The impact of humidity on evaporative cooling in small desert birds exposed to high air temperatures

Environmental temperatures that exceed body temperature (Tb) force endothermic animals to rely solely on evaporative cooling to dissipate heat. However, evaporative heat dissipation can be drastically reduced by environmental humidity, imposing a thermoregulatory challenge. The goal of this study was to investigate the effects of humidity on the thermoregulation of desert birds and to compare the sensitivity of cutaneous and respiratory evaporation to reduced vapor density gradients. Rates of evaporative water loss, metabolic rate, and Tb were measured in birds exposed to humidities ranging from ∼2 to 30 g H2O m23 (0%–100% relative humidity at 307C) at air temperatures between 447 and 567C. In sociable weavers, a species that dissipates heat primarily through panting, rates of evaporative water loss were inhibited by as much as 36% by high humidity at 487C, and these birds showed a high degree of hyperthermia. At lower temperatures (407–447C), evaporative water loss was largely unaffected by humidity in this species. In Namaqua doves, which primarily use cutaneous evaporation, increasing humidity reduced rates of evaporative water loss, but overall rates of water loss were lower than those observed in sociable weavers. Our data suggest that cutaneous evaporation is more efficient than panting, requiring less water to maintain Tb at a given temperature, but panting appears less sensitive to humidity over the air temperature range investigated here.DST/National Research Foundation Centre of Excellencehttp://www.journals.uchicago.edutoc/pbz/tm201

Mapping evaporative water loss in desert passerines reveals an expanding threat of lethal dehydration

Extreme high environmental temperatures produce a variety of consequences for wildlife, including mass die-offs. Heat waves are increasing in frequency, intensity, and extent, and are projected to increase further under climate change. However, the spatial and temporal dynamics of die-off risk are poorly understood. Here, we examine the effects of heat waves on evaporative water loss (EWL) and survival in five desert passerine birds across the southwestern United States using a combination of physiological data, mechanistically informed models, and hourly geospatial temperature data. We ask how rates of EWL vary with temperature across species; how frequently, over what areas, and how rapidly lethal dehydration occurs; how EWL and die-off risk vary with body mass; and how die-off risk is affected by climate warming. We find that smaller-bodied passerines are subject to higher rates of mass-specific EWL than larger-bodied counterparts and thus encounter potentially lethal conditions much more frequently, over shorter daily intervals, and over larger geographic areas. Warming by 4 °C greatly expands the extent, frequency, and intensity of dehydration risk, and introduces new threats for larger passerine birds, particularly those with limited geographic ranges. Our models reveal that increasing air temperatures and heat wave occurrence will potentially have important impacts on the water balance, daily activity, and geographic distribution of arid-zone birds. Impacts may be exacerbated by chronic effects and interactions with other environmental changes. This work underscores the importance of acute risks of high temperatures, particularly for small-bodied species, and suggests conservation of thermal refugia and water sources

Avian thermoregulation in the heat : evaporative cooling in five Australian passerines reveals within-order biogeographic variation in heat tolerance

Evaporative heat loss pathways vary among avian orders, but the extent to which evaporative cooling capacity and heat tolerance vary within orders remains unclear. We quantified the upper limits to thermoregulation under extremely hot conditions in five Australian passerines: yellow-plumed honeyeater (Lichenostomus ornatus; ∼17 g), spiny-cheeked honeyeater (Acanthagenys rufogularis; ∼42 g), chestnut-crowned babbler (Pomatostomus ruficeps; ∼52 g), grey butcherbird (Cracticus torquatus; ∼86 g) and apostlebird (Struthidea cinerea; ∼118 g). At air temperatures (Ta) exceeding body temperature (Tb), all five species showed increases in Tb to maximum values around 44–45°C, accompanied by rapid increases in resting metabolic rate above clearly defined upper critical limits of thermoneutrality and increases in evaporative water loss (EWL) to levels equivalent to 670–860% of baseline rates at thermoneutral Ta. Maximum cooling capacity, quantified as the fraction of metabolic heat production dissipated evaporatively, ranged from 1.20 to 2.17, consistent with the known range for passerines, and well below the corresponding ranges for columbids and caprimulgids. Heat tolerance limit (HTL, the maximum Ta tolerated) scaled positively with body mass, varying from 46°C in yellow-plumed honeyeaters to 52°C in a single apostlebird, but was lower than that of three southern African ploceid passerines investigated previously. We argue this difference is functionally linked to a smaller scope for increases in EWL above baseline levels. Our data reiterate the reliance of passerines in general on respiratory evaporative heat loss via panting, but also reveal substantial within-order variation in heat tolerance and evaporative cooling capacity.The National Science Foundation [IOS- 1122228 to B.O.W.].http://jeb.biologists.org2018-07-30am2017Zoology and Entomolog

The Impact of Humidity on Evaporative Cooling in Small Desert Birds Exposed to High Air Temperatures

