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 No rth America have been increasingly reported since 1995. In the St Lawrence River basin (Canada) elevated frequencies of limb and eye deformities in mu dpuppies (Necturus maculosus) and leopard frogs (Rana pipiens) were recorde d in the early 1990s. A caging study was conducted during 1998 to verify th e rates recorded in leopard frogs and pursue the potential causes of deform ities seen in juveniles and adults. Week-old leopard frog tadpoles were col lected from a reference wetland and maintained through to metamorphosis in cages in previously identified high risk wetlands. Deformity frequencies we re measured and compared with frequencies measured in wild populations of l eopard frogs inhabiting the same wetlands. The results of caging studies an d sampling of wild populations were also compared with corresponding data c ollected from a reference wetland. No deformities were observed in caged or wild reference animals. Very low deformity frequencies (up to 2.2%) were o bserved 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 sit es. 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 t he potential for production of deformities in the waters of a given wetland , but tended to underestimate the rates calculated for samples of wild popu lations. The ramifications of the first-year findings for similar assessmen ts of amphibian deformity rates and establishment of cause-effect linkages are discussed.