In the case of the disappearing red-legged frog, bullfrogs, rather than mosquitofish, seem to be major culprits, according to a new UC Davis study. Since the turn of the century, California's native red-legged frog -- made famous by Mark Twain in his "leaping frog" tale -- has been declining in numbers. The victim of hunters and lost habitat, the frog was listed as a threatened species in 1996. It has nearly disappeared from the Central Valley, and its range is now restricted to the coastal foothills between Bodega Bay and Santa Barbara. Non-native species, including bullfrogs introduced for hunting and mosquitofish introduced for mosquito control, also were suspected in the red-legged frog decline. To test this notion, UC Davis ecologist Sharon Lawler conducted a study in cooperation with the San Joaquin County Mosquito Abatement District. She established 12 earthen ponds and added 700 hatchling red-legged tadpoles to each pond. Three of the ponds also received bullfrog tadpoles, three received adult mosquitofish, three received both and three received neither. Lawler discovered that only 5 percent of the red-legged frog tadpoles survived in ponds with bullfrogs, however tadpole survival wasn't affected by mosquitofish. "We can't let the mosquitofish completely off the hook because almost all of the tadpoles had been bitten by the fish, and juvenile frogs from mosquitofish ponds were smaller," said Lawler. She recently reported her findings at the annual meeting of the Ecological Society of America.