The Costs of Being Cool: Panting Thresholds, Thermal Limits, and Evaporative Cooling In Southwestern

Loughran, Caleb L.

Wolf, Blair O.

University of New Mexico

Department of Biology

University of New Mexico

Albuquerque, NM USA

loughran@unm.edu

In the American Southwest, operative environmental temperatures (Te) often exceed what is physiologically tenable for many reptilian species. When faced with extreme environmental temperatures, lizards must either retreat to thermal refugia or attempt to maintain or lower body temperature (Tb) through evaporative processes such as open-mouth panting. Currently, the capacities for evaporative heat dissipation are largely unknown as are the relative efficiencies of evaporative cooling for various species. To better understand the role of panting in body temperature defense against extreme environmental temperatures, we measured thermoregulatory performance for a variety of lizard species native to the southwest. We used flow-through respirometry to gather data on standard metabolic rate (SMR) and evaporative water loss (EWL) at air temperatures (Ta) that ranged from 35°C to 50°C. Concurrently, we used a live-streaming camera to monitor lizard activity, and panting initiation, and Tb in real-time using thermocouples that were inserted into the lizard’s cloaca. We found SMR and EWL, increased steeply following the onset of panting, with the ability to maintain a gradient between Ta and Tb strongly associated with EWL rate. Species inhabiting hot desert environments had higher panting thresholds, were much more efficient at dissipating heat, and tolerated higher Ta’s and Tb’s for longer periods than species adapted to more mesic environments. Identifying the onset of panting, the capacity for heat defense and its relationship to critical thermal limits should provide valuable insight into how climate warming may impact lizard activity and hence water and energy budgets under future climates.