Water storage compromises walking endurance in an active forager: evidence of a trade-off between osmoregulation and locomotor performance.

Trade-offs between locomotor performance and load-carrying in animals are well-established and often result from requisite life processes including reproduction and feeding. Osmoregulation, another necessary process, may involve storage of fluid in the urinary bladder of some species. The purpose of this study was to determine whether storage of urine in the urinary bladder reduces walking endurance in an actively foraging lizard. The results of our paired-design study indicate that the volume of fluid stored in the urinary bladder (36.5+/-1.6 ml) contributed a significant load (9.2% of body mass) to the lizards. This load resulted in a disproportionate 24.5+/-2.8% decrement in walking endurance. Specifically, Gila monsters walked at a fixed pace for a significantly shorter duration when the urinary bladder contained fluid (26+/-2.0 min) compared to when the bladder was empty (34.3+/-2.3 min). Since fluid stored in the bladder contributes to osmoregulation in this species, our results indicate the presence of a trade-off between osmoregulation and endurance in Gila monsters. Bearing other loads (e.g., a clutch or meal) influences the evolution of life-history traits and foraging strategy; thus the negative effect of fluid storage on endurance may also have evolutionary implications.