Size-specific Fecundity of the Desert Tortoise (Gopherus agassizii)

We determined the annual fecundity of desert tortoises using x-rays from 1993 through 1995 at Yucca Mountain, Nevada. The smallest tortoise to reproduce was 209 mm carapace length (CL); eleven smaller tortoises did not reproduce. The mean age of first reproduction was estimated to be 19-20 years. Clutch size and annual fecundity were related to female size, but annual clutch frequency was not Clutch size ranged from 1 to 10 eggs; annual clutch frequency ranged from 0 to 2. Mean clutch size of tortoises with a single clutch was 0.9 eggs larger than that of tortoises with two clutches. Annual fecundity ranged from 0 to 16 eggs. Predicted annual fecundity was three eggs for a 208 mm CL tortoise and increased by one egg for each 7 mm increase in CL. Annual fecundity of tortoises in this study was compared with that of tortoises studied in Goffs, California, during 1983-1985. The regressions of fecundity on female size had similar slopes, but tortoises at Goffs produced an average of 4.5 eggs more than tortoises of the same size in our study. Tortoises at Goffs, however, were significantly smaller than tortoises in our study.