New Discovery to Help Threatened Desert Tortoises

Desert tortoises are among the species of wildlife threatened by the vast amount of housing development occurring in places like the Las Vegas region.  To help minimize the amount of tortoises wiped out by development the U.S. Fish and Wildlife Service has been relocating endangered tortoises to nearby protected areas with the belief that the closer animals are relocated to where they were found, the better their chances for survival.  Unfortunately, these efforts frequently have low levels of success, with substantial numbers of animals not surviving the relocation. But now a discovery from a new study has revealed that distance is not what determines whether relocated endangered animals will survive the move–it is the animals with more genetic variation who have the best chances for survival.  This discovery can have a significant impact on future conservation efforts.


Over the past several years, tortoises that were given up as pets, or removed from places like developments in suburban Las Vegas and solar farms in the desert, were surrendered to the U.S. Fish and Wildlife Service.  From 1997 to 2014, the U.S. Fish and Wildlife Service moved more than 9,100 Mojave desert tortoises to the 100-square-kilometer (about 39 miles square) Large Scale Translocation Site. The newcomers, many of which were abandoned pets or had been displaced by development, joined nearly 1,500 desert tortoises already living there.

Study overview

Over the years of relocating tortoises, the U.S. Fish and Wildlife Service took blood samples to screen for diseases and marked each animal before releasing them into the Ivanpah Valley protective site, which enabled the animals to be tracked in later surveys. UCLA researchers then sequenced the blood samples drawn from 79 tortoises that were released to the site and were known to be alive in 2015, and from another 87 known to have died after they were released at the site.

The researchers compared translocated tortoises that lived or died over the same time period after being relocated to the site. They found that survivors averaged 23% greater heterozygosity than those that perished. Simply put, tortoises with more genetic variation had higher survival rates.


Conventional wisdom has been that tortoises from areas closest to the translocation site would fare best. But a new study has found no connection between the tortoises’ place of origin and their chances of survival. It did, however, uncover a far better predictor: Tortoises with a lot of genetic variation were much more likely to survive after their relocation.

What’s next

The researchers wanted to make tortoise conservation efforts more effective, and uncover trends that would help other species as well.  Although the Large Scale Translocation Site provided an intriguing dataset, it’s not the same as a controlled experiment. Additional studies are needed to understand why more heterozygous tortoises have a higher survival probability and precisely how much of an increase in genetic variation improves a tortoise’s odds of surviving.

“When thinking about moving animals or plants out of danger, or repopulating an area emptied by wildfire, now we can easily and economically measure genetic variability to better gauge the survival probability of those translocated individuals. It’s not the only criteria, but it’s an important piece of the puzzle.”

-Dr. Brad Shaffer, researcher and professor of ecology and evolutionary biology and director of the UCLA La Kretz Center for California Conservation Science

Journal Reference: Peter A. Scott, Linda J. Allison, Kimberleigh J. Field, Roy C. Averill-Murray, H. Bradley Shaffer. Individual heterozygosity predicts translocation success in threatened desert tortoises. Science, 2020; 370 (6520): 1086.  DOI: 10.1126/science.abb0421