Functional role of metabolic suppression in avian thermoregulation in the heat.

Hypometabolism arising from active metabolic suppression occurs in several contexts among endotherms, particularly during heterothermic states such as torpor. However, observed Q ≈ 1 for avian resting metabolic rate within the thermoneutral zone, values far below the Q = 2-3 expected on the basis of Arrhenius effects, suggests hypometabolism also plays a role in birds' thermoregulation at environmental temperatures approaching or exceeding normothermic body temperature (T). We evaluated the occurrence of hypometabolism during heat exposure among birds by re-analysing literature data to quantify changes in T and resting metabolic rate (RMR) near the upper boundary of the thermoneutral zone, at air temperatures (T) between the inflection above which T increases above normothermic levels (T) and the upper critical limit of thermoneutrality (T).

An evaluation of a biophysical model for predicting avian thermoregulation in the heat.

Survival and reproduction of endotherms depend on their ability to balance energy and water exchange with their environment, avoiding lethal deficits and maximising gains for growth and reproduction. At high environmental temperatures, diurnal endotherms maintain body temperature (Tb) below lethal limits via physiological and behavioural adjustments. Accurate models of these processes are crucial for predicting effects of climate variability on avifauna.

Daily Torpor in Birds and Mammals: Past, Present, and Future of the Field.

Torpor is an incredibly efficient energy-saving strategy that many endothermic birds and mammals use to save energy, by lowering their metabolic rates, heart rates, and typically body temperatures. Over the last few decades, the study of daily torpor-in which torpor is used for less than 24 hours per bout-has advanced rapidly. The papers in this issue cover the ecological and evolutionary drivers of torpor, as well as some of the mechanisms governing torpor use.

Evaporative cooling via panting and its metabolic and water balance costs for lizards in the American Southwest.

In lizards there is considerable variation in the ability to dissipate environmental/endogenous heat loads through evaporative cooling via panting, which effects how long lizards can spend exposed to high solar heat loads. We recently described the differing capacities of lizards to depress body temperature (Tb) through evaporative cooling via panting. Here, we link panting and Tb depression with rates of evaporative water loss and its metabolic costs under high heat loads.

Keeping your cool: thermoregulatory performance and plasticity in desert cricetid rodents.

Small mammals in hot deserts often avoid heat via nocturnality and fossoriality, and are thought to have a limited capacity to dissipate heat using evaporative cooling. Research to date has focused on thermoregulatory responses to air temperatures (Ta) below body temperature (Tb). Consequently, the thermoregulatory performance of small mammals exposed to high Ta is poorly understood, particularly responses across geographic and seasonal scales.

Thermoregulation in desert birds: scaling and phylogenetic variation in heat tolerance and evaporative cooling.

Evaporative heat dissipation is a key aspect of avian thermoregulation in hot environments. We quantified variation in avian thermoregulatory performance at high air temperatures () using published data on body temperature (), evaporative water loss (EWL) and resting metabolic rate (RMR) measured under standardized conditions of very low humidity in 56 arid-zone species. Maximum during acute heat exposure varied from 42.

Nitrogen stable isotope turnover and discrimination in lizards.

Rationale: Nitrogen stable isotope ratio (δ N) processes are not well described in reptiles, which limits reliable inference of trophic and nutrient dynamics. In this study we detailed δ N turnover and discrimination (Δ N) in diverse tissues of two lizard species, and compared these results with previously published carbon data (δ C) to inform estimates of reptilian foraging ecology and nutrient physiology.

Methods: We quantified N incorporation and discrimination dynamics over 360 days in blood fractions, skin, muscle, and liver of Sceloporus undulatus and Crotaphytus collaris that differed in body mass.

Extreme and variable torpor among high-elevation Andean hummingbird species.

Torpor is thought to be particularly important for small endotherms occupying cold environments and with limited fat reserves to fuel metabolism, yet among birds deep torpor is both rare and variable in extent. We investigated torpor in hummingbirds at approximately 3800 m.a.

The functional significance of panting as a mechanism of thermoregulation and its relationship to the critical thermal maxima in lizards.

Because most desert-dwelling lizards rely primarily on behavioral thermoregulation for the maintenance of active body temperature, the effectiveness of panting as a thermoregulatory mechanism for evaporative cooling has not been widely explored. We measured changes in body temperature () with increasing air temperature () for 17 species of lizards that range across New Mexico and Arizona and quantified the temperatures associated with the onset of panting, and the capacity of individuals to depress below while panting, and estimated the critical thermal maxima (CT) for each individual. We examined these variables as a function of phylogeny, body mass and local acclimatization temperature.

Avian mortality risk during heat waves will increase greatly in arid Australia during the 21st century.

Intense heat waves are occurring more frequently, with concomitant increases in the risk of catastrophic avian mortality events via lethal dehydration or hyperthermia. We quantified the risks of lethal hyperthermia and dehydration for 10 Australian arid-zone avifauna species during the 21st century, by synthesizing thermal physiology data on evaporative water losses and heat tolerance limits. We evaluated risks of lethal hyperthermia or exceedance of dehydration tolerance limits in the absence of drinking during the hottest part of the day under recent climatic conditions, compared to those predicted for the end of this century across Australia.

Metabolic rate is negatively linked to adult survival but does not explain latitudinal differences in songbirds.

