Seasonal metabolic adjustments in an avian evolutionary relict restricted to mountain habitat.

For endotherms, maintaining body temperature during cold winters is energetically costly.Greater increase in winter maximum thermogenic capacity (Msum) has typically been correlated with improved cold tolerance. However, seasonal studies have shown equivocal direction change in basal metabolic rate (BMR) in winter, perhaps explained by latitude or phylogeny. We examined seasonal metabolic responses in the Cape rockjumper (Chaetops frenatus; "rockjumper"), a range-restricted mountain bird. We hypothesized that, given their mountain habitat preference, rockjumpers would be physiologically specialized for cooler air temperatures compared to other subtropical passerines. We measured body condition (using the ratio of Mb/tarsus), BMR, and Msum, in wild-living rockjumpers during winter and summer (n = 12 adults in winter -- 4 females, 8 males; n = 12 adults in summer -- 6 females, 6 males). We found birds had lesser BMR and thermal conductance, and greater Msum and body condition, in winter compared to summer. These changes may help rockjumpers conserve energy in winter while still allowing birds to produce more metabolic heat during the coldest air temperatures. When compared with existing data on avian seasonal metabolic adjustments, rockjumper BMR fit general patterns observed in passerines, but their Msum was low compared with other members of the oscine Passeriformes. These patterns may be explained by the narrow temperature range of their habitat not requiring cold-adjustment, or perhaps by their basal placement within passerine phylogeny. Further work on the physiological phenotypic plasticity in habitat specialists across different latitudinal zones and taxa is needed to better understand the relationship between metabolism, habitat, and phylogeny.

An oversimplification of physiological principles leads to flawed macroecological analyses.

Macrophysiological analyses are useful to predict current and future range limits and improve our understanding of endotherm macroecology, but such analyses too often rely on oversimplifications of endothermic thermoregulatory and energetic physiology, which lessens their applicability. We detail some of the major issues with macrophysiological analyses based on the classic Scholander-Irving model of endotherm energetics in the hope that it will encourage other research teams to more appropriately integrate physiology into macroecological analyses.

The role of ambient temperature and body mass on body temperature, standard metabolic rate and evaporative water loss in southern African anurans of different habitat specialisation.

Temperature and water availability are two of the most important variables affecting all aspects of an anuran's key physiological processes such as body temperature (T b), evaporative water loss (EWL) and standard metabolic rate (SMR). Since anurans display pronounced sexual dimorphism, evidence suggests that these processes are further influenced by other factors such as vapour pressure deficit (VPD), sex and body mass (M b). However, a limited number of studies have tested the generality of these results across a wide range of ecologically relevant ambient temperatures (T a), while taking habitat use into account. Thus, the aim of this study was to investigate the role of T a on T b, whole-animal EWL and whole-animal SMR in three wild caught African anuran species with different ecological specialisations: the principally aquatic African clawed frog (Xenopus laevis), stream-breeding common river frog (Amietia delalandii), and the largely terrestrial raucous toad (Sclerophrys capensis). Experiments were conducted at a range of test temperatures (5-35 °C, at 5 °C increments). We found that VPD better predicted rates of EWL than T a in two of the three species considered. Moreover, we found that T b, whole-animal EWL and whole-animal SMR increased with increasing T a, while T b increased with increasing M b in A. delalandii and S. capensis but not in X. laevis. Whole-animal SMR increased with increasing M b in S. capensis only. We did not find any significant effect of VPD, M b or sex on whole-animal EWL within species. Lastly, M b did not influence T b, whole-animal SMR and EWL in the principally aquatic X. laevis. These results suggest that M b may not have the same effect on key physiological variables, and that the influence of M b may also depend on the species ecological specialisation. Thus, the generality of M b as an important factor should be taken in the context of both physiology and species habitat specialisation.

31° South: The physiology of adaptation to arid conditions in a passerine bird.

