Home Range Size and Movements by Desert Tortoises, Gopherus agassizii, in the Eastern Mojave Desert

Abstract

We constructed minimum convex polygon (MCP) home ranges for free-ranging desert tortoises from a natural population adjacent to the Desert Tortoise Conservation Center, near Las Vegas, NV. Home range area estimates were not significantly different from those estimated for other desert tortoises in the Mojave and Sonoran deserts. Male tortoises had significantly larger and more variable home ranges in a combined statistical analysis of this study with those of Burge (1977) and Barrett (1990). Bootstrap analysis of the MCP polygon areas suggested substantial autocorrelation of the tortoise sightings despite a mean interval between recaptures of 3.2 days, violating an assumption of nearly all home range estimation techniques and predisposing to underestimation of the true home range area. Extending the interval between recaptures would severely limit the number of points that could be obtained on an individual tortoise in a single activity season. We also created "by eye" minimum polygons to compare with MCP's for the same tortoises. This comparison suggests that MCP's for desert tortoises include, as substantial fractions of their total area (12-56%, mean = 35%), areas with no evidence that tortoises use them. Movements between resightings vary with the sex of the animal (male > female) and interval since previous sighting. The distance of movements was approximately exponentially distributed, with short movements more common than longer movements, predisposing home range estimates for desert tortoises to be autocorrelated. We urge the consideration of home range as an indicator of size of the areas traversed by a tortoise and the patterns of movement in different individuals, sexes, and/or populations with less emphasis on the biological interpretation of area as a resource or characteristic of the animal.