Cutaneous Evaporative Water Loss Research Articles

Partitioning of evaporative water loss into respiratory and cutaneous pathways in Wahlberg's epauletted fruit bats (Epomophorus wahlbergi).

The relative contributions of respiratory and cutaneous evaporation to total evaporative water loss (TEWL) and how the partitioning of these two avenues varies with environmental temperature has received little attention in bats. We trained Wahlberg's epauletted fruit bats (Epomophorus wahlbergi) captured in Pretoria, South Africa, to wear latex masks while hanging in respirometry chambers, and we measured respiratory evaporative water loss (REWL) and cutaneous evaporative water loss (CEWL) over air temperatures (Ta) from 10° to 40°C. The bats' normothermic body temperature (Tb) was approximately 36°C, which increased at higher Ta to 40.5° ± 1.0°C at Ta ≈ 40°C. Both TEWL and resting metabolic rate (RMR) increased sharply at Ta >35°C, with a mean TEWL at 40°C equivalent to 411% of that at 30°C. The increase in TEWL was driven by large increases in both CEWL and REWL. CEWL comprised more than 50% of TEWL over the entire Ta range, with the exception of Ta ≈ 40°C, where REWL accounted for 58% of evaporative water loss. Surface area-specific CEWL increased approximately sixfold with increasing Ta. Thermoregulation at Ta approaching or exceeding Tb involved a considerable energetic cost, with RMR at Ta ≈ 40°C exceeding by 24% that measured at Ta ≈ 10°C. Our data do not support recent arguments that respiratory gas exchange across the wing membranes represents 5%-10% of the total in E. wahlbergi.

Microclimate preferences correlate with contrasted evaporative water loss in parapatric vipers at their contact zone

Terrestrial ectotherms predominantly use behavioural means to thermoregulate and thereby optimize performances. However, thermoregulation can impart physiological challenges to other critical processes such as water balance by increasing evaporative water loss (EWL). Like thermoregulation, water balance is influenced by both external factors (e.g., microhabitat and environmental constraints) and endogenous traits (e.g., evaporative water loss rates, dehydration tolerance). Although thermoregulation and water balance are tightly linked, the role of water balance is often overlooked when evaluating species climatic adaptation and response to global warming. We studied two congeneric viperid species (the Aspic Viper, Vipera aspis (L., 1758), and the Common Viper, Vipera berus (L., 1758)) with contrasted climatic affinities (south European versus boreal, respectively). These parapatric species are syntopic in narrow contact zones where microhabitat partitioning has been reported. We compared total EWL and cutaneous evaporative water loss (CEWL) of the two species and monitored the thermal and hydric conditions of the microhabitats used in syntopic populations. We found that the boreal V. berus has greater EWL, both total and cutaneous. Accordingly, this species selected more humid microhabitats throughout the year. Humidity appears to be an important determinant of habitat selection, and therefore, V. berus is likely vulnerable to changing precipitation at the southern limit of its distribution.

Effects of Water Vapor Density on Cutaneous Resistance to Evaporative Water Loss and Body Temperature in Green Tree Frogs (Hyla cinerea)

Increased cutaneous resistance to evaporative water loss (Rc) in tree frogs results in decreased water loss rate and increased body temperature. We examined sensitivity of Rc to water vapor density (WVD) in Hyla cinerea by exposing individual frogs and agar models to four different WVD environments and measuring cutaneous evaporative water loss rate and body temperature simultaneously using a gravimetric wind tunnel measuring system. We found that water loss rate varied inversely and body temperature directly with WVD but that models were affected to a greater extent than were animals. Mean Rc was significantly different between the highest WVD environment and each of the three drier environments but did not differ among the drier environments, indicating that Rc initially increases and then reaches a plateau in response to decreasing WVD. Rc was equivalent when calculated using either WVD difference or WVD deficit as the driving force for evaporation. We also directly observed secretions from cutaneous glands while measuring body temperature and tested secretions and skin samples for the presence of lipids. We found that irregular transient body temperature depressions observed during wind tunnel trials occur due to evaporative cooling from intermittent skin secretions containing lipids, although we were unable to identify lipid-secreting glands.

