SEASONAL AND ANNUAL ENERGY BUDGETS OF FEMALE DESERT TORTOISES (GOPHERUS AGASSIZII)

Abstract

How do female desert tortoises (Gopherus agassizii) reproduce every year despite variability in winter rainfall and food availability? To answer this question, I measured energy budgets of individual female desert tortoises from July 1987 to July 1989. Females produced eggs in years with low levels of winter annual plants by relaxing their control of energy and water homeostasis. They tolerated large deficits and surpluses in their body dry‐matter composition (both nonlipid and lipid) on a seasonal, annual, and longer time scale. They could increase body energy content (lipid and nonlipid energy) before winter and use this reserve (especially nonlipid energy) the following spring to produce eggs. Females used high‐protein foods and rainwater, when available, to achieve energy surpluses that helped them survive periods of low resource availability (e.g., during hibernation and droughts). Adjusting seasonal and annual field metabolic rates (FMR) and food requirements to levels of food availability, they still managed to produce eggs, even in a drought year. Egg production in 1988 (mean ± 1 sd: 3.56 ± 1.94 eggs, N = 9) did not differ from that in 1989 (3.00 ± 2.69 eggs, N = 9); both were lower than during 1983–1987 (6.75 ± 3.05 eggs, N = 100). Energy per se did not limit egg production in 1988 and 1989, but the availability of nonlipid energy (probably protein) limited egg production in 1988 and was limiting in spring 1989. Yet, water was the primary resource limiting egg production in 1989. Females forgoing egg production in 1989 accumulated body nonlipid energy and lost less total body water than did females producing eggs. Females that produced eggs in 1989 forfeited body nonlipid energy. In 1988 and 1989, the paucity of new annual plants in spring contributed to a lower egg production. The amount of annuals that germinate in summer can affect egg production, because females stored nonlipid energy during summer 1988 when eating these annuals, and allocated this energy to eggs in the following spring (1989). Tortoises stored lipids during summer when consuming dry annuals. These lipids are critical for surviving the winter, but females forfeited body water and nonlipid dry matter to digest the dry annuals. Reproductive effort (RE) was higher during the drought year (July 1988–July 1989: RE = 26.1%) than during the wetter year (July 1987–July 1988: RE = 13.0%) because tortoises reduced FMR by 70–90% in 1989. Compared to two other chelonians, RE of desert tortoises was consistent with the K‐selected trend of lower RE for larger, long‐lived, and late‐maturing species. Forfeiting body condition to produce only a few eggs, even under a great environmental stress, was consistent with a life history strategy called bet hedging.

Effects of climatic variation on field metabolism and water relations of desert tortoises

“…The three ®eld sites were located in the Mojave Desert of California (for details see Peterson 1994aPeterson , 1996aHenen 1997). The site in the western Mojave (at the Desert Tortoise Research Natural Area or DTNA, near California City; 35°15¢N, 117°51¢W) and one in the northeastern Mojave Desert (in Ivanpah Valley, about midway between Baker, California and Las Vegas, Nevada; 35°22¢N, 115°21¢W) were chosen to maximize the likelihood of observing dierences in seasonal rainfall, while minimizing other climatic and physiographic dierences (Peterson 1994a).…”

“…Ultimately, 12 males and 10 females at DTNA, 15 males and 12 females at Ivanpah, and 13 males and 18 females at Gos were involved in the study. All animals carried radiotransmitters weighing approximately 110 g glued to their carapaces (see Peterson 1996a;Henen 1997 for details).…”

“…Tortoises were injected with 37 MBq (1 millicurie) per kilogram body mass of tritiated water, and either 3 ml kg A1 (high level) or 0.16 ml kg A1 (low level) of 95 atom% 18 O water. After allowing a 14±18 h equilibration period for isotopes to mix thoroughly in body water (Henen 1997), initial blood samples were taken and tortoises were weighed and immediately released at the site of their capture. Tortoises were recaptured opportunistically at approximately 1-to 5-month intervals, when their weights were recorded and blood samples were taken (for details see Peterson 1996a;Henen 1997).…”

“…Deserts, by de®nition, are dry, unpredictable habitats, so food and water supplies are often scarce (Nagy 1988). Desert tortoises (Gopherus agassizii) possess a variety of morphological, physiological, and behavioral characteristics which enable them to survive and reproduce in their harsh environment (Nagy and Medica 1986;Peterson 1994a;Henen 1997). Their former abundance in the Mojave Desert (Corn 1994;Berry and Medica 1995;but see Bury and Corn 1995) testi®es to their success.…”

“…Three populations of desert tortoises in the Mojave Desert of California have been studied continuously for several years (Peterson 1994a(Peterson ,b, 1996aHenen 1997). One of these years (1992) included an El NinÄ o southern oscillation (ENSO) event (Glantz 1996).…”