Assessment of rates of deformity in wild frog populations using in situ cages: a case study of Leopard Frogs (Rana pipiens) in Ontario, Canada

High rates of deformity in wild amphibian populations from north-eastern North America have been increasingly reported since 1995. In the St Lawrence River basin (Canada) elevated frequencies of limb and eye deformities in mudpuppies (Necturus maculosus) and leopard frogs (Rana pipiens) were recorded in the early 1990s. A caging study was conducted during 1998 to verify the rates recorded in leopard frogs and pursue the potential causes of deformities seen in juveniles and adults. Week-old leopard frog tadpoles were collected from a reference wetland and maintained through to metamorphosis in cages in previously identified high risk wetlands. Deformity frequencies were measured and compared with frequencies measured in wild populations of leopard frogs inhabiting the same wetlands. The results of caging studies and sampling of wild populations were also compared with corresponding data collected from a reference wetland. No deformities were observed in caged or wild reference animals. Very low deformity frequencies (up to 2.2%) were observed in frogs caged in high risk wetlands, but greater frequencies (3.4-10%) were observed in wild young-of-the-year frogs captured at the same sites. The types of deformities were similar among groups; they included fused, missing or extra digits and disproportionate hindlimb length or eye pupil size. In addition, mortality rates were elevated in two cages in high risk wetlands. In general, the caging procedure was effective in establishing the potential for production of deformities in the waters of a given wetland, but tended to underestimate the rates calculated for samples of wild populations. The ramifications of the first-year findings for similar assessments of amphibian deformity rates and establishment of cause-effect linkages are discussed.