Comparing the Contributions of Cutaneous and Respiratory Evaporative Water Loss to Cooling in Anna’s Hummingbirds (Calypte anna) Under Fasted and Fed Conditions

Author: Cecilia Doan

Faculty Supervisor: Derrick Groom

Department: Biology

Endothermic animals evaporate water from the body to cool down through two primary routes: cutaneous and respiratory. In small species like Anna's hummingbirds (Calypte anna), the high surface area to volume ratio presents an opportunity for the skin to be a major site of evaporative cooling. Moreover, the surplus of water from their diet may permit high rates of evaporation from both routes. This study investigates the importance of the two evaporative water loss routes of Anna's hummingbirds in fed and fasted states. We hypothesize that cutaneous evaporation has a high contribution to total evaporation. We also predict that water loss rates are higher under the fed (hydrated) than fasted state. Metabolic and evaporation rates from the head and body were measured in a partitioned chamber across four temperatures. Birds were fed before entry into respirometry, and the transition from fed to fasted states was recorded over two hours. Results showed cutaneous evaporation did not significantly change in response to temperature or hydration, suggesting it’s not as important as respiratory water loss. Furthermore, our data demonstrates there was possibly no impact of hydration. Fed animals expired less water than fasted animals, likely due to the oxygen-saving capabilities of carbohydrate metabolism.