Nationwide Frog Abnormalities Stable at 2 Percent in US

A decade-long study has revealed that only 2 percent of frogs in national wildlife refuges have physical abnormalities such as missing or deformed legs.

The results suggest that a 1990s study detailing the incidence of frog abnormalities in wildlife refuges may be rare. There were just a handful of "hotspots" where frog abnormalities were consistent enough to be linked to local causes. The so-called hotspot regions include the Mississippi River Valley, California and south-central and eastern Alaska.

"We now know what the baseline is and the 2 percent level is relatively good news, but some regions need a deeper look," said Marcel Holyoak, professor of environmental science and policy at the University of California, Davis, and a co-author on the study.

The fieldwork was conducted at wildlife reserves around the US between 2000 and 2009, where researchers collected more than 68,000 frogs and toads to study.

Rather than attempting to identify specific causes of the frog abnormalities, the researchers sought to determine where the abnormalities were occurring. Holyoak said that once there is an understanding of where these frog and toad abnormalities are occurring, researchers can home in on why they are happening.

Holyoak said the results show that these hotspots occur in specific places, but within the hotspots the rate of malformations change over time.

"We see them at an elevated frequency one year or for a few years, and then they recover," he said.

The most common abnormalities were missing or shortened toes or legs, and skin cysts, the researchers found.

Among all the abnormalities found in frogs surveyed nationwide, only 12 cases of frogs with extra legs were observed.

As to why the abnormalities occur, several suggestions have been put forward, including agricultural parasites, pollution from industry, ultraviolet exposure and naturally occurring metals leaking into bodies of water in the frogs' habitat.

The research is published in the journal PLOS One.