Allyson Walsh, associate director for the zoo's Institute for Conservation Research, said just 30 percent of the residents are receiving medical treatment, though some others have been quarantined and need new evaluations.
"The ones that don't get better and that are sick and suffering will probably be euthanized because that's the sensible thing to do," she said.
She disputed the notion that budget cuts are forcing the reptiles to be put down. Although the center has housed sickly tortoises for years, Walsh said they eventually would have been euthanized anyway.
Walsh said sick tortoises cannot be adopted out and she has not been contacted by any researchers interested in taking in the sick animals.
"That's a possibility but we wouldn't transfer an animal to anyone who was doing destructive research," she said.
The right thing to do for a sick animal is euthanize it, she said.
Seth Webster disagrees.
Webster, a 36 year old programmer from New York, created a Change.org petition that together with a similar one on the site has drawn more than 3,000 signatures. He said he is working with a Florida tortoise refuge that recently bought land in Nevada to see if Fish and Wildlife will transfer the tortoises, or at least let an outside evaluator decide which animals are so sick they should be killed.
"Animals have a very strong will to survive," he said. "These tortoises live to 100 years. If we euthanize him, are we robbing him of 30 years? It doesn't seem fair to euthanize them just because the tortoises are sick and someone ran out of money."
Desert tortoises have made their rocky homes in Utah, California, Arizona and Nevada for 200 million years. But the prehistoric animal has some unfortunate evolutionary quirks, including a susceptibility to flu-like respiratory infections and difficulties settling in to new homes. They are also sensitive to change as the tortoises sometimes dehydrate themselves by voiding a year's worth of stored water when handled.