Abstract:
Evaporative heat dissipation is a key aspect of avian thermoregulation
in hot environments.We quantified variation in avian thermoregulatory
performance at high air temperatures (Ta) using published data on
body temperature (Tb), evaporative water loss (EWL) and resting
metabolic rate (RMR) measured under standardized conditions of very
low humidity in 56 arid-zone species. Maximum Tb during acute heat
exposure varied from 42.5±1.3°C in caprimulgids to 44.5±0.5°C in
passerines. Among passerines, both maximum Tb and the difference
between maximum and normothermic Tb decreased significantly with
body mass (Mb). Scaling exponents for minimum thermoneutral EWL
and maximum EWL were 0.825 and 0.801, respectively, even though
evaporative scope (ratio of maximum to minimum EWL) varied widely
among species. Upper critical limits of thermoneutrality (Tuc) varied by
>20°C and maximumRMR during acute heat exposure scaled toMb
0.75
in both the overall data set and among passerines. The slope of RMR
at Ta>Tuc increased significantly with Mb but was substantially higher
among passerines, which rely on panting, comparedwith columbids, in
which cutaneous evaporation predominates. Our analysis supports
recent arguments that interspecific within-taxon variation in heat
tolerance is functionally linked to evaporative scope and maximum
ratios of evaporative heat loss (EHL) to metabolic heat production
(MHP). We provide predictive equations for most variables related to
avian heat tolerance. Metabolic costs of heat dissipation pathways,
rather than capacity to increase EWL above baseline levels, appear to
represent the major constraint on the upper limits of avian heat
tolerance.