Arid environments provide ideal ground for investigating the mechanisms of adaptive evolution. High temperatures and low water availability are relentless stressors for many endotherms, including birds; yet birds persist in deserts. While physiological adaptation probably involves metabolic phenotypes, the underlying mechanisms (plasticity, genetics) are largely uncharacterized. To explore this, we took an intraspecific approach that focused on a species that is resident over a mesic to arid gradient, the Karoo scrub-robin (Cercotrichas coryphaeus). Specifically, we integrated environmental (climatic and primary productivity), physiological (metabolic rates: a measure of energy expenditure), genotypic (genetic variation underlying the machinery of energy production) and microbiome (involved in processing food from where energy is retrieved) data, to infer the mechanism of physiological adaptation. We that found the variation in energetic physiology phenotypes and gut microbiome composition are associated with environmental features as well as with variation in genes underlying energy metabolic pathways. Specifically, we identified a small list of candidate adaptive genes, some of them with known ties to relevant physiology phenotypes. Together our results suggest that selective pressures on energetic physiology mediated by genes related to energy homeostasis and possibly microbiota composition may facilitate adaptation to local conditions and provide an explanation to the high avian intraspecific divergence observed in harsh environments.

An experimental test of the allotonic frequency hypothesis to isolate the effects of light pollution on bat prey selection.

Artificial lights may be altering interactions between bats and moth prey. According to the allotonic frequency hypothesis (AFH), eared moths are generally unavailable as prey for syntonic bats (i.e., bats that use echolocation frequencies between 20 and 50 kHz within the hearing range of eared moths) due to the moths' ability to detect syntonic bat echolocation. Syntonic bats therefore feed mainly on beetles, flies, true bugs, and non-eared moths. The AFH is expected to be violated around lights where eared moths are susceptible to exploitation by syntonic bats because moths' evasive strategies become less effective. The hypothesis has been tested to date almost exclusively in areas with permanent lighting, where the effects of lights on bat diets are confounded with other aspects of human habitat alteration. We undertook diet analysis in areas with short-term, localized artificial lighting to isolate the effects of artificial lighting and determine if syntonic and allotonic bats (i.e., bats that use echolocation frequencies outside the hearing range of eared moths) consumed more moths under conditions of artificial lights than in natural darkness. We found that syntonic bats increased their consumption of moth prey under experimentally lit conditions, likely owing to a reduction in the ability of eared moths to evade the bats. Eared moths may increase in diets of generalist syntonic bats foraging around artificial light sources, as opposed to allotonic species and syntonic species with a more specialized diet.

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 (Ta) exceed body temperature (Tb). 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. We used phylogenetically informed regressions to compare EWL and evaporative cooling efficiency [ratio of evaporative heat loss (EHL) and metabolic heat production (MHP); EHL/MHP] among nocturnal and diurnal birds at high Ta We analyzed variation in three response variables: (1) slope of EWL at Ta between 40 and 46°C, (2) EWL at Ta=46°C and (3) EHL/MHP at Ta=46°C. Nocturnality emerged as a weak, negative predictor, with nocturnal species having slightly shallower slopes and reduced EWL compared with diurnal species of similar mass. In contrast, nocturnal activity was positively correlated with EHL/MHP, indicating a greater capacity for evaporative cooling in nocturnal birds. However, our analysis also revealed conspicuous differences among nocturnal taxa. Caprimulgids and Australian owlet-nightjars had shallower slopes and reduced EWL compared with similarly sized diurnal species, whereas owls had EWL rates comparable to those of diurnal species. Consequently, our results did not unequivocally demonstrate more economical cooling among nocturnal birds. Owls predominately select refugia with cooler microclimates, but the more frequent and intense heat waves forecast for the 21st century may increase microclimate temperatures and the necessity for active heat dissipation, potentially increasing owls' vulnerability to dehydration and hyperthermia.

