Insatiable Invaders : Biologists Fear Nothing Can Stop the Spread of African Clawed Frogs


Almost no one speaks kindly of the African clawed frog, a weirdly exotic amphibian that came as an invited guest and has become Orange County’s most unwelcome resident.

Fifty years ago, the smooth-skinned creatures were imported to North America for aquariums and to play a scientific role in human pregnancy testing. But when pet owners tired of the creatures’ rude habits, and other pregnancy tests were discovered, the African frogs were unceremoniously dumped into the wild.

That was a Gargantuan mistake.

Over the years, the frogs--described as gluttonous predators--have propagated like crazy, armies of them taking over local drainage ditches, ponds and swimming pools. They have fanned out across the county via flood control channels and other waterways, attacking native species and giving people the creeps as they go.


The experts don’t know how Orange County’s problems with the amphibian compare with clawed frog woes in other counties or elsewhere in the nation. The strange frogs have vexed wildlife officials in Los Angeles and San Diego counties, and they have been spotted as far afield as Arizona and Washington, D.C.

But one thing is certain: They especially love Orange County.

“They’re kind of like the frog from outer space,” said Bill Bretz, manager of the San Joaquin Marsh Reserve, where “millions and millions” of the pests now live. “I don’t think they’ll ever be gotten rid of. I think they’re part of our landscape now.”

While most frogs feast on insects, this tongueless amphibian prowls the water for fish, other frogs and tadpoles and its own young.

Trying to halt the onslaught, biologists have unleashed an arsenal on the frogs, including traps, paralyzing chemicals, electrical zaps and even pellet guns, with a stunning lack of success.

The frogs just keep coming. And the damage continues.

“They’re just in frog heaven down there,” said Dave Dick, a Fish and Game spokesman in Sacramento. “I don’t think you’re going to get anything that’s quite as incompatible with our native wildlife as this guy.”

The frog’s voracious appetite for native wildlife is jeopardizing efforts to restore three Laguna Canyon lakes to their natural condition, according to biologist Elisabeth Brown, who will propose a pilot program to begin trapping the frogs there.

“They are not cute green things,” Brown said of the bug-eyed frogs, which have an olive-brown upper body and a whitish underbelly, sometimes with brown spots. “They are a terrible predator and they have an amazing way of eating.”

One of the frogs’ favored means of devouring dinner is by chomping onto their prey and then ripping a bite lose by pushing themselves backward with the large flippers on their back feet.

With finger-like appendages on their front legs, the frogs are named for the black “claws” on the three innermost toes of each hind foot.

Even in the tadpole stage, these critters behave peculiarly, Brown said, hanging upside down in ponds with the tips of their tiny tails spinning like helicopter rotors.

And the frogs are as sturdy as they are strange.

“They’ve found them under ice [in Washington, D.C.], so they can survive in extreme conditions,” said Jim St. Amant, a state Department of Fish and Game fisheries field supervisor who found the first clawed frog in a flood-control channel near Westminster in 1968 and spent 15 years trying to get rid of them. “So there’s no end to where they can go.”

Everybody, it seems, has a story about the clawed frog.

In 1984, literally millions of them were discovered in a Newport Beach reservoir, grossing out residents who prefer not to have frogs languishing in their source of domestic water.

Biologists in the Santa Clarita Valley have been battling the frog since 1989, because they say it is imperiling a tiny endangered fish known as the unarmoured three-spine stickleback, found only where the Santa Clara River flows through Soledad Canyon. They tried zapping the frogs with electric current but could electrocute only those frogs “dumb enough to get into range,” a state fishery biologist said.

The frogs are so thick in the Upper Newport Bay Ecological Reserve that when a volunteer naturalist left a net out to trap crayfish overnight, she instead captured 69 clawed frogs.

“She has the record,” said John Scholl, a state Fish and Game wildlife interpreter at the reserve, where there is an ongoing effort to snag the frogs. “We now call her the frog queen.”

As the African frogs have multiplied in the San Joaquin Freshwater Marsh next to UC Irvine, other species have vanished. Even the once-ubiquitous bullfrogs have disappeared--usually being eaten in the tadpole stage.

“A place where you might have had a half-dozen [native amphibian species] now just has clawed frogs in it,” Bretz said. “Who knows how we’re altering the ecosystems by the loss of these other amphibians?”

And while other wildlife populations dwindle, the hearty African frogs display a mind-boggling adaptability.

Largely aquatic creatures, they can burrow into mud during dry spells and hibernate for weeks. Or, they can hop for miles over dry land to find water. With their admirable ability to absorb oxygen from water through their skin, the frogs can also stay submerged for hours without drowning. They even seem to thrive in polluted waters.

