Effects of artificial rain on survivorship, body condition, and growth of head-started desert tortoises (Gopherus agassizii) released to the open desert

We subjected neonate Desert Tortoises (Gopherus agassizii) that hatched inside fenced, predator-resistant field enclosures containing natural vegetation to either a natural rainfall regime or a regime of natural rainfall plus irrigation (supplemental precipitation) over a five-year period, to test the hypothesis that mimicking an above-average rainfall regime in years of average or low natural rainfall will improve rates of survival and growth. We also tested the hypothesis that survivorship of released 1-yr olds will be high, due to a decline in predation susceptibility once the vulnerable nesting and hatchling phases are completed. Survivorship inside the enclosures during the first year of life was high (averaging 90%) in both groups, even during a record low rainfall year, but growth rates were always substantially higher (2 to 16-[1] greater) in rain-supplemented juveniles. Body condition index (CI) measurements indicated that first-year juveniles without added rain were able to maintain body conditions similar to rain-supplemented juveniles during two average rainfall years, but not during a drought year. Older juveniles without added rain died during the latter part of the 16 mo drought, suggesting that the high drought survivorship of first-year non-supplemented juveniles may be related to the yolk they carried after hatching, along with possible behavioral and physiological diffrerences. Nearly all yearlings that were set free (in autumn) were dead within 6 mo, regardless of whether they had supplemental rain or not during their first year inside enclosures, and regardless of whether they were released near the head-start enclosures or a kilometer away. The main cause of mortality was predation, primarily by ravens. The poor survival of released yearling tortoises and the drought-induced death of nearly all older captive juveniles raised without added precipitation lead us to recommend that rain supplementation and delayed release be incorporated in the protocol for head-starting Desert Tortoises