Blair O. Wolf

Climate change increases the likelihood of catastrophic avian mortality events during extreme heat waves

Severe heat waves have occasionally led to catastrophic avian mortality in hot desert environments. Climate change models predict increases in the intensity, frequency and duration of heat waves. A model of avian evaporative water requirements and survival times during the hottest part of day reveals that the predicted increases in maximum air temperatures will result in large fractional increases in water requirements (in small birds, equivalent to 150–200 % of current values), which will severely reduce survival times during extremely hot weather. By the 2080s, desert birds will experience reduced survival times much more frequently during mid-summer, increasing the frequency of catastrophic mortality events.

The Allometry of Avian Basal Metabolic Rate: Good Predictions Need Good Data

Basal metabolic rate (BMR) is often predicted by allometric interpolation, but such predictions are critically dependent on the quality of the data used to derive allometric equations relating BMR to body mass (Mb). An examination of the metabolic rates used to produce conventional and phylogenetically independent allometries for avian BMR in a recent analysis revealed that only 67 of 248 data unambiguously met the criteria for BMR and had sample sizes with n ≥ 3. The metabolic rates that represented BMR were significantly lower than those that did not meet the criteria for BMR or were measured under unspecified conditions. Moreover, our conventional allometric estimates of BMR (W; log BMR = −1.461 + 0.669log Mb) using a more constrained data set that met the conditions that define BMR and had n ≥ 3 were 10%–12% lower than those obtained in the earlier analysis. The inclusion of data that do not represent BMR results in the overestimation of predicted BMR and can potentially lead to incorrect conclusions concerning metabolic adaptation. Our analyses using a data set that included only BMR with n ≥ 3 were consistent with the conclusion that BMR does not differ between passerine and nonpasserine birds after taking phylogeny into account. With an increased focus on data mining and synthetic analyses, our study suggests that a thorough knowledge of how data sets are generated and the underlying constraints on their interpretation is a necessary prerequisite for such exercises.

Thermal Effects of Radiation and Wind on a Small Bird and Implications for Microsite Selection

The physical environmental factors (air temperature, solar radiation, wind speed) that define specific microclimates and their effects on water and energy budgets of small birds are of major importance to our understanding of avian thermal biology. We examined the effects of solar radiation, wind speed, and their interaction on metabolic rates in the Verdin, Auriparus flaviceps. Daytime resting metabolic rates and evaporative water loss rates as a function of air temperature, as well as basal metabolic rate, were also measured to allow estimation of water and energy flux rates in diverse microclimates. In the absence of solar radiation, as wind speed was increased from 0.4 to 3.0 m/s, metabolic rate increased 14%. Exposure to simulated solar radiation significantly reduced metabolic heat production at all wind speeds measured except 3.0 m/s. Solar heat gain (SHG) was estimated for an irradiance of 1000 W/m2, similar to that commonly observed in nature. At 0.4 m/s wind speed and 1000 W/m2 irradiance, SHG may reduce metabolic rate by 46%. SHG declines precipitously as wind speed is increased, and at 3.0 m/s, metabolic rate is only reduced by 3%. Analyses of changes in thermostatic costs associated with microclimate selection in winter suggest that Verdins may reduce metabolic rate by as much as 50% by shifting from a shaded, windy site to one protected from the wind and exposed to 1000 W/m2 solar radiation. Similar analyses for Verdins during the summer suggest that microsite selection can result in significant water savings. By remaining out of the sun and wind, Verdins can reduce their rate of evaporative water loss by at least a factor of four. This analysis clearly dem- onstrates the potential importance of daytime microclimate selection to balancing water and energy budgets in small birds.

