We measured survival, growth and body condition of eight hatchling cohorts of desert tortoises in captivity over 11 years to evaluate head-starting methods. At 11 years of age, seven times as many of the first cohort had survived than if they were free-living tortoises. Improvements in predator control, food and water supplementation and pen structure increased survival from seven to 10 times that under wild conditions. Annual survival averaged 96%. Carapace length (CL) increased 6.95 mm per year, similar to that of free-living tortoises. Annual growth rates varied with calendar year (possibly reflecting food and water supply), age, cohort (year hatched), mother, and in four dry years, with crowding. Most of the first cohort grew to a releasable size (CL >100 mm) by their ninth year. Body Condition Indices (CI) remained high, indicating little dehydration despite droughts in eight of the 11 years, because irrigation offered drinking opportunities. Head-started tortoises developed fully-hardened shells (>98% of adult shell hardness) earlier (10.1 vs 11.6 years), but at a larger CL (117 vs 104 mm), than did free-living tortoises. Selective feeding in head-start pens decreased subsequent germination of favored wildflower species, apparently by reducing the natural seedbank. Consequently we reseeded and irrigated each autumn to promote subsequent spring food supply. We irrigated in early summer to enable drinking and ensuing consumption of dry, dead plants and Bermuda grass hay, a supplement. These procedures can greatly improve juvenile survivorship, and increase numbers of hard-shelled, midsized juveniles to help augment wild populations.

We released and monitored 53 juvenile Agassiz’s Desert Tortoises (Gopherus agassizii), aged two to 15 y, that were hatched and head-started inside predator-resistant field enclosures. We set free these tortoises under a variety of conditions to evaluate effects of release distance, season of release, and age/body size on homing behavior and survivorship. Some juveniles moved large distances following release, but homing itself was undetectable. The lack of homing behaviors was likely due to release distance and selection of release sites out of the line-of-sight of natal enclosures. The use of small halfway-house enclosures to accustom some relocated juveniles to release sites for four months before release (i.e. “soft release”) had no effect on subsequent movements or survivorship during the first year following release. Survivorship was not affected by distance of release from natal enclosures, which ranged from 546 m to 1.4 km. Survivorship through one year was similar for juveniles released in spring or autumn. After two years, most small juveniles had been killed by predators, but survivorship increased with body size and age. Juveniles over approximately 100 mm MCL (midline carapace length) and nine years of age when released exhibited high survivorship. However, following a long drought the previous two years, predation by Coyotes (Canis latrans) was heavy on these larger juveniles in the third year after release. Thus, survivorship after three years was relatively low (34%) with the youngest, smallest cohort (two years old when released) exhibiting the lowest survivorship (4%). We recommend releasing headstarted tortoises after they attain a body size of larger than 100 mm MCL and selecting release sites at least 546 m from enclosures.