Movement and habitat use of reintroduced bolson tortoises in southern New Mexico
Principle Investigators
Dr. Abby Lawson, New Mexico State University
Dr. Magnus McCaffrey, Turner Institute of Ecoagriculture
Dr. Chris Wiese, Turner Institute of Ecoagriculture
Graduate Student
Heather Zimba, MS candidate
Rationale.
The Bolson tortoise is the largest and rarest of the six Gopherus species native to North America. Today, however, fewer than 2,500 individuals remain in the wild, all of which are restricted to discontinuous basins in the Mapimí sub-province of north-central Mexico, representing >90% extirpation from their Pleistocene range. The Turner Endangered Species Fund (TESF) operates the only large-scale ex situ breeding program aimed at conserving Bolson tortoises and establishing assurance populations within their Pleistocene range in the United States. Reintroduction is an increasingly important conservation tool for long-lived, imperiled species, yet evaluating early success remains challenging when demographic responses require decades to detect. TESF biologists released 101
transmittered juvenile tortoises to the wild on the Armendaris Ranch in 2021 and 2022 and intend to release an additional 70 individuals between fall 2023 and spring 2024. Since this translocation project is taking place in the historical range of Bolson tortoises, there is a large gap in our understanding as to what constitutes ideal habitat for this species. To begin to address this knowledge deficit we investigated post-release settlement, displacement, and daily movement patterns of head-started juvenile Bolson tortoises (Gopherus flavomarginatus) released in southern New Mexico from 2021–2024 as part of the first experimental reintroduction following extirpation ≥11,000 years ago.
Outcomes.
Using VHF radiotelemetry to monitor 141 juveniles across six cohorts over multiple years post-release, we found that fall-released tortoises settled more rapidly than spring releases, though settlement distances were similar; higher temperatures delayed spring settlement while greater shrub cover heterogeneity accelerated it. Linear displacement over multiple years was generally low but variable, with spring cohorts showing increasing displacement while fall cohorts plateaued, indicating greater long-term site fidelity. Throughout the multi-year monitoring period, daily movement probability increased with precipitation, moderate temperatures, shrub cover, and time since release, while step length decreased with higher local burrow density and increased with age. Contrary to predictions, conspecific presence did not increase movement distances, suggesting benefits of proximity to active burrows such as thermal refugia access. Our results demonstrate that movement-based metrics in the first several years following release provide early, informative indicators of reintroduction progress for long-lived species, revealing that both spring and fall releases are viable with distinct tradeoffs, and highlighting the importance of release timing, vegetation structure, and climate conditions in shaping post-release behavior and emphasize the need for long-term monitoring to fully evaluate reintroduction success.
Zimba, H. 2026. Site settlement, displacement, and daily movement of released juvenile bolson tortoises in southern New Mexico. MS Thesis, New Mexico State University, Las Cruces, New Mexico.
