Both male and female reproductive organs were found in one-third of the 87 field mice, house mice, harvest mice and California voles trapped last year on land that once collected toxic runoff at the Kesterson National Wildlife Refuge. Just 3 percent of rodents trapped there showed such characteristics in 1995. We're not sure what it means, because it's new, and the phenomenon hasn't been widely reported. Technically, the rodents seem to be pseudo-hermaphroditic, according to biologist Gary Santolo, who led the investigation. They appear to be males from the outside, and lack a vagina, but have a fully developed set of female organs internally.
Some scientists say something in the environment is causing the outbreak.
Animals in the area have been monitored yearly since an investigation in 1985 revealed that selenium, a byproduct of agricultural runoff, was killing or deforming thousands of birds. Massive quantities of selenium, pesticides and other toxic chemicals were being deposited in the refuge in ponds that served as a sump for farm runoff. The refuge no longer encompasses the ponds. Selenium is not believed to be the cause of the hermaphroditic rodents. All of them had elevated levels of the element in their livers, but studies of selenium's effect on lab mice have not induced similar deformities.