Can birds do it too? Evidence for convergence in evaporative water loss regulation for birds and mammals

Abstract

Birds have many physiological characteristics that are convergent with mammals. In the light of recent evidence that mammals can maintain a constant insensible evaporative water loss (EWL) over a range of perturbing environmental conditions, we hypothesized that birds might also regulate insensible EWL, reflecting this convergence. We found that budgerigars () maintain EWL constant over a range of relative humidities at three ambient temperatures. EWL, expressed as a function of water vapour pressure deficit, differed from a physical model where the water vapour pressure deficit between the animal and the ambient air is the driver of evaporation, indicating physiological control of EWL. Regulating EWL avoids thermoregulatory impacts of varied evaporative heat loss; changes in relative humidity had no effect on body temperature, metabolic rate or thermal conductance. Our findings that a small bird can regulate EWL are evidence that this is a common feature of convergently endothermic birds and mammals, and may therefore be a fundamental characteristic of endothermy.

High air temperatures induce temporal, spatial and social changes in the foraging behaviour of wild zebra finches

“…The importance of water and the physiological constraints of heat were further supported by our observations of birds during the hottest periods of the day, and the hottest days, when we encountered zebra finches sitting in small groups near the water. We found that by the time air temperatures reached 35 °C, a significant proportion of birds were panting, in line with previous studies (Schleucher, 1993;Zann, 1996), as well as wing spreading or fanning (Eto, Withers, & Cooper, 2017;O'Connor et al, 2018;Tieleman & Williams, 1999). Above 41 °C individuals also exhibited two other behaviours, which we were unable to find any reference to, in a heat dissipation context.…”

High air temperatures induce temporal, spatial and social changes in the foraging behaviour of wild zebra finches

“…The importance of water and the physiological constraints of heat were further supported by our observations of birds during the hottest periods of the day, and the hottest days, when we encountered zebra finches sitting in small groups near the water. We found that by the time air temperatures reached 35 °C, a significant proportion of birds were panting, in line with previous studies (Schleucher, 1993;Zann, 1996), as well as wing spreading or fanning (Eto, Withers, & Cooper, 2017;O'Connor et al, 2018;Tieleman & Williams, 1999). Above 41 °C individuals also exhibited two other behaviours, which we were unable to find any reference to, in a heat dissipation context.…”

Two independent approaches to assessing the constancy of evaporative water loss for birds under varying evaporative conditions

“…That the mesic-zone red-capped parrot can control EWL over a range of RH is consistent with previous studies demonstrating constancy of EWL over a range of RH for the arid-zone budgerigar (Eto et al, 2017) and two other mesic-zone parrots (eastern rosella and red-rumped parrots; .…”

“…We confirmed this constancy of EWL using a helox atmosphere as a second methodology to perturb the evaporative environment, as has been established previously for mammals; a rodent (Cooper and Withers, 2014) and a marsupial . Our results for this mesic-habitat parrot are consistent with the hypothesis that EWL control functions in the context of thermoregulatory control rather than as a water-conserving mechanism at low RH (Eto et al, 2017;. Evaporative water loss of parrot carcasses measured at 24°C is indicated with grey symbols.…”

Small Alpine Marsupials Regulate Evaporative Water Loss, Suggesting a Thermoregulatory Role Rather than a Water Conservation Role

“…Similar independence of RH with EWL has been reported for other mammals (Cooper and Withers 2008Withers and Cooper 2014). Inhibition of EWL by increasing RH was observed only at the highest RH, where a small ΔWVP near saturation makes the maintenance of constant EWL challenging, as has also been observed for some other small mammals and a bird Withers, 2008, 2020;Withers and Cooper, 2014;Eto et al 2017). It is typical for physiologically regulated variables to deviate from constancy under more extreme environmental conditions e.g.…”

High air temperatures induce temporal, spatial and social changes in the foraging behaviour of wild zebra finches

