SREL Reprint #3616

The effect of size on postrelease survival of head-started Mojave desert tortoises

Pearson A. McGovern1,2, Kurt A. Buhlmann1, Brian D. Todd3, Clinton T. Moore4, J. Mark Peaden3,5,
Jeffrey Hepinstall-Cymerman2, Jacob A. Daly6, and Tracey D. Tuberville1

1Savannah River Ecology Lab, University of Georgia, P.O. Drawer E, Aiken, South Carolina 29802
2Warnell School of Forestry and Natural Resources, University of Georgia,
180 E Green Street, Athens, Georgia 30602
3Department of Wildlife, Fish and Conservation Biology, University of California, Davis,
One Shields Ave, Davis, California 95616
4U.S. Geological Survey, Georgia Cooperative Fish and Wildlife Research Unit,
Warnell School of Forestry and Natural Resources, University of Georgia,
180 E Green Street, Athens, Georgia 30602
5Department of Biology, Rogers State University, 1701 W Will Rogers Boulevard, Claremore, Oklahoma 74017
6U.S. Army Garrison Fort Hunter Liggett, Directorate of Public Works, Environmental Division,
Fort Hunter Liggett, California 93928

Abstract: Captive-rearing conservation programs focus primarily on maximizing postrelease survival. Survival increases with size in a variety of taxa, often leading to the use of enhanced size as a means to minimize postrelease losses. Head-starting is a specific captive-rearing approach used to accelerate growth in captivity prior to release in the wild. We explored the effect of size at release, among other potential factors, on postrelease survival in head-started Mojave desert tortoises Gopherus agassizii. Juvenile tortoises were reared for different durations of captivity (2–7 y) and under varying husbandry protocols, resulting in a wide range of juvenile sizes (68–145 mm midline carapace length) at release. We released all animals (n = 78) in the Mojave National Preserve, California, United States, on 25 September 2018. Release size and surface activity were the only significant predictors of fate during the first year postrelease. Larger sized head-starts had higher predicted survival rates when compared with smaller individuals. This trend was also observed in animals of the same age but reared under different protocols, suggesting that accelerating the growth of head-started tortoises may increase efficiency of head-starting programs without decreasing postrelease success. Excluding five missing animals, released head-starts had 82.2% survival in their first year postrelease (September 2018–September 2019), with all mortalities resulting from predation. No animals with >90-mm midline carapace length were predated by ravens. Our findings suggest the utility of head-starting may be substantially improved by incorporating indoor rearing to accelerate growth. Target release size for head-started chelonians will vary among head-start programs based on release site conditions and project-specific constraints.

Keywords: desert tortoise; species recovery; head-starting; size; survival

SREL Reprint #3616

McGovern, P. A., K. A. Buhlmann, B. D. Todd, C. T. Moore, J. M. Peaden, J. Hepinstall-Cymerman, J. A. Daly, and T. D. Tuberville. 2020. The effect of size on postrelease survival of head-started Mojave desert tortoises. Journal of Fish and Wildlife Management 11(2): 494-506.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).