A regional survey of malformed frogs in Minnesota (USA) (Minnesota malformed frogs)

In late 1995, school children discovered malformedfrogs in a south central Minnesota pond. Press coverage resultedin numerous citizen reports of frog malformation across Minnesotain 1996. After some initial site investigation, 3 affected frogsites and 4 nearby reference sites were selected for moredetailed evaluation. Field biologists made 89 visits to studysites beginning spring 1997 through fall 1999 to examine thenumber and type of frog malformations. Over 5,100 Leopardfrogs (Rana pipiens) were captured and examined atall study sites. Water elevations and associated littoralinundation were recorded from 1997-2000. Results indicate thatmalformation occurred at all study sites above historicalbackground levels. Rana pipiens malformation across allsites over three seasons averaged 7.9% and ranged from 0 to 7% at reference sites and 4 to 23% at affected sites. At onenorthern Minnesota site, mink frog (Ranaseptentrionalis) malformation was 75% in 1998. A sitecharacteristic common to the most affected sites was an elasticzone of littoral inundation. Climate driven hydrologic variationlikely influenced water depth and associated breeding locations.