Sex differences in the thermoregulation and evaporative water loss of a heterothermic bat, Lasiurus cinereus, during its spring migration

SUMMARY This study quantifies sex differences in thermoregulation and water loss of a small (20-35 g) insectivorous heterothermic mammal, the hoary bat Lasiurus cinereus, during its spring migration. We measured body temperature, metabolic rate and evaporative water loss, and calculated wet thermal conductance, for bats exposed to air temperatures ranging from 0 to 40°C for periods of 2-5 h. Pregnant females maintained normothermic body temperatures (35.7±0.7°C; mean ± s.e.m.) independent of air temperature. In contrast, males became torpid during the majority (68%) of exposures to air temperatures <25°C. The thermal neutral zone (TNZ) ranged between approximately 30°C and 34°C in both sexes and, within the TNZ, females had lower mass-specific metabolic rates (6.1±0.2 mW g-1) than males (9.0±0.9 mW g-1). Wet thermal conductance values in torpid bats (0.7±0.5 mW g-1 deg.-1) were lower than those of normothermic individuals (1.1±0.3 mW g-1 deg.-1). Mass-specific rates of evaporative water loss in males were consistently higher than in females at most air temperatures and rates of water loss in torpid bats were 63±6% of normothermic values. These results suggest that male and pregnant female L. cinereus employ different thermoregulatory strategies during their spring migration. Females defend normothermic body temperatures, presumably to expedite embryonic growth, while males use torpor, presumably to minimize energy and water deficits. These variable thermoregulatory strategies may reflect continental differences in the summer distribution of the sexes.

Shifting species interactions in terrestrial dryland ecosystems under altered water availability and climate change

Species interactions play key roles in linking the responses of populations, communities, and ecosystems to environmental change. For instance, species interactions are an important determinant of the complexity of changes in trophic biomass with variation in resources. Water resources are a major driver of terrestrial ecology and climate change is expected to greatly alter the distribution of this critical resource. While previous studies have documented strong effects of global environmental change on species interactions in general, responses can vary from region to region. Dryland ecosystems occupy more than one‐third of the Earths land mass, are greatly affected by changes in water availability, and are predicted to be hotspots of climate change. Thus, it is imperative to understand the effects of environmental change on these globally significant ecosystems.

Use of saguaro fruit by white-winged doves: isotopic evidence of a tight ecological association

Abstract We report the use of stable isotope and crop content analyses to quantify the use of saguaro (Carnegiea gigantea) nectar and fruit by migratory desert white-winged doves (Zenaida asiatica mearsnii). Saguaro resources had characteristically 13C-enriched CAM values (δ13C=–12.8±0.7‰ SD VPDB and –13.1±0.5‰ SD VPDB for nectar and fruit, respectively) relative to other food plants used by doves (δ13CC3=–24.9±3.3‰ SD VPDB). The water contained in saguaro nectar and fruit was deuterium enriched (δD=19.6±2.0‰ SD VSMOW and 48.4±1.6‰ SD VSMOW for nectar and fruit, respectively) relative to other water sources (ranging from –41 to –19‰ VSMOW). During the fruiting season, there was a positive correlation between δ13C in dove liver tissues and percent of saguaro in crop contents. A two-point mixing model indicated that during the peak of saguaro fruit use, most of the carbon incorporated in dove tissues was from saguaro. Desert white-winged doves appear to be saguaro specialists. Averaged over the period when doves were resident, saguaro comprised about 60% of the total carbon incorporated into dove tissues. Tissue δ13C and δD of body water showed a significant positive correlation, indicating that doves were using saguaro as a source of both nutrients and water. However, at the peak of saguaro utilization, the doves’ body-water δD was more positive (by about 20‰) than saguaro fruit water. We hypothesize that this enrichment is due to fractionated evaporative water losses by doves. Using dove carbon isotope data and a two end-point mixing model we estimate that, on average, doves consume the equivalent of 128 saguaro fruits per season; each fruit contains on average 26.0±14.8 g SD of pulp (wet mass) of which 19.4 g is water. Stable isotopes have been used to produce qualitative re-constructions of animal diets. Our study shows that they can be used to provide quantitative estimates of the flow of nutrients from resources into consumers as well.