“…The importance of water and the physiological constraints of heat were further supported by our observations of birds during the hottest periods of the day, and the hottest days, when we encountered zebra finches sitting in small groups near the water. We found that by the time air temperatures reached 35 °C, a significant proportion of birds were panting, in line with previous studies (Schleucher, 1993;Zann, 1996), as well as wing spreading or fanning (Eto, Withers, & Cooper, 2017;O'Connor et al, 2018;Tieleman & Williams, 1999). Above 41 °C individuals also exhibited two other behaviours, which we were unable to find any reference to, in a heat dissipation context.…”

Two independent approaches to assessing the constancy of evaporative water loss for birds under varying evaporative conditions

“…That the mesic-zone red-capped parrot can control EWL over a range of RH is consistent with previous studies demonstrating constancy of EWL over a range of RH for the arid-zone budgerigar (Eto et al, 2017) and two other mesic-zone parrots (eastern rosella and red-rumped parrots; .…”

“…We confirmed this constancy of EWL using a helox atmosphere as a second methodology to perturb the evaporative environment, as has been established previously for mammals; a rodent (Cooper and Withers, 2014) and a marsupial . Our results for this mesic-habitat parrot are consistent with the hypothesis that EWL control functions in the context of thermoregulatory control rather than as a water-conserving mechanism at low RH (Eto et al, 2017;. Evaporative water loss of parrot carcasses measured at 24°C is indicated with grey symbols.…”

Small Alpine Marsupials Regulate Evaporative Water Loss, Suggesting a Thermoregulatory Role Rather than a Water Conservation Role

“…Similar independence of RH with EWL has been reported for other mammals (Cooper and Withers 2008Withers and Cooper 2014). Inhibition of EWL by increasing RH was observed only at the highest RH, where a small ΔWVP near saturation makes the maintenance of constant EWL challenging, as has also been observed for some other small mammals and a bird Withers, 2008, 2020;Withers and Cooper, 2014;Eto et al 2017). It is typical for physiologically regulated variables to deviate from constancy under more extreme environmental conditions e.g.…”

High air temperatures induce temporal, spatial and social changes in the foraging behaviour of wild zebra finches

“…The importance of water and the physiological constraints of heat were further supported by our observations of birds during the hottest periods of the day, and the hottest days, when we encountered zebra finches sitting in small groups near the water. We found that by the time air temperatures reached 35 °C, a significant proportion of birds were panting, in line with previous studies (Schleucher, 1993;Zann, 1996), as well as wing spreading or fanning (Eto, Withers, & Cooper, 2017;O'Connor et al, 2018;Tieleman & Williams, 1999). Above 41 °C individuals also exhibited two other behaviours, which we were unable to find any reference to, in a heat dissipation context.…”

Two independent approaches to assessing the constancy of evaporative water loss for birds under varying evaporative conditions

“…That the mesic-zone red-capped parrot can control EWL over a range of RH is consistent with previous studies demonstrating constancy of EWL over a range of RH for the arid-zone budgerigar (Eto et al, 2017) and two other mesic-zone parrots (eastern rosella and red-rumped parrots; .…”

“…We confirmed this constancy of EWL using a helox atmosphere as a second methodology to perturb the evaporative environment, as has been established previously for mammals; a rodent (Cooper and Withers, 2014) and a marsupial . Our results for this mesic-habitat parrot are consistent with the hypothesis that EWL control functions in the context of thermoregulatory control rather than as a water-conserving mechanism at low RH (Eto et al, 2017;. Evaporative water loss of parrot carcasses measured at 24°C is indicated with grey symbols.…”

Small Alpine Marsupials Regulate Evaporative Water Loss, Suggesting a Thermoregulatory Role Rather than a Water Conservation Role

“…Similar independence of RH with EWL has been reported for other mammals (Cooper and Withers 2008Withers and Cooper 2014). Inhibition of EWL by increasing RH was observed only at the highest RH, where a small ΔWVP near saturation makes the maintenance of constant EWL challenging, as has also been observed for some other small mammals and a bird Withers, 2008, 2020;Withers and Cooper, 2014;Eto et al 2017). It is typical for physiologically regulated variables to deviate from constancy under more extreme environmental conditions e.g.…”