DAVIS, J.R.*; DENARDO, D.F.; Arizona State Univ.: The contribution of urinary bladder water to hydrostasis in the Gila Monster (Heloderma suspectum)

The infrequent, yet predictable seasonal rainfall of the Sonoran Desert results in feast or famine hydric conditions for many desert species. As a result, the hydration state of some Sonoran Desert organisms may fluctuate considerably over the course of a year. Since hydrostasis is critical to many physiological functions, a few species have specific morphologic adaptations to reduce broad fluctuations in hydration state. For example, water stored in the bladder of the Desert Tortoise (Gopherus agassizii) contributes to its survival during drought. We have shown that Gila Monsters (Heloderma suspectum) can also transfer water from the urinary bladder into the circulation, yet the ecological relevance of this finding is untested. We hypothesized that Gila Monsters can use water stored in the urinary bladder as a reservoir during drought conditions to attenuate osmotic fluctuations and thus help maintain hydrostasis. We tested this hypothesis in the laboratory by manipulating bladder volume of Gila Monsters and serially measuring plasma osmolarity during a dehydration period. We anesthetized twelve Gila Monsters, emptied the urinary bladder of each using a urethral catheter, and randomly designated each as either �empty� or �supplemented�. We infused 30ml of sterile saline via the catheter into the bladder of �supplemented� individuals. Animals were then maintained without access to food or water, and plasma, body mass, and tail volume were collected every two weeks until animals were dehydrated to 80% of initial body mass. We examined whether the two groups differed in the rate of change in plasma osmolarity, mass, or tail volume. The results of this study will help clarify the previously undescribed role of the Gila Monster urinary bladder as an adaptation for dealing with seasonal aridity.