Twenty-Third Annual Meeting and Symposium of the
Desert Tortoise Council, April 3-5, 1998
Abstracts

In 1996 and 1997, 102 tortoises were marked during two 45-day censuses on a one square kilometer plot in the Tucson Mountains as part of a three year project to study the desert tortoise at Saguaro National Park. Frequency of observation of individual tortoises was correlated with both body size and with average distance from the edge of the plot. The relationship between body size and number of prominent growth rings indicates a growth rate similar to that obtained from recapture data if each ring is counted as one year. Most tortoises greater than 20 years could not be aged precisely because inner rings were worn away and outer rings could not be resolved however it was clear that they were at least 20 years old. The age distribution was evaluated alternatively by calculating a Lincoln Index estimate for each age class and by using simple models to adjust the age frequency distribution for effects of body size and distance to the plot boundary. Preliminary results indicate possible high variability of annual productivity and a low mortality rate for juvenile and immature tortoises. The age distribution of tortoise shell remains recovered in the Tucson Mountains indicates that egg and hatchling mortality is highest although sample sizes are small. Validity of age determination from growth rings needs to be tested in other populations where many annual recaptures of young tortoises are expected. Methods for the adjustment of age distributions need to be refined.