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

During high-temperature periods, endothermic animals evaporate water from the body surface to cool down. The two primary routes of evaporative water loss are: (1) evaporating water from the skin; and (2) evaporation from the respiratory surfaces during breathing and panting. In smaller avian species like Anna's hummingbirds ( Calypte anna), the higher cutaneous surface area to volume ratio presents an opportunity for the skin to become an important avenue for evaporative cooling, possibly becoming a major site. Additionally, hummingbirds feed on water-rich nectar. As a result, they eliminate surplus water through urination, but significant evaporation can also occur through their skin and respiratory system. Thus, total evaporative water loss could rise significantly for these animals when in a hydrated state. Here, this study investigates the relative importance of the two evaporative water loss routes (cutaneous and respiratory) of Anna's hummingbirds in fed and fasted states. We hypothesize that cutaneous evaporation has a high contribution to total evaporation. Moreover, we predict that evaporative water loss rates are higher under the fed (hydrated) condition relative to fasted states. Metabolic and evaporation rates from the head and the body were measured separately in a partitioned chamber at 30°C. Birds were fed immediately prior to entry into respirometry, and the transition from fed to fasted states was recorded over a two-hour period. Total evaporative water loss nearly doubled in the fed versus fasted condition (1.75 ± 0.85 mg H2O/min and 0.97 ± 0.28 mg H2O/min, respectively), with respiratory water loss being significantly higher in the fed compared to the fasted condition (0.87 ± 0.38 mg H2O/min versus 0.42 ± 0.12 mg H2O/min, respectively). On the other hand, the body had a large, but non-significant, difference in evaporative water loss rates between the fed and fasted conditions (0.88 ± 0.55 mg H2O/min and 0.57 ± 0.17 mg H2O/min, respectively). Importantly, cutaneous evaporative water loss made up half of the total evaporation in both the fed and fasted conditions. These results demonstrate that evaporative water loss is modulated by hydration status at both the respiratory and cutaneous surfaces. Furthermore, cutaneous evaporation makes up approximately the same proportion of total evaporation to other birds at thermoneutral temperatures. Future work manipulating ambient temperature in conjunction with hydration status will further our understanding of the relative importance of cutaneous evaporation for these small-bodied birds. Funded by the Genentech Foundations Scholarship, CSUPERB New Investigator Grant, and San Francisco State University. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.