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 (Ta) in ∼100-g representatives of three orders, namely, the African cuckoo (Cuculus gularis, Cuculiformes), lilac-breasted roller (Coracias caudatus, Coraciiformes) and Burchell's starling (Lamprotornis australis, Passeriformes). All three species initiated respiratory mechanisms to increase evaporative heat dissipation when body temperature (Tb) approached 41.5°C in response to increasing Ta, with gular flutter observed in cuckoos and panting in rollers and starlings. Resting metabolic rate and evaporative water loss increased by quantitatively similar magnitudes in all three species, although maximum rates of evaporative water loss were proportionately lower in starlings. Evaporative cooling efficiency [defined as the ratio of evaporative heat loss (EHL) to metabolic heat production (MHP)] generally remained below 2.0 in cuckoos and starlings, but reached a maximum of ∼3.5 in rollers. The high value for rollers reveals a very efficient evaporative cooling mechanism, and is similar to EHL/MHP maxima for similarly sized columbids which very effectively dissipate heat via cutaneous evaporation. This unexpected phylogenetic variation among the orders tested in the physiological mechanisms of heat dissipation is an important step toward determining the evolution of heat tolerance traits in desert birds.

Seasonal behavioral responses of an arid-zone passerine in a hot environment.

Many arid-zone animals have to forage under extremely hot conditions to maintain water and energy balance. The effect of high air temperatures (Tair) on the behavioral patterns of small endothermic animals-characterized by their high energy and water demands-will provide a valuable framework for understanding species vulnerability to climate warming. We determined the seasonal behavioral responses to changes in Tair in a~10-g arid-zone passerine, the rufous-eared warbler (Malcorus pectoralis), in the Karoo semi-desert, South Africa. Rufous-eared warblers showed significant temperature-dependence in their behavior in summer, but not in winter. During summer, the warblers frequently experienced Tair exceeding 40°C in the shade. For all observations <26°C compared to >36°C, the warblers showed reductions in preening (40% decrease), foraging effort (56% decrease), and foraging success (15% decrease), as well as a significant increase in time spent engaged in evaporative cooling behavior. Moreover, as Tair increased the warblers shifted increasingly off the ground and out of the full sun, into microsites in the shade (131% increase) and in shrubs (23% increase). In this regard, behavior varied seasonally, with the time spent in the shade 23% higher, and foraging effort 28% higher, in summer compared to winter across a range of moderate Tair (15-30°C). Our findings emphasize the link between behavior and temperature in small birds inhabiting hot, arid environments, as well as the importance of understanding these responses for predicting biologically meaningful responses (and hence, vulnerability) of arid-zone avian communities to climactic shifts.

Avian thermoregulation in the heat: evaporative cooling capacity in an archetypal desert specialist, Burchell's sandgrouse (Pterocles burchelli).

Sandgrouse (Pterocliformes) are quintessential examples of avian adaptation to desert environments, but relatively little is known about the limits to their heat tolerance and evaporative cooling capacity. We predicted that evaporative cooling in Burchell's sandgrouse (Pterocles burchelli) is highly efficient and provides the basis for tolerance of very high air temperature (Ta). We measured body temperature (Tb), resting metabolic rate (RMR) and evaporative water loss (EWL) at Ta between 25°C and ∼58°C in birds exposed to successive increments in Ta Normothermic Tb averaged 39.0°C, lower than typical avian values. At Ta>34.5°C, Tb increased linearly to a maximum of 43.6°C at Ta=56°C. The upper critical limit of thermoneutrality (Tuc) was Ta=43.8°C, closely coinciding with the onset of panting and gular flutter. Above the Tuc, RMR increased 2.5-fold to 2.89 W at Ta=56°C, a fractional increase far exceeding that of many other species under comparable conditions. Rates of EWL increased rapidly at Ta>42.9°C to 7.84±0.90 g h(-1) at Ta=56°C, an 11-fold increase above minimal levels. Maximum evaporative cooling efficiency (ratio of evaporative heat loss to metabolic heat production) was 2.03, but could be as high as 2.70 if our assumption that the birds were metabolising lipids is incorrect. Thermoregulation at very high Ta in P. burchelli was characterised by large increases in RMR and EWL, and is much less efficient than in taxa such as columbids and caprimulgids.

Avian thermoregulation in the heat: efficient evaporative cooling allows for extreme heat tolerance in four southern hemisphere columbids.

