We studied two closely situated (6.4 km distance) coastal populations of the Gopherus berlandieri in Cameron County, Texas. The Yturria Ranch population was monitored from 1972 through 1987, and the Reed Ranch population was monitored from 1977 through 1987. Tortoises were located by searching between grid lines 18.3 m apart that encompassed 3.3 ha on the Yturria Ranch and 2.0 ha on the Reed Ranch. Nine (3.7%) of 244 tortoises marked on the Yturria Ranch and 16 (10.5%) of 151 marked on the Reed Ranch died during the study. Timing of known deaths were distinctly different for the two sites, with nine tortoises found dead on the Yturria Ranch from 1983–87, whereas all known tortoise deaths occurred on the Reed Ranch from 1977–79. Annual survivorship estimates were higher for males and females on the Yturria Ranch, but survivorships at both sites were similar. Abundance of prickly pear (Opuntia engelmannii), on which tortoises feed, on the Yturria Ranch site may have contributed to a higher survivorship by providing high water content biomass during droughts and by providing cover that reduced predation.
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Vol. 45 • No. 1