“They’re so tough that you can just keep them in a bottle of water,” said St. Amant, who moved to Arkansas after hoisting the white flag to the frogs in 1990, when he retired from the Department of Fish and Game. “They’re just unbelievable.”

Dick said state officials are still concerned about the proliferation of the frogs in California. But budget constraints prevent greater efforts to eradicate the pest.

The frogs dodge would-be predators by secreting a frothy, foul-tasting mucus from their skin. Although their tadpoles are vulnerable, almost nothing will eat a developed clawed frog.

“I’ve even seen a great blue heron trying to swallow an African clawed frog one evening and it tried to spit it back out,” Scholl said.

Worried about the African transplant’s effect on natural habitats, experts in the 1970s began searching for something that would eat the frog. They tried to feed the frogs to a variety of fish and turtles and even tried coaxing a crocodile to eat one.

“Nothing would eat them,” said Robert Fisher, a UC San Diego researcher who began studying the frogs 10 years ago. “Or things would eat them and then they would spit them back out.”

Fisher, who has captured and marked the frogs and then tracked their movement in Orange County, said they have expanded mostly in low-lying areas where development has flourished. They favor man-made waterways with slow-moving or standing water, such as drainage ditches or golf courses, over natural creek beds, which are more likely to flood or dry up altogether.

“All the golf courses in Irvine are full of clawed frogs,” Fisher said.

As development continues in South County and expands toward the Santa Ana Mountains, the frogs are bound to move to the higher elevations, Fisher said, where they can “knock out the native amphibians and fish.”

But some say the frog is less threatening than its lore suggests.

“It’s an imported species with a reputation a lot stronger than its actual damage potential,” said Patrick Moore, a Fish and Game spokesman in Long Beach. “There are many exotic species that we have problems with along the same lines.”

Still, it seems that the frogs have found the welcome mat only in laboratories, where they are used as research tools.

UCI professor Katumi Sumikawa said the frogs’ oocytes--immature eggs--are large, hearty and ideal for use in brain protein studies underway in the psychobiology department. Students inject the oocytes with ribonucleic acid--an essential component of all cells--and the oocyte then produces brain protein.

When the frogs were first brought to North America, scientists used them to determine pregnancy. A woman’s urine was injected into the frog, which would ovulate if the woman was pregnant.

Although it has been illegal since about 1970 to buy, sell or trade clawed frogs in California, they were once sold in tropical fish stores. As people tired of their pet frogs--or got fed up with them eating everything else in the aquarium--they began releasing the creatures in local waterways, experts say.

Some people, hoping to sell the clawed amphibians to researchers, might even have tried breeding the frogs in local ponds, wildlife officials say, exacerbating the proliferation problem.

St. Amant said no one has tracked the frog’s movement nationwide, and it is impossible to determine precisely where the frogs first landed, how they have migrated or where they are creating the most serious problems. But wildlife officials say the one plucked from the Westminster channel in 1968 was the first documented clawed frog captured on the loose in California.

Brown, who became acquainted with the African frogs as a biology student and is now involved in helping restore the Laguna Lakes, said the exotic species was first noticed in Laguna Canyon after flooding about three years ago.

“I got some photographs and thought, ‘Oh God, that’s bad news,’ ” she said.

Then, this spring, following the winter’s heavy rains, Brown discovered pools of water “just loaded with tadpoles of this frog.”

As chairwoman of the Coastal Greenbelt Authority, which manages the Laguna Coast Wilderness Park where the lakes are located, Brown will propose the trapping program to the authority and then launch a search for funding.

“My goal is to try to clear them out this winter, to lower the population before they start breeding again this spring,” Brown said.

Despite past failures at trying to weed out the frogs, local biologists say they applaud Brown’s determination to wage war on the pest, even if only on a small battlefield in Laguna Canyon.

Recently, it had seemed like most people had given up, said Gary Reynolds, an Orange County Vector Control District biologist who is tired of having the mosquito fish that the district deposits in local ponds eaten by African frogs.

“It was kind of like our Africanized honeybees that are coming. They felt it would be fruitless to try to destroy them in the entire county,” Reynolds said. “They just figure we have to live with them.”


African Clawed Frog

These amphibians which were used in pregnancy tests during the 1960s, have proliferated in area lakes and waterways.

Size: 4 to 10 inches long.

Coloring: Olive-brown upper body and white underbelly which sometimes is flecked with brown spots.

Habitat: Flood-control channels, ponds and lakes.

Diet: Fish, tadpoles, other frogs and own young.

Natural enemies: None; repels would-be attackers by secreting foul-tasting mucus from skin.

Why the name?: Finger-like appendages on front legs, claws on three innermost toes of each hind foot.

Source: State Department of Fish and Game