A battle between the desert tortoise and the United States military has reached a détente in the sun-parched Mojave Desert of southern California.

At issue was, as with most conflicts, territory. For years the tortoise and Marine Corps Air Ground Combat Center at Twentynine Palms in California had coexisted peacefully.  But after the US Congress voted in 2013 to expand the combat center for live-ammunition desert training missions, the tortoises faced jeopardy.

But as the desert tortoises are mere civilians in this conflict, the marines that run the combat center are now on a mission to finalize the transfer of hundreds of the animals to a secure location.

It's the largest relocation of tortoises ever. It even has a name: Operation Desert Tortoise.

Rough times for tortoises

The desert tortoise, which roams the Mojave and Sonoran deserts, is "threatened" under the United States Endangered Species Act.

Ileene Anderson, a scientist with the Center for Biological Diversity, said biologists began seeing die-offs in the western Mojave in the late 1970s due to a respiratory pathogen, possibly introduced through interaction with other formerly captive tortoises.

"People were taking tortoises from the deserts and keeping them as pets, and sometimes they would drop them back into nature," Anderson said. "It's possible that mingling with other species in captivity could have helped spread the virus - but it's not known."

Now, because of the tortoises' protected status, it's illegal to even touch them, except as part of scientific operations.

Predators like coyotes are also a problem, especially for young tortoises with soft, tasty underbellies. But it's the human footprint, including urban development, illegal livestock grazing and even recreational off-road vehicles, which have taken the biggest toll on the population.

With humans come trash, telephone poles and hungry ravens looking for a tasty morsel of tortoise, according to Jane Hendron, a spokesperson for the US Fish and Wildlife Service (FWS) in Carlsbad, California.

"Ravens are opportunistic, and they follow humans and the trash they leave behind. They find telephone polls a good perch to look for tortoises on the ground," Herndon said.

And because tortoises don't start reproducing until the age of 16, it takes a while to replenish a decimated population. So, with all of the stressors facing the tortoises - including years of drought, which prevents females from laying eggs - the last thing this population needed was the threat of military vehicles.

Risky mission

Keeping the tortoises safe is more complicated than simply moving them to another part of the desert. It's been a nearly three-year planning operation.

In 2014, authorized biologists began attaching a radio transmitter to each tortoise they found, before releasing them back into the wild. The most critical phase of Operation Desert Tortoise began on April 8, 2017, when more than 100 biologists fanned out in the desert lands west and south of the command center to find tortoises with radio transmitters.

Biologists then gave all tortoises full health assessments, including blood work and a physical exam, before packing 930 animals into plastic bins. Helicopters carried the bins 20 kilometers (12 miles) away, to one of five sites identified as supportable tortoise habitat. Authorized biologists hand-carried each tortoise to its designated release point, and observed its immediate behavior after release.

"Throughout the collection and release process, each tortoise was checked multiple times for identity and health status," Christiansen said.

The combat center is keeping radio transmitters on about 20 percent of the translocated animals, as well as on an equal number of tortoises already living in the relocation sites, Christiansen added. The tortoises in the new areas were also checked for respiratory and other illnesses, to avoid infecting their new neighbors.

Big price tag

The total cost of Operation Desert Tortoise is nearly $50 million (45.8 million euros) - but that also funds a 30-year monitoring program.

Both Christiansen and FWS said moving the animals wouldn't hurt the survival of the species.

But there is some evidence that moving tortoises, even under the most careful planning, could be harmful. A 2014 article published in Animal Conservation highlighted serious health threats that could result from desert tortoise translocation.

Then there is the legacy of Fort Irwin. In 2008, the movement of desert tortoises from Fort Irwin National Training Center, also in the Mojave Desert, to areas south of the installation, was halted after biologists found higher-than-expected death rate among tortoises that had been moved.

But a study published in 2010 in "Endangered Species Research" blamed drought conditions, not disease or inadequate planning, for the high death rate. The study said the tortoises generally fared no worse than those living in wild areas of the Mojave Desert.

There goes the neighborhood

Hendron said that much has been learned from each successive translocation of tortoises, and she pointed out several things that Operation Desert Tortoise is doing differently than in the Fort Irwin case.

One difference is the use of a "head start" facility at Twentynine Palms. Any tortoise too small to support a radio transmitter was moved to the combat center's tortoise research and captive rearing site, where it will be held and monitored until it's large enough to head out on its own.

Hendron added that every effort has been made to keep tortoise "neighborhoods" together in the move. This can decrease stress of finding their way in the new (though very similar) landscape.

Anderson said that, even with all the measures to preserve the neighborhoods and minimize stress, the tortoises still might face difficulty.

"They don't know safe spaces where they can avoid predators in this new landscape, and they won't know where the rainwater pools are, so they can get a drink," Anderson said.

Anderson added that if the tortoises don't adapt to the new "neighborhood" right away, they might try to return home.  "When moved, tortoises often try to go back - and we've seen some navigate up to fifteen kilometers back to their home base."

But, she added, even though the tortoises are mobile, they are pretty slow.