SREL Reprint #3505

Effects of short-term, outdoor head-starting on growth and survival in the Mojave Desert Tortoise (Gopherus agassizii)

Tracey D. Tuberville1, Kurt A. Buhlmann1, Rahel Sollmann2, Melia G. Nafus2,3, J. Mark Peaden2,
Jacob A. Daly1,4, and Brian D. Todd2

1University of Georgia’s Savannah River Ecology Laboratory, Post Office Drawer E,
Aiken, South Carolina 29802, USA
2Department of Wildlife, Fish and Conservation Biology, University of California, Davis,
One Shields Ave, Davis, California 95616, USA
3Fort Collins Science Center, U.S. Geological Survey, 2150 Centre Avenue Building C,
Fort Collins, Colorado 80526, USA
4Parks Reserve Forces Training Area, Directorate of Public Works, Environmental Division,
Dublin, California 94568, USA

Abstract: The combination of life-history traits that makes some turtle species vulnerable to population declines also limits their ability to recover even after threats have been addressed. Because juvenile turtle survival is typically lower than adult survival, head-starting, the process of rearing juveniles through one of their most vulnerable periods, may be a useful recovery tool. We evaluated short-term, outdoor head-starting in Mojave Desert Tortoises (Gopherus agassizii) by comparing growth and survival among three treatments: (1) juveniles reared in outdoor predator-resistant enclosures and receiving low (LOW) or (2) high levels of rain supplementation (HIGH); and (3) free-ranging animals released 0–18 mo after hatching (FIELD). Juveniles from the HIGH treatment had higher annual growth (12.7 mm midline carapace length [MCL] per year) than juveniles from the LOW or FIELD treatments (10.7 mm). Annual growth also varied among years, presumably due to variation in rainfall. Annual survival was high (0.94 ± 0.01) for both LOW and HIGH treatments; MCL at hatching had a weak positive effecton survival probability (effect size: 0.42 ± 0.35). Annual survival of FIELD animals averaged 0.48 ± 0.09. There was no effect of size at release (40.8–61.5 mm MCL) on post-release survival of FIELD animals, suggesting that the greatest benefit of short-term outdoor head-starting is increasing survival during the head-start period. Although releasing at larger sizes (100 mm MCL) has been recommended, slow growth in tortoises would require extended outdoor head-starting periods. Indoor rearing, which has been successfully implemented with other turtle species, may increase growth rates of juvenile Desert Tortoises and warrants future study as a conservation technique.

Keywords: Mojave Desert; population manipulations; species recovery; Testudinidae

SREL Reprint #3505

Tuberville, T. D., K. A. Buhlmann, R. Sollmann, M. G. Nafus, J. M. Peaden, J. A. Daly, and B. D. Todd. 2019. Effects of short-term, outdoor head-starting on growth and survival in the Mojave Desert Tortoise (Gopherus agassizii). Herpetological Conservation and Biology 14(1): 171-184.

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