Birds show phylogenetic variation in the relative importance of respiratory versus cutaneous evaporation, but the consequences for heat tolerance and evaporative cooling capacity remain unclear. We measured evaporative water loss (EWL), resting metabolic rate (RMR) and body temperature (Tb) in four arid-zone columbids from southern Africa [Namaqua dove (Oena capensis, ∼37 g), laughing dove (Spilopelia senegalensis, ∼89 g) and Cape turtle dove (Streptopelia capicola, ∼148 g)] and Australia [crested pigeon (Ocyphaps lophotes), ∼186 g] at air temperatures (Ta) of up to 62°C. There was no clear relationship between body mass and maximum Ta tolerated during acute heat exposure. Maximum Tb at very high Ta was 43.1±1.0, 43.7±0.8, 44.7±0.3 and 44.3±0.8°C in Namaqua doves, laughing doves, Cape turtle doves and crested pigeons, respectively. In all four species, RMR increased significantly at Ta above thermoneutrality, but the increases were relatively modest with RMR at Ta=56°C being 32, 60, 99 and 11% higher, respectively, than at Ta=35°C. At the highest Ta values reached, evaporative heat loss was equivalent to 466, 227, 230 and 275% of metabolic heat production. The maximum ratio of evaporative heat loss to metabolic production observed in Namaqua doves, 4.66, exceeds by a substantial margin previous values reported for birds. Our results support the notion that cutaneous evaporation provides a highly efficient mechanism of heat dissipation and an enhanced ability to tolerate extremely high Ta.

Avian thermoregulation in the heat: scaling of heat tolerance and evaporative cooling capacity in three southern African arid-zone passerines.

Many birds can defend body temperature (Tb) far below air temperature (Ta) during acute heat exposure, but relatively little is known about how avian heat tolerance and evaporative cooling capacity varies with body mass (Mb), phylogeny or ecological factors. We determined maximum rates of evaporative heat dissipation and thermal end points (Tb and Ta associated with thermoregulatory failure) in three southern African ploceid passerines, the scaly-feathered weaver (Sporopipes squamifrons, Mb≈10 g), sociable weaver (Philetairus socius, Mb≈25 g) and white-browed sparrow-weaver (Plocepasser mahali, Mb≈40 g). Birds were exposed to a ramped profile of progressively increasing Ta, with continuous monitoring of behaviour and Tb used to identify the onset of severe hyperthermia. The maximum Ta birds tolerated ranged from 48°C to 54°C, and was positively related to Mb. Values of Tb associated with severe heat stress were in the range of 44 to 45°C. Rates of evaporative water loss (EWL) increased rapidly when Ta exceeded Tb, and maximum evaporative heat dissipation was equivalent to 141-222% of metabolic heat production. Fractional increases in EWL between Ta<40°C and the highest Ta reached by each species were 10.8 (S. squamifrons), 18.4 (P. socius) and 16.0 (P. mahali). Resting metabolic rates increased more gradually with Ta than expected, probably reflecting the very low chamber humidity values we maintained. Our data suggest that, within a taxon, larger species can tolerate higher Ta during acute heat stress.

The role of thermal physiology in recent declines of birds in a biodiversity hotspot.

We investigated whether observed avian range contractions and population declines in the Fynbos biome of South Africa were mechanistically linked to recent climate warming. We aimed to determine whether there were correlations between preferred temperature envelope, or changes in temperature within species' ranges, and recent changes in range and population size, for 12 Fynbos-resident bird species, including six that are endemic to the biome. We then measured the physiological responses of each species at air temperatures ranging from 24 to 42°C to determine whether physiological thermal thresholds could provide a mechanistic explanation for observed population trends. Our data show that Fynbos-endemic species occupying the coolest regions experienced the greatest recent reductions in range and population size (>30% range reduction between 1991 and the present). In addition, species experiencing the largest increases in air temperature within their ranges showed the greatest declines. However, evidence for a physiological mechanistic link between warming and population declines was equivocal, with only the larger species showing low thermal thresholds for their body mass, compared with other birds globally. In addition, some species appear more vulnerable than others to air temperatures in their ranges above physiological thermal thresholds. Of these, the high-altitude specialist Cape rockjumper (Chaetops frenatus) seems most at risk from climate warming. This species showed: (i) the lowest threshold for increasing evaporative water loss at high temperatures; and (ii) population declines specifically in those regions of its range recording significant warming trends. Our findings suggest that caution must be taken when attributing causality explicitly to thermal stress, even when population trends are clearly correlated with rates of warming. Studies explicitly investigating the mechanisms underlying such correlations will be key to appropriate conservation planning.

