ECOLOGICAL ENERGETICS OF THE DESERT TORTOISE (GOPHERUS-AGASSIZII) - EFFECTS OF RAINFALL AND DROUGHT

To elucidate ecological effects of variation in the temporal distribut ion of a limiting resource (water in the Mojave Desert), energetics of two free-living populations of desert tortoises (Gopherus [= Xerobate s] agassizii) were studied concurrently over 18 mo with use of doubly- labeled water. Field metabolic rates (FMR) and feeding rates (estimate d from rates of water influx and rates of change in dry mass) were hig hly variable. This variability was manifested at several levels, inclu ding seasonal changes within populations, year-to-year differences wit hin populations, and differences between populations. Underlying obser ved patterns and contrasts was considerable variation among individual s. Much of the variation in energetic variables was associated with a single climatic variable, rainfall. Seasonal, annual, and interpopulat ion differences in FMR and foraging rates corresponded to differences in availability of free-standing water from rainstorms. At least some of the differences among individuals were apparently due to difference s in proclivity or ability to drink. Tortoises had very low FMRs relat ive to other reptiles, which allowed them to tolerate long periods of chronic energy shortage during a drought. Calculations suggested that tortoises experienced a net loss of energy on their spring diet of suc culent annual plants. If so, tortoises require drier forage to accrue an energy profit, which emphasizes their reliance on drinking rainwate r (which can be stored in the bladder and resorbed later to hydrate dr y forage). Further, it suggests that growth (as protein deposition) an d net acquisition of energy may be temporally decoupled in desert tort oises, which has potential consequences for geographic variation in li fe history traits. Energy acquisition and expenditure in desert tortoi ses are thus strongly constrained by the contingencies of rainfall, bo th indirectly through effects on availability and quality of food, and directly through reliance on free-standing water for drinking, which is apparently necessary for achieving a net annual energy profit.