History of Minnesota Frog Abnormalities: Do Recent Findings Represent a New Phenomenon?

Two lines of investigation were used to determine whether recent Minnesota reports of frog abnormalities differ from historical reports: (1) museum collection studies paired with follow-up field surveys, and (2) comparison of recent and earlier field surveys in west-central Minnesota. For the museum study I examined 2433 northern leopard frogs (Rana pipiens) in the Bell Museum of Natural History collections of the University of Minnesota, collected during 1958-63. 0.7% of frogs were found to have abnormalities, consisting of 0.5% predator amputations and 0.2% malformations. Three types of malformation were found: missing hindlimbs, vestigial feet, and fused digits. Conspicuously absent from the museum collections were the malformation categories of extra limbs, split limbs, webbing of hindlimbs (cutaneous fusions), and bent/twisted limbs (anteversions). Postmetamorphic field surveys at some of the museum collection sites were attempted in 1997. One finding was that frogs were much more easily collected in 1958-63! During surveys in 1997 I found frogs at only 5 of 14 museum sites on record. Fifteen malformed frogs were recorded among 611 examined (2.5%), compared to one malformed frog among 276 (0.4%) in the corresponding museum collections. Field surveys of more museum sites are in progress. Frog surveys conducted in west-central Minnesota from 1976-97 were also analyzed for differences in abnormality frequencies and types. Nine sites were identified as having reasonable (\u3e50) sample sizes in both pre-1993 and post-1995 surveys. Pre-1993 data included three abnormal frogs among 1772 examined (0.2%), compared to 59 abnormal frogs among 2548 (2.3%) in 1996-97. Only 2 categories of abnormality were found in the pre-1993 data, partial limbs and vestigial foot. I conclude that recent findings of anuran abnormalities in Minnesota do represent a new phenomenon. Frog abnormalities were more frequent, more varied, more severe, and more widely distributed in 1996 and 1997 than in 1958-93