South Dakota is 10 mm (Underhill, 1960). The largest toad collected had a SVL of