Partitioning of evaporative water loss in white-winged doves: plasticity in response to short-term thermal acclimation

SUMMARY We investigated changes in the relative contributions of respiratory evaporative water loss (REWL) and cutaneous evaporative water loss (CEWL) to total evaporative water loss (TEWL) in response to short-term thermal acclimation in western white-winged doves Zenaida asiatica mearnsii. We measured REWL, CEWL, oxygen consumption and carbon dioxide production in a partitioned chamber using flow-through respirometry. In doves housed for 2-4 weeks in a room heated to ca. 43°C during the day, TEWL increased from 5.5±1.3 mg g-1 h-1 at an air temperature (Ta) of 35°C to 19.3±2.5 mg g-1 h-1 at Ta=45°C. In doves housed at room temperature for the same period, TEWL increased from 4.6±1.1 mg g-1 h-1 at Ta=35°C to 16.1±4.6 mg g-1 h-1 at Ta=45°C. The CEWL of heat-acclimated doves increased from 3.6±1.2 mg g-1 h-1 (64% of TEWL) at 35°C to 15.0±2.1 mg g-1 h-1 (78% of TEWL) at Ta=45°C. Cool-acclimated doves exhibited more modest increases in CEWL, from 2.7±0.7 mg g-1 h-1 at Ta=35°C to 7.8±3.4 mg g-1 h-1 at Ta=45°C, with the contribution of CEWL to TEWL averaging 53% over this Ta range. Cool-acclimated doves became mildly hyperthermic (body temperature Tb=42.9±0.4°C) and expended 35% more energy relative to heat-acclimated doves (Tb=41.9±0.6°C) at Ta=45°C, even though TEWL in the two groups was similar. In each of the two groups, metabolic rate did not vary with Ta, and averaged 7.1±0.5 mW g-1 in cool-acclimated doves and 6.3±0.8 mW g-1 in heat-acclimated doves. The differences in TEWL partitioning we observed between the two experimental groups resulted from a consistently lower skin water vapour diffusion resistance (rv) in the heat-acclimated doves. At Ta=45°C, rv in the cool-acclimated doves was 120±81 s cm-1, whereas rv in the heat-acclimated doves was 38±8 s cm-1. Our data reveal that in Z. a. mearnsii, TEWL partitioning varies in response to short-term thermal acclimation.

Lipid content of terrestrial arthropods in relation to body size, phylogeny, ontogeny and sex

Energy storage in arthropods has important implications for survival and reproduction. The lipid content of 276 species of adult arthropods with wet mass in the range 0.2–6.13 g is determined to assess how lipid mass scales with body mass. The relative contribution of lipids to total body mass is investigated with respect to phylogeny, ontogeny and sex. The lipid content of adult insects, arachnids, and arthropods in general shows an isometric scaling relationship with respect to body mass (M) (Marthropod lipid = −1.09 ×Mdry1.01 and Marthropod lipid = −1.00 ×Mlean0.98). However, lipid allocation varies between arthropod taxa, as well as with sex and developmental stage within arthropod taxa. Female insects and arachnids generally have higher lipid contents than males, and larval holometabolous insects and juvenile arachnids have higher lipid contents than adults.

How important are columnar cacti as sources of water and nutrients for desert consumers? A review.

Succulent CAM plants, such as columnar cacti, are important physiognomic elements of many arid lands. Although, these plants are often ecologically important because they provide abundant resources in the form of nectar and fruit, their contribution to the energy, nutrient and water budgets of consumers has not been quantified. We describe an isotopic approach that allows quantifying the ecological importance of CAM succulents. We first briefly review our work on the interaction between saguaros, an archetypical CAM succulent, and the desert doves that feed on its fruit. We then describe the potential importance of saguaro fruit as a function of its abundance, macronutrient composition, and seasonal availability. We argue that the resources provided by saguaros do much to satisfy the energy and water requirements of the birds that reside in hot subtropical deserts during the summer. We then describe the carbon isotope composition of saguaros and of the plant community in which they are imbedded and use two species of desert doves to illustrate how stable isotopes can reveal the importance of a single plant as a source of carbon and water for consumers. The second section of this review presents new data on the importance of saguaros for the entire community of birds that inhabit the Sonoran Desert during the summer. We show how the resources of saguaro reach across dietary guilds and account for a large proportion of the diet of many insectivorous species as well as that of granivorous and frugivorous species. We demonstrate that many of these species probably obtain significant water as well as nutrients from saguaro fruit. Finally, we point out the current limitations of using deuterium as a water tracer in animal systems.

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

ABSTRACT 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. Highlighted Article: Heat tolerance varies with body size in African weavers in the Kalahari Desert.