Egg Size and Annual Egg Production by Female Desert Tortoises (Gopherus agassizii): The Importance of Food Abundance, Body Size, and Date of Egg Shelling

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 summer. Due to El Niño 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 summer 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 produced 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.