Seventy critically endangered Mojave Desert tortoises have emerged from their winter burrows after being reintroduced into their native habitat on Edwards Air Force Base in southern California, a press release from the San Diego Zoo Wildlife Alliance said.
The alliance and The Living Desert Zoo and Gardens, a global authority on desert plant and wildlife conservation, are working on the desert tortoise recovery programme in partnership with Edwards Air Force Base, the U.S. Fish and Wildlife Service, and the U.S. Geological Survey, using “headstarting” techniques designed to increase the survival rate for young tortoises.
The cohort of young tortoises was the first in the programme to be reintroduced into the wild after receiving a combination of indoor- and outdoor-rearing. Within 24 hours of re-entering their native habitat, the tortoises were exhibiting positive natural behaviours by constructing new burrows or modifying existing ones for shelter.
Now, six months later, they have emerged after winter brumation—a state of deep sleep specific to reptiles—and are spending time basking at the entrances to their burrows, the release said. Scientists are conducting health assessments and replacing radio transmitters on the tortoises to continue monitoring their success and better understand the juvenile age class.
“We have worked so hard to get here, and we have been through so much together,” said Melissa Merrick, PhD, associate director of recovery ecology at the San Diego Zoo Wildlife Alliance. “We have had to re-strategize, switch plans, react, and adapt to so many emerging situations, and we have all done it successfully to get to this point.”
Merrick said the second group of young headstart tortoises had just arrived from The Living Desert and would spend the next six months with the San Diego Zoo Wildlife Alliance before joining their predecessors in the wild. “It’s an exciting time for the programme,” she said.
Scientists track gravid (egg-carrying) desert tortoises, monitor the adult females as they lay eggs in human care, and rear the hatchlings for up to two years. The hatchlings are reared indoors at The Living Desert for six months, and then in a protected outdoor environment at Edwards Air Force Base.
By reintroducing the tortoises into their native habitat when they are no longer at their most vulnerable, conservationists are giving the juvenile tortoises a fighting chance at survival—as evidenced by the success of the first reintroduced cohort. Indoor rearing at The Living Desert enables the tortoises to grow to three to five times the size they would be at this stage in their native habitat, making them less vulnerable to predation.
The Living Desert Zoo and Gardens is a nonprofit, accredited member of the Association of Zoos and Aquariums, ensuring the highest standards in all aspects of animal care, education, conservation, public service, and operations. The organization is active in conservation research, habitat protection, breeding programmes and education initiatives around the world as well as in its own community. The zoo has been a top destination in the Palm Springs area for 50 years.
Team members from the San Diego Zoo Wildlife Alliance, The Living Desert, and Edwards Air Force Base were present for the reintroduction of the 70 young tortoises into the Mojave Desert—a full-circle moment for those who cared for the tortoises in their first year of life. More than a year earlier, scientists of the alliance were excavating nests in the middle of the night in emergency circumstances due to a dangerous heat wave, and wildlife care staff at The Living Desert were preparing to receive the tortoises a month early and care for those still in varying stages of hatching.
“Our headstarting programme is essential for maximizing the success of young tortoises as we release them into the wild,” said James Danoff-Burg, PhD, director of conservation at The Living Desert Zoo and Gardens. “Mortality of juvenile desert tortoises is dramatic, often approaching 100% in areas where ravens are overpopulated due to humans providing them food via their open trash containers.”
Danoff-Burg said the benefits of providing a head start are clear as within six months of life, the tortoises get to the approximate size of a two-and-a-half-year-old tortoise in the wild. “Larger tortoises are much more resistant to raven and coyote predation than smaller ones, and our headstarting programme ensures that not only normal juvenile mortality rates in the wild are reduced, so is predation,” he said. “Headstarting makes reintroductions of desert tortoises more successful.”
This keystone species in the southwest USA serves as a prime example of the dangers of a changing climate. In September, the nests of a second headstart cohort—laid in protected outdoor habitats at Edwards Air Force Base—were again excavated in emergency circumstances. Predatory ants and fly larvae attacked hatchling tortoises as they emerged from their shells. It is believed that the arrival of Hurricane Hilary created extra moist conditions, making the desert a breeding ground for these insects.
The surviving tortoises received veterinary care at The Living Desert. The cohort—following in the footsteps of their predecessors—was reared indoors at The Living Desert for the first six months of their life. This month, they will be transferred to the outdoor headstart habitats at Edwards Air Force Base managed by San Diego Zoo Wildlife Alliance, where they will spend the next six months. At approximately one year of age, they will be reintroduced into their native habitat.
Once common throughout the Mojave and Sonoran deserts of California, Nevada, and Arizona, desert tortoise populations have declined by an estimated 90% in the last 20 years. California’s Mojave Desert tortoise, federally protected in the USA since 1989, is categorized as critically endangered on the International Union for Conservation of Nature’s Red List of Threatened Species, and faces threats, including habitat loss and fragmentation, disease, human-subsidized predators, and climate change.
The U.S. Fish and Wildlife Service’s evaluation of population trends from 2018 indicates the species is on a path to extinction under current conditions. However, with continued successful efforts to address the threats they face, there is hope this trend can be reversed.