Survival rates vary dramatically among species and predictably across latitudes, but causes of this variation are unclear. The rate-of-living hypothesis posits that physiological damage from metabolism causes species with faster metabolic rates to exhibit lower survival rates. However, whether increased survival commonly observed in tropical and south temperate latitudes is associated with slower metabolic rate remains unclear.

Cooling requirements fueled the collapse of a desert bird community from climate change.

Climate change threatens global biodiversity by increasing extinction risk, yet few studies have uncovered a physiological basis of climate-driven species declines. Maintaining a stable body temperature is a fundamental requirement for homeothermic animals, and water is a vital resource that facilitates thermoregulation through evaporative cooling, especially in hot environments. Here, we explore the potential for thermoregulatory costs to underlie the community collapse of birds in the Mojave Desert over the past century in response to climate change.

The Physiology of Heat Tolerance in Small Endotherms.

Understanding the heat tolerances of small mammals and birds has taken on new urgency with the advent of climate change. Here, we review heat tolerance limits, pathways of evaporative heat dissipation that permit the defense of body temperature during heat exposure, and mechanisms operating at tissue, cellular, and molecular levels.

Foraging strategies of individual silky pocket mice over a boom-bust cycle in a stochastic dryland ecosystem.

Small mammals use multiple foraging strategies to compensate for fluctuating resource quality in stochastic environments. These strategies may lead to increased dietary overlap when competition for resources is strong. To quantify temporal contributions of high (C) versus low quality (C) resources in diets of silky pocket mice (Perognathus flavus), we used stable carbon isotope (δC) analysis of 1391 plasma samples collected over 2 years.

Avian thermoregulation in the heat: is evaporative cooling more economical in nocturnal birds?

Evaporative cooling is a prerequisite for avian occupancy of hot, arid environments, and is the only avenue of heat dissipation when air temperatures () exceed body temperature (). Whereas diurnal birds can potentially rehydrate throughout the day, nocturnal species typically forgo drinking between sunrise and sunset. We hypothesized that nocturnal birds have evolved reduced rates of evaporative water loss (EWL) and more economical evaporative cooling mechanisms compared with diurnal species, permitting nocturnal species to tolerate extended periods of intense heat without becoming lethally dehydrated.

Avian thermoregulation in the heat: metabolism, evaporative cooling and gular flutter in two small owls.

The thermoregulatory responses of owls to heat stress have been the subject of few studies. Although nocturnality buffers desert-dwelling owls from significant heat stress during activity, roost sites in tree and cactus cavities or in deep shade provide only limited refuge from high environmental temperatures during the day. We measured thermoregulatory responses to acute heat stress in two species of small owls, the elf owl () and the western screech-owl (), which occupy the Sonoran Desert of southwestern North America, an area of extreme heat and aridity.

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.

Avian thermoregulation in the heat: phylogenetic variation among avian orders in evaporative cooling capacity and heat tolerance.

Little is known about the phylogenetic variation of avian evaporative cooling efficiency and heat tolerance in hot environments. We quantified thermoregulatory responses to high air temperature () in ∼100-g representatives of three orders, namely, the African cuckoo (, Cuculiformes), lilac-breasted roller (, Coraciiformes) and Burchell's starling (, Passeriformes). All three species initiated respiratory mechanisms to increase evaporative heat dissipation when body temperature () approached 41.

Avian thermoregulation in the heat: evaporative cooling capacity of arid-zone Caprimulgiformes from two continents.

Birds in the order Caprimulgiformes (nightjars and allies) have a remarkable capacity for thermoregulation over a wide range of environmental temperatures, exhibiting pronounced heterothermy in cool conditions and extreme heat tolerance at high environmental temperatures. We measured thermoregulatory responses to acute heat stress in three species of Caprimulgiformes that nest in areas of extreme heat and aridity, the common poorwill (: Caprimulgidae) and lesser nighthawk (: Caprimulgidae) in the Sonoran Desert of Arizona, and the Australian owlet-nightjar (: Aegothelidae) in the mallee woodlands of South Australia. We exposed wild-caught birds to progressively increasing air temperatures () and measured resting metabolic rate (RMR), evaporative water loss (EWL), body temperature () and heat tolerance limit (HTL; the maximum reached).

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, evaporative water loss and body temperature at air temperatures () between 30 and 52°C. We found marked increases in resting metabolism above the upper critical temperature (), which for six of the seven species fell within a relatively narrow range (36.

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 (; ∼17 g), spiny-cheeked honeyeater (; ∼42 g), chestnut-crowned babbler (; ∼52 g), grey butcherbird (; ∼86 g) and apostlebird (; ∼118 g). At air temperatures () exceeding body temperature (), all five species showed increases in 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 Maximum cooling capacity, quantified as the fraction of metabolic heat production dissipated evaporatively, ranged from 1.

Seasonal Metabolic Acclimatization Varies in Direction and Magnitude among Populations of an Afrotropical Passerine Bird.

Avian metabolic responses demonstrate considerable diversity under fluctuating environmental conditions, a well-studied example being the seasonal upregulation of basal metabolic rate (BMR) and summit metabolism (M) in temperate species experiencing harsh winters. Fewer studies have examined seasonal metabolic acclimatization in subtropical or tropical species. We investigated seasonal metabolic variation in an Afrotropical ploceid passerine, the white-browed sparrow-weaver (Plocepasser mahali; ∼47 g), at three sites along a climatic gradient of approximately 7°C in winter minimum air temperature (T).