By Ben Neary
NMWF Conservation Director
Biologists in southern New Mexico have been nurturing a population of rare tortoises over the past nearly 20 years. Starting with 30 adults, they’ve raised hundreds of juveniles on a private ranch and now are on track to relocate a group to the Sevilleta National Wildlife Refuge this fall.
Dr. Chris Wiese, a senior biologist with the Turner Endangered Species Fund, directs the program to restore the endangered Bolson Tortoise, the largest and rarest tortoise species native to North America.
Wiese will talk about the tortoise restoration project as the featured speaker for the New Mexico Wildlife Federation’s free Wildlife Wednesday series in July. Her free presentation will start at 5:30 p.m., July 10, at the Marble Brewery Northeast Heights Taproom, 9904 Montgomery Blvd., NE., in Albuquerque.
Wiese and her husband, biologist Scott Hillard, are the principle biologists working on the Bolson Tortoise project. They have worked mainly on the Armendaris Ranch, owned by billionaire philanthropist Ted Turner, the sponsor of the restoration project. Wiese started as a volunteer in 2007 and has been directing the tortoise-breeding program since 2012. Wiese noted that Javier Gonzalez, a field biologist, is an important member of the recovery team.
The Bolson Tortoise once ranged from what is now Texas east to parts of Arizona. Its range extended from the Chihuahuan deserts of southern New Mexico down into Mexico. Today, biologists estimate the wild population at only about 2,500 adults, all in Mexico.
“The range of the Bolson Tortoise, about 10,000 to 12,000 years ago, began to shrink tremendously,” Wiese said. “And the only extant wild population is in an area called the Bolsón de Mapimí, which is where the Bolson Tortoise gets its name.”
The wild tortoise population is in north-central Mexico where the provinces of Coahuila, Chihuahua and Durango come together.
The Turner organization and the U.S. Fish and Wildlife Service reached a formal agreement last year specifying that the federal agency was interested in restoring the Bolson Tortoise to New Mexico, a place where it hasn’t existed likely for thousands of years. Under that agreement, the USFWS and the Turner organization became official partners in recovering the tortoise.
Work to conserve the tortoises on the Turner ranches started in 2006, when 30 tortoises that had been kept on a private ranch in Arizona were moved to New Mexico. Hillard has been involved in the project since before the tortoises were relocated, Wiese said.
“Twenty-six of those tortoises came to the Armendaris Ranch and four went to the Living Desert Zoo and Garden State Park in Carlsbad,” Wiese said. “We started breeding them and today we still have twenty-three of the original twenty-six. And we now have an additional seven hundred juveniles. They’ve been doing well.”
The tortoise restoration project has expanded beyond raising the animals in protected enclosures.
“In the spring of 2021, we started putting some animals outside of enclosures, with transmitters, so we’re calling it an experimental release,” Wiese said. “Because we needed to basically start to understand the parameters of releasing. And when you put some juveniles out there, how far do they walk, and where do they go, and how do they do? Do they get eaten by predators? All these kinds of things.”
The initial 2021 release was over 100 tortoises and there have been a couple of similar releases since then. Wiese said about 80 percent of them stay within a radius of three-quarters of a mile of the place where they were released.
Wiese and others have experimented to determine the optimum size of the juvenile tortoises for release. “The smallest tortoises get basically hammered,” she said. “They disappear. They get eaten by everything.”
Tortoises become a little more resistant to predator attacks when they reach about the size of native box turtles, Wiese said. She said the tortoises they’re releasing now are around 4 inches along the midline carapace. The largest mature tortoises measure over 15 inches along their midline carapace, she said.
Three years isn’t long enough to know where the tortoises will ultimately decide to settle, Wiese said. “Some are moving, because perhaps they’re reaching sexual maturity, and perhaps they’re looking for a new village to settle in. Perhaps in another decade, we’ll understand why these guys moved, but the vast majority that we put on the ground stay within a mile of where we put them.”
It’s unclear how long Bolson Tortoises can live, but Wiese said the largest adults that came from Arizona are probably now around 70 years old. She said the first young born after the tortoises came from Arizona are only now reaching sexual maturity, at around 17 years old.
“When you’re working with tortoises, you have to be very patient. Everything takes many years,” Wiese said.
Wiese and Hillard have been working with the U.S. Fish and Wildlife Service and plan to release young tortoises at the Sevilleta National Wildlife Refuge this fall. She said they will be placed in a release pen, where they will be protected to make sure they do well there. The plan is to add more tortoises next year.
“Our approach has been to put out a smaller number and make sure that we’re not finding things, for example the presence of predators we didn’t know about, and they all get eaten,” Wiese said. “And then when we find that they’re doing well in the area that we’ve chosen for them, we add more to that.”
The tortoises are herbivores and can go many months without drinking water. They spend much of their time in burrows, often returning to the same burrow each winter.
“The ultimate goal, from the beginning, is to establish free-living Bolson Tortoise colonies as assurance colonies in the U.S.,” Wiese said. “We believe that this tortoise is part of the Chihuahuan desert ecosystem, and it belongs here. And we want to see it not get extirpated completely, or even worse, disappear completely from the face of the earth.”
In addition to supporting the tortoise project, Turner, one of the largest landowners in the country, has supported several other projects to restore endangered species on his New Mexico ranches, including the black-footed ferret and the aplomado falcon.
Wiese said Turner’s continued commitment to the tortoise program over decades shows that he doesn’t shy away from a challenge. “He is committed to these species, and it doesn’t matter if it takes a long time and if it’s difficult,” she said.