Environmental temperatures that exceed body temperature (Tb) force endothermic animals to rely solely on evaporative cooling to dissipate heat. However, evaporative heat dissipation can be drastically reduced by environmental humidity, imposing a thermoregulatory challenge. The goal of this study was to investigate the effects of humidity on the thermoregulation of desert birds and to compare the sensitivity of cutaneous and respiratory evaporation to reduced vapor density gradients. Rates of evaporative water loss, metabolic rate, and Tb were measured in birds exposed to humidities ranging from ∼2 to 30 g H2O m23 (0%–100% relative humidity at 307C) at air temperatures between 447 and 567C. In sociable weavers, a species that dissipates heat primarily through panting, rates of evaporative water loss were inhibited by as much as 36% by high humidity at 487C, and these birds showed a high degree of hyperthermia. At lower temperatures (407–447C), evaporative water loss was largely unaffected by humidity in this species. In Namaqua doves, which primarily use cutaneous evaporation, increasing humidity reduced rates of evaporative water loss, but overall rates of water loss were lower than those observed in sociable weavers. Our data suggest that cutaneous evaporation is more efficient than panting, requiring less water to maintain Tb at a given temperature, but panting appears less sensitive to humidity over the air temperature range investigated here.DST/National Research Foundation Centre of Excellencehttp://www.journals.uchicago.edutoc/pbz/tm201

Avian thermoregulation in the heat : resting metabolism, evaporative cooling and heat tolerance in Sonoran Desert songbirds

We examined thermoregulatory performance in seven Sonoran Desert passerine bird species varying in body mass from 10 to 70 g – lesser goldfinch, house finch, pyrrhuloxia, cactus wren, northern cardinal, Abert’s towhee and curve-billed thrasher. Using flow-through respirometry, we measured daytime resting metabolism, evaporativewater loss and body temperature at air temperatures (Tair) between 30 and 52°C. We found marked increases in resting metabolism above the upper critical temperature (Tuc), which for six of the seven species fell within a relatively narrow range (36.2–39.7°C), but which was considerably higher in the largest species, the curvebilled thrasher (42.6°C). Resting metabolism and evaporative water loss were minimal below the Tuc and increased with Tair and body mass to maximum values among species of 0.38–1.62 Wand 0.87– 4.02 g H2O h−1, respectively. Body temperature reached maximum values ranging from 43.5 to 45.3°C. Evaporative cooling capacity, the ratio of evaporative heat loss to metabolic heat production, reached maximum values ranging from 1.39 to 2.06, consistent with known values for passeriforms and much lower than values in taxa such as columbiforms and caprimulgiforms. These maximum values occurred at heat tolerance limits that did not scale with body mass among species, but were ∼50°C for all species except the pyrrhuloxia and Abert’s towhee (48°C). High metabolic costs associated with respiratory evaporation appeared to drive the limited heat tolerance in these desert passeriforms, compared with larger desert columbiforms and galliforms that use metabolically more efficient mechanisms of evaporative heat loss.The National Science Foundation [grant number IOS-1122228 to B.O.W.].http://jeb.biologists.org2018-09-15am2018Zoology and Entomolog

Avian thermoregulation in the heat : evaporative cooling capacity and thermal tolerance in two Australian parrots

Avian orders differ in their thermoregulatory capabilities and tolerance of high environmental temperatures. Evaporative heat loss, and the primary avenue whereby it occurs, differs amongst taxa. Although Australian parrots (Psittaciformes) have been impacted by mass mortality events associated with extreme weather events (heat waves), their thermoregulatory physiology has not been well-characterized. We quantified the upper limits to thermoregulation under extremely hot conditions in two Australian parrots: the mulga parrot (Psephotellus varius; ~55 g) and the galah (Eolophus roseicapilla; ~265 g). At air temperatures (Ta) exceeding body temperature (Tb), both species showed increases in Tb to maximum values around 43–44°C, accompanied by rapid increases in resting metabolic rate above clearly defined upper critical limits of thermoneutrality and increases in evaporative water loss (EWL) to levels equivalent to 700–1000% of baseline rates at thermoneutral Ta. Maximum cooling capacity, quantified as the fraction of metabolic heat production dissipated evaporatively, ranged from 1.71 to 1.79, consistent with the known range for parrots, similar to the corresponding range in passerines, and well below the corresponding ranges for columbids and caprimulgids. Heat tolerance limit (HTL, the maximum Ta tolerated) ranged from 44-55°C, similar to the range reported for passerines, but lower than reported for columbids and caprimulgids. Our data suggest that heat tolerance in parrots is similar to that of passerines. We argue that understanding how thermoregulatory capacity and heat tolerance vary across avian orders is vital for predicting how climate change and the associated increase in frequency of extreme weather events may impact avian populations in the future.This material is based on work supported by the National Science Foundation (IOS-1122228 to B.O.W.). Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.http://jeb.biologists.org2019-03-01am2018Zoology and Entomolog