You've got to respect frogs. The amphibians have an astonishing variety of survival skills.
For example, gastric brooding frogs swallow their fertilized eggs to protect them from predators. When the eggs hatch and the tadpoles mature, the mother safely regurgitates her froglets.
Such adaptations have kept frogs around for millions of years, but a recent checkup has scientists concerned. They fear a sudden mass extinction of amphibians on a scale and pace not experienced since the age of dinosaurs.
Nearly a third of the world's 6,000 species of frogs, toads and salamanders are in danger of disappearing, according to the Global Amphibian Assessment.
In the past 20 to 30 years, about 120 species of frogs are believed to have become extinct, the assessment found.
Human destruction of habitat is one reason for the loss. But there is a new enemy for which there may be no defense.
On the trail in Central America
In 1998, Karen Lips, a young biologist from Southern Illinois University, helped identify a fungus that seemed to be killing off entire species of amphibians, including, possibly, the gastric brooding frog mentioned above.
It's called the chytrid fungus, and how it kills is a mystery.
The best guess, according to Joe Mendelson, curator of herpetology at Zoo Atlanta, is that it attacks keratin, a protein that waterproofs the parts of a frog's skin most subject to wear and tear. The loss of keratin, it's believed, might throw off the critical water balance in a frog's body.
"That's just a guess," Mendelson said, "but the best one we have right now."
For years, the fungus was one step ahead of Lips and her team of biologists. They'd visit a forest where frogs were thriving, then return a couple of years later to find most had disappeared -- and with them the evidence of their fungal demise.
Lips' team set up a stakeout in a Panamanian rain forest called El Cope. There had been previous die-offs, beginning in the Monteverde Cloud Forest of Costa Rica, and it seemed the fungus was heading southeast. The fungus hadn't yet arrived in El Cope, but Lips said she suspected the rain forest would be its next stop.
Over six years, Lips and her team walked the forest, finding 64 thriving species of amphibians and no sign of the fungus, but one day that all changed.
In September 2004, one of Lips' graduate students found a frog sitting on a rock in the middle of the day -- unusual because that type of frog was only active at night. The frog wasn't moving, and it was behaving as if it had the flu. It became the first frog on the group's stakeout to test positive for the fungus.
Soon after the first infected frog was found, dead frogs started turning up every day, Lips said. But within four months, she said, "We ran out of dead frogs to count."
No more could be found. Lips and her team packed up their research and headed home.
The results of their work have now been published in the Proceedings of the National Academy of Sciences. The findings are important because they establish how quick and devastating a killer this fungus, which is spreading around the world, has become, Mendelson said.
'Noah's Ark' solution
Frogs are considered a good indicator of the health of the Earth. They breathe through permeable skin and live both on land and in water. They likely would be the first type of species to suffer from pollution.
They are also a critical link in the food chain. Lips and her colleagues reported that in one Panamanian region that experienced a massive frog die-off, the snakes that feed on frogs began starving to death. Birds and mammals that eat the frogs there may also be in decline, Lips said.
The question remains why, after such a long history of adaptation and survival, are entire species succumbing to a fungus?
One explanation has troubling implications for humans, said Claude Gascon, an ecologist with the group Conservation International.
The concern, Gascon said, is that between loss of habitat, toxins in the environment and global warming, "the Earth has been so stressed by mankind's footprint that, in the case of frogs, the chytrid fungus was the tipping point that indicates a dangerous turn in the overall health of the Earth's ability to sustain us all."
Lips' research established the path of the fungus, which is moving southeast through Central America. Her map should help scientists anticipate where the fungus is heading next.
Her findings have triggered a mission to rescue frogs in the path of the fungus, led by Zoo Atlanta's Mendelson and Ron Gagliardo of The Atlanta Botanical Garden. The approach, reminiscent of Noah's Ark, involves capturing enough males and females from each species, and breeding them in captivity, to establish survival colonies.
The hope is this approach can buy enough time for scientists to figure out how to protect frogs from the fungus and reintroduce them safely into the wild, wherever in the world they are threatened.