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 m(-3) (0%-100% relative humidity at 30°C) at air temperatures between 44° and 56°C. 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 48°C, and these birds showed a high degree of hyperthermia. At lower temperatures (40°-44°C), 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.

Thermoregulation in African Green Pigeons (Treron calvus) and a re-analysis of insular effects on basal metabolic rate and heterothermy in columbid birds.

Columbid birds represent a useful model taxon for examining adaptation in metabolic and thermal traits, including the effects of insularity. To test predictions concerning the role of insularity and low predation risk as factors selecting for the use of torpor, and the evolution of low basal metabolic rate in island species, we examined thermoregulation under laboratory and semi-natural conditions in a mainland species, the African Green Pigeon (Treron calvus). Under laboratory conditions, rest-phase body temperature (T b) was significantly and positively correlated with air temperature (T a) between 0 and 35 °C, and the relationship between resting metabolic rate (RMR) and T a differed from typical endothermic patterns. The minimum RMR, which we interpret as basal metabolic rate (BMR), was 0.825 ± 0.090 W. Green pigeons responded to food restriction by significantly decreasing rest-phase T b, but the reductions were small (at most ~5 °C below normothermic values), with a minimum T b of 33.1 °C recorded in a food-deprived bird. We found no evidence of the large reductions in T b and metabolic rate and the lethargic state characteristic of torpor. The absence of torpor in T. calvus lends support to the idea that species restricted to islands that are free of predators are more likely to use torpor than mainland species that face the risk of predation during the rest-phase. We also analysed interspecific variation in columbid BMR in a phylogenetically informed framework and verified the conclusions of an earlier study which found that BMR is significantly lower in island species compared to those that occur on mainlands.

Body temperature patterns in two syntopic elephant shrew species during winter.

We measured body temperature (T(b)) in free-ranging individuals of two species of elephant shrews, namely western rock elephant shrews (Elephantulus rupestris) and Cape rock elephant shrews (E. edwardii), during winter in a winter-rainfall region of western South Africa. These syntopic species have similar ecologies and morphologies and thus potential for large overlaps in diet and habitat use. Unexpectedly, they displayed different T(b) patterns. Western rock elephant shrews were heterothermic, with all individuals decreasing T(b) below 30°C on at least 34% of nights. The level of heterothermy expressed was similar to other species traditionally defined as daily heterotherms and was inversely related to T(a), as is commonly seen in small heterothermic endotherms. In contrast, Cape rock elephant shrews rarely allowed their T(b) to decrease below 30°C. The level of heterothermy was similar to species traditionally defined as homeotherms and there was no relationship between the level of heterothermy expressed and T(a). In both species, the minimum daily T(b) was recorded almost exclusively at night, often shortly before sunrise, although in some individuals minimum T(b) occasionally occurred during the day. The interspecific variation in T(b) patterns among Elephantulus species recorded to date reiterates the importance of ecological determinants of heterothermy that interact with factors such as body mass and phylogeny.

Adaptive thermoregulation in endotherms may alter responses to climate change.

Climate change is one of the major issues facing natural populations and thus a focus of recent research has been to predict the responses of organisms to these changes. Models are becoming more complex and now commonly include physiological traits of the organisms of interest. However, endothermic species have received less attention than have ectotherms in these mechanistic models. Further, it is not clear whether responses of endotherms to climate change are modified by variation in thermoregulatory characteristics associated with phenotypic plasticity and/or adaptation to past selective pressures. Here, we review the empirical data on thermal adaptation and acclimatization in endotherms and discuss how those factors may be important in models of responses to climate change. We begin with a discussion of why thermoregulation and thermal sensitivity at high body temperatures should be co-adapted. Importantly, we show that there is, in fact, considerable variation in the ability of endotherms to tolerate high body temperatures and/or high environmental temperatures, but a better understanding of this variation will likely be critical for predicting responses to future climatic scenarios. Next, we discuss why variation in thermoregulatory characteristics should be considered when modeling the effects of climate change on heterothermic endotherms. Finally, we review some biophysical and biochemical factors that will limit adaptation and acclimation in endotherms. We consider both long-term, directional climate change and short-term (but increasingly common) anomalies in climate such as extreme heat waves and we suggest areas of important future research relating to both our basic understanding of endothermic thermoregulation and the responses of endotherms to climate change.

