Biology ETDs

Publication Date

Winter 12-22-2022


In resting animals, water use positively correlates with metabolic rate, for example smaller animals using proportionally more water per gram of body mass. However, animals also must endure heat and exertion, and evaporative cooling requires additional water use that may not scale similarly with body size. How evaporative water loss allometrically scales with body mass during heat-stressed exercise is poorly resolved, particularly for birds, yet is critically important for understanding the consequences of climate warming on the fitness of bird populations. Here, we evaluated how air temperature (Ta) influenced evaporative water loss during exercise (EWLexercise) across tropical bird species that varied 18-fold in body mass (Mb). We also tracked changes in body temperature (Tb exercise) and metabolic rate during exercise (MRexercise) to better understand the physiological consequences of alternative scaling relationships between EWLexercise and Mb. Using push open-flow respirometry, we measured EWLexercise, MRexercise, and Tb exercise, across air temperatures from 15-30oC in 12 species of passerines from a humid, subtropical forest in Kinabalu Park, Borneo, Malaysia. Across body masses of 6.5–116 g, larger species required more evaporative cooling and exhibited higher rates of evaporative water loss during activity at warm (>23oC) air temperatures. Differences in EWLexercise across body masses were likely due to lower surface area -to-volume ratios of larger species. Tb exercise were more labile with Ta in small species than in large species. Heat loading provided an alternative to water loss in small species, and led to the use of conductive cooling as an alternative to evaporative cooling. Larger birds were more susceptible to water loss during exertion at warmer air temperatures. This finding could help to explain the evolution toward smaller body sizes under Anthropocene warming, a widely reported phenomenon, and predict declines of larger-bodied birds under future climate warming.

Project Sponsors

National Science Foundation




Climate change; Allometry; Evaporative cooling; Air temperature; Exercise Metabolic rate; Body temperature; Hop-flutter wheel

Document Type


Degree Name


Level of Degree


Department Name

UNM Biology Department

First Committee Member (Chair)

Christopher Witt

Second Committee Member

Jennifer Rudgers

Third Committee Member

Alexander Gerson