Seasonal Patterns of Body Condition, Hydration State, and Activity of Gila Monsters (Heloderma suspectum) at a Sonoran Desert Site

—Animals in nature use synergistic behavioral and physiological responses to cope with variation in resource availability. We used a combination of traditional tools (i.e., radiotelemetry, bodycondition measurements, plasma osmometry, and direct observation) and contemporary techniques (i.e., implanted temperature loggers and portable ultrasonography) to identify seasonal patterns of body condition, hydration state, and surface activity of 16 free-living Gila Monsters during two active seasons. Despite seasonal drought each year, Gila Monster snout–vent length increased during the study; yet body mass, tail volume, and hydration state decreased. Generally, surface activity was associated with rainy periods, and males were significantly more active than females but only during the reproductive season. Our results indicate that Gila Monsters combine flexible behavioral patterns (i.e., the timing and duration of surface activity), resource storage and economical use, and tolerance of substantial physiological disturbance to endure seasonal resource limitations at a site in the Arizona-Upland subdivision of the Sonoran Desert. Success of animals in hot deserts in part relies on elegantly integrated behavioral and physiological responses to maintain homeostasis despite limited and highly seasonal water and food availability. Reptiles are considered especially well suited for life in deserts because of low metabolic demands and low evaporative water loss (EWL) rates (Pough, 1980; Huey, 1982; Avery, 1982; Bradshaw, 1997). Although we have identified these and other behavioral and physiological processes used by reptiles to survive, our understanding of long-term integrated responses to resource variation in nature is still lacking. The Desert Tortoise (Gopherus agassizii) provides a rare example of such an integrated understanding because it has received considerable attention from biologists for decades because of its conservation status (Nagy and Medica, 1986; Peterson, 1996a,b; Tracy et al., 2006). The Desert Tortoise uses a survival approach that relies on low water and energy demands (Nagy and Medica, 1986; Henen et al., 1998), long-term water and energy storage (Peterson, 1996b), careful microclimate selection including extensive refuge use (Bulova, 2002), and tolerance of significant perturbations to water and energy balance caused by drought (Peterson, 1996a). Other reptiles demonstrate characteristics required for a similar survival strategy in resource-limited environments, yet the collective behavioral and physiological responses used in nature are known for few species. The Gila Monster (Heloderma suspectum) is a large (adult body mass 5 350–700 g and snout– vent length; SVL 5 260–360 mm), long-lived (.20 yr), and venomous lizard (Bogert and Martin del Campo, 1956; Beck, 2005) that provides an ideal model for long-term integrative behavioral and physiological studies. Gila Monsters experience limited and seasonal water and food availability because the species is geographically distributed predominantly in the Sonoran Desert of Arizona and Mexico, with populations on the fringe of the distribution extending slightly into adjacent deserts (extreme eastern Mojave Desert and western Chihuahuan Desert) (Beck, 2005). Although most published field studies on the species have been conducted outside the core (i.e., Sonoran Desert) distribution (e.g., Washington County, Utah: Beck, 1990; Grant County, New Mexico: Beck and Jennings, 2003; Clark County, Nevada: Gienger and Tracy, 2003), several characteristics of the species have been established. Gila Monsters use shelters extensively (Arizona: Lowe et al., 1986; Utah: Beck, 1990; New Mexico: Beck and Jennings, 2003; current Arizona site: Gallardo, 2003), store energy caudally (Bogert and Martin del Campo, 1956; Beck, 2005), and have relatively low preferred and activity body temperatures (28–29uC) compared to other desert lizards (Utah: Beck, 1990; New Mexico: 2005). Laboratory studies have also identified a suite of intrinsic characteristics 1 Corresponding Author. Present address: Rhodes College, Department of Biology, 2000 North Parkway, Memphis, Tennessee 38112-1690 USA; E-mail: [email protected] and Memphis Zoo, 2000 Prentiss Place, Memphis, Tennessee 38112-1690 USA; E-mail: [email protected] Journal of Herpetology, Vol. 44, No. 1, pp. 83–93, 2010 Copyright 2010 Society for the Study of Amphibians and Reptiles