Torpor in dark times: patterns of heterothermy are associated with the lunar cycle in a nocturnal bird.

Many studies have shown that endotherms become more heterothermic when the costs of thermoregulation are high and/or when limited energy availability constrains thermoregulatory capacity. However, the roles of many ecological variables, including constraints on foraging opportunities and/or success, remain largely unknown. To test the prediction that thermoregulatory patterns should be related to foraging opportunities in a heterothermic endotherm, we examined the relationship between the lunar cycle and heterothermy in Freckled Nightjars (Caprimulgus tristigma), which are visually orienting, nocturnal insectivores that are dependent on ambient light to forage. This model system provides an opportunity to assess whether variation in foraging opportunities influences the expression of heterothermy. The nightjars were active and foraged for insects when moonlight was available but became inactive and heterothermic in the absence of moonlight. Lunar illumination was a much stronger predictor of the magnitude of heterothermic responses than was air temperature (T(a)). Our data suggest that heterothermy was strongly related to variation in foraging opportunities associated with the lunar cycle, even though food abundance appeared to remain relatively high throughout the study period. Patterns of thermoregulation in this population of Freckled Nightjars provide novel insights into the environmental and ecological determinants of heterothermy, with the lunar cycle, and not T(a), being the strongest predictor of torpor use.

A new comparative metric for estimating heterothermy in endotherms.

A major focus in the study of endothermic thermoregulation has been the description of thermoregulatory patterns used by various species and/or populations. Compared with ectotherms, relatively few attempts have been made to study the thermoregulation of endotherms in an adaptive framework. We believe that one of the main factors limiting this area of research has been the lack of an appropriate metric to directly compare body temperature (T(b)) variation across all endothermic species. Thus, we present a simple comparative metric, the heterothermy index (HI), to quantify the expression of heterothermy by endotherms during a given time frame. Key advantages of HI are that (1) it represents a new analytical technique that has different strengths than the metrics commonly used to describe variation in T(b), (2) it allows for evaluation of nonenergetic costs and benefits that affect the expression of heterothermy, and (3) it has the potential to unify research on homeotherms and heterotherms through quantitative comparative analyses that examine the entire continuum of thermoregulatory patterns. In short, we suggest that our metric provides a means to overcome one of the hurdles presently slowing the advancement of research on endothermic thermoregulation beyond the simple description of thermoregulatory patterns.

Do owls use torpor? Winter thermoregulation in free-ranging pearl-spotted owlets and African scops-owls.

Numerous avian taxa use torpor, which involves pronounced reductions in body temperature (T(b)) to below normothermic levels. However, the occurrence of this phenomenon in owls (Strigidae) remains largely unknown. We investigated winter patterns of thermoregulation in the crepuscular 80-g pearl-spotted owlet Glaucidium perlatum and the strictly nocturnal 61-g African scops-owl Otus senegalensis by obtaining telemetric measurements of skin temperature (T(skin)) from free-ranging individuals in the Kalahari Desert of southern Africa. Pearl-spotted owlets remained homeothermic throughout the study period, whereas African scops-owls routinely used shallow torpor, with T(skin) reduced by 3.3 degrees -8.6 degrees C (pooled mean, 5.3+/- 1.1 degrees C) below normothermic levels for 3-4 h after sunrise. The mean lowest T(skin) recorded in three African scops-owl individuals was 29.0 degrees C +/- 0.1 degrees C. The thermoregulatory differences between these two species may be related to their diets and activity patterns. African scops-owls are almost exclusively insectivorous and experience a marked reduction in food availability on cold winter nights. In contrast, pearl-spotted owlets have more flexible activity patterns and include larger or diurnal vertebrate prey in their diet.