Journal of Herpetology, Vol. 33, No. 3, pp. 394-408, 1999 Copyright 1999 Society for the Study of Amphibians and Reptiles Egg Size and Annual Egg Production by Female Desert Tortoises (Gopherus agassizii): The Importance of Food Abundance, Body Size, and Date of Egg Shelling I. R. WALLIS,1 B. T. HENEN2 AND K. A. NAGY3 Department of Biology and Environmental Biology Division, Laboratory of Structural Biology and Molecular Medicine, University of California, Los Angeles, 900 Veteran Avenue, Los Angeles, California 90024-1786, USA ABSTRACT.-We studied egg production in two Californian populations of desert tortoises, (Gopherus agassizii) in 1992 and 1993. One population inhabited the Desert Tortoise Research Natural Area (DTNA) in the western Mojave Desert, where most of the rain falls in the winter. The second population lived near Goffs, in the eastern Mojave, where annual precipitation is divided more evenly between winter and sum- mer. Due to El Niio conditions, heavy winter rains fell at both sites in both years (1991-1992 and 1992- 1993). Consequently, the biomass of spring annuals and annual egg production by tortoises were high in both years at both sites. There were no differences in reproductive output between years so we pooled data for both years to examine the relationship between egg-laying parameters (clutch size and frequency, annual egg production, egg size, etc.) and female size. Variation in annual egg production was due mainly to variation in clutch size, not clutch frequency. Annual egg production per female was lower at DTNA than at Goffs, because some adult females at DTNA did not produce eggs in some years. Females that did lay eggs produced the same number of eggs per year at both sites, even though females at Goffs were smaller (midline carapace length = 214 mm) than females at DTNA (MCL = 234 mm). Despite correction for these body size differences, the eggs produced at Goffs were smaller in all dimensions than eggs produced at DTNA. Smaller eggs and presumably smaller neonates may be related to the greater predictability of sum- mer rain and consequent greater food supply for emergent hatchlings at Goffs. For adult females, food supply probably limits reproduction only during drought years. How can individual females vary their annual reproductive output? Our more extensive data for DTNA tortoises showed that larger females pro- duced larger clutch sizes. In addition, by statistically removing the effects of body size we showed that larger clutches contained smaller eggs. Moreover, larger females produced eggs earlier in the year giving them a better opportunity to produce a second clutch that year. Thus, timing of first clutch was important Still, much of the variation in reproductive output was not explained. Other characteristics of individuals (e.g, age, genetics, physiological maturity, home range quality, or forage selection) may explain some of the variation in reproductive output Rainfall in the Mojave Desert is unpredictable and varies greatly with time and location. While the long-term average annual precipitation is about 150 mm, annual precipitation ranges from as little as three millimeters to as much as 400 mm. Rain often falls primarily in the winter, promoting a flush of annual plants in the spring (Beatley, 1974), but seasonal rainfall patterns usually vary considerably from year to year. Rainfall also differs between the eastern and western regions of the Mojave Desert (Nagy and Medica, 1986; Peterson, 1996a). Rainfall from July through September, when many desert tor- 1 Present Address: Divisionof Botany and Zoology, Australian National University, Canberra ACT 0200, Australia. 2 Present Address: Smithsonian Institution, National Zoological Park, Department of Zoological Research, 3001 Connecticut Avenue, Washington, District of Co- lumbia, 20008, USA E-mail: henenbt@aol.com 3 To whom correspondence should be sent. E-mail: kennagy@biology.ucla.edu toise eggs hatch, frequently exceeds 40 mm in the eastern Mojave, whereas the western Mojave averages 20 mm or less (National Oceanic and Atmospheric Administration, 1995). Thus, sum- mer annuals are more likely to germinate and grow in the Eastern Mojave and, as a result, tor- toise hatchlings in the Eastern Mojave are more likely than their western counterparts to find food and water before hibernating in autumn. We wondered whether variation in tortoise re- production matched current resource availabili- ty for adults or food and water availability for hatchlings. The nutrients for reproduction may come from nutrients consumed from the environment and from the animal's body stores. However, an animal confronted with unpredictable food and water supplies should not draw too heavily upon body reserves if doing so threatens its life. This is especially true for female desert tortoises because: (1) they require many years to mature (ca. 15 yr. Woodbury and Hardy, 1948) and, (2)