Federal officials believe fewer than 200 of the federally endangered species of frog exist in the wild, the Los Angeles Times reported. Their natural habitat of the San Gabriel Mountains in California was fire-stripped in 2009, killing many of the frogs.
Authorities announced Tuesday that the effort to revive the species at the Fresno Chaffee Zoo suffered a setback when 104 yellow-legged frogs died mysteriously.
"We have two frogs left. We're trying to determine exactly what happened," said Scott Barton, director of the zoo, which is highly regarded for amphibian husbandry. "We were thrown a curve ball with a species that was new to us. It's been a humbling experience."
The Fresno zoo is not the only facility that has had setbacks in the task of breeding the frogs. Tadpoles released by the San Diego Zoo last year have not been seen since. Seven frogs living at the San Diego Zoo died of fungal infections in 2006.
With extremely low numbers, the yellow-legged frog is one of the most endangered amphibians on the planet.
U.S. Geological Survey ecologist Adam Backlin said "saving the mountain yellow-legged frog from extinction is turning out to be more difficult than anyone anticipated."