Climate influences thermal balance and water use in African and Asian elephants: physiology can predict drivers of elephant distribution

Elephant movement patterns in relation to surface water demonstrate that they are a water-dependent species. Thus, there has been interest in using surface water management to mitigate problems associated with localized elephant overabundance. However, the physiological mechanisms underlying the elephant's water dependence remain unclear. Although thermoregulation is likely an important driver, the relationship between thermoregulation, water use and climate has not been quantified. We measured skin surface temperature of and cutaneous water loss from 13 elephants (seven African, 3768±642 kg; six Asian, 3834±498 kg) and determined the contribution of evaporative cooling to their thermal and water budgets across a range of air temperatures (8-33°C). We also measured respiratory evaporative water loss and resting metabolic heat production on a subset of elephants (N=7). The rate of cutaneous evaporative water loss ranged between 0.31 and 8.9 g min(-1) m(-2) for Asian elephants and 0.26 and 6.5 g min(-1) m(-2) for African elephants. Simulated thermal and water budgets using climate data from Port Elizabeth, South Africa, and Okaukuejo, Namibia, suggested that the 24-h evaporative cooling water debt incurred in warm climates can be more than 4.5 times that incurred in mesic climates. This study confirms elephants are obligate evaporative coolers but suggests that classification of elephants as water dependent is insufficient given the importance of climate in determining the magnitude of this dependence. These data highlight the potential for a physiological modeling approach to predicting the utility of surface water management for specific populations.

Habitat Selection by Sea Kraits (Laticauda spp.) at Coastal Sites of Orchid Island, Taiwan

Three species of amphibious sea kraits (Laticauda spp.) spend variable time at sea and require fresh water for water balance. Both the rate of cutaneous evaporative water loss and the extent of terrestriality are known to differ among them. Laticauda semifasciata has the greatest rate of water loss and the least extent of terrestriality, whereas L. colubrina exhibits the reverse and L. laticaudata is intermediate. These sea kraits tend to be more abundant at places where there are sources of fresh water, but other factors also influence their distribution. To further clarify the habitat requirements, we investigated the abundance of each species of sea krait at six different habitats and the availability of each type of habitat on Orchid Island, Taiwan. The six habitats were high coral reef without fresh water (HR) and with fresh water (HRF); low coral reef without fresh water (LR) and with fresh water (LRF); sand or gravel coast, which has no coral reef, without fresh water (NR) and with fresh water (NRF). The extent of safety judged from the relative availability of retreat sites, from high to low, was HR, LR, and NR among these habitats. More than 75% of individuals counted for each species were found in HRF. We found no sea kraits in NRF and NR. The most available habitat was LR, but no L. laticaudata or L. semifasciata were found in this habitat. We found 3.3% and 16.7% of L. colubrina in LR and HR, respectively. For L. colubrina, the second abundant habitat was HR, whereas for L. laticaudata and L. semifasciata, the second abundant habitat was LRF. We conclude that both safety (availability of retreat sites) and fresh water are important to the habitat selection of sea kraits. Compared with other species, L. colubrina is characterized by a greater extent of terrestrial habit and possibly greater variety of access to sources of fresh water.

Cutaneous Resistance to Evaporative Water Loss in the Crab-eating Frog (Fejervarya cancrivora)

Abstract Resistance to cutaneous evaporative water loss has been examined in anuran amphibians that occupy a variety of habitats, except for the ocean. We measured evaporative water loss rate and cutaneous resistance to evaporation in the Crab-eating Frog, Fejervarya cancrivora, a euryhaline species that enters the sea. Studies were conducted on Hoga Island, southeastern Sulawesi, Indonesia. Area-specific evaporative water loss rate was significantly lower in living F. cancrivora than in agar replicas of the frogs and total water loss rate was dependent on body mass. Cutaneous resistance to evaporative water loss was 0.27 sec cm−1, which is the lowest significant resistance known for frogs. Cutaneous resistance may have evolved as a way for F. cancrivora to reduce water loss while on land, thereby offsetting high osmotic losses to seawater before frogs have fully acclimated to higher salinities.

Respiratory evaporative water loss during hovering and forward flight in hummingbirds

Hummingbirds represent an end point for small body size and water flux in vertebrates. We explored the role evaporative water loss (EWL) plays in management of their large water pool and its use in dissipating metabolic heat. We measured respiratory evaporative water loss (REWL) in hovering hummingbirds in the field (6 species) and over a range of speeds in a wind tunnel (1 species) using an open-circuit mask respirometry system. Hovering REWL during the active period was positively correlated with operative temperature (T e) likely due to some combination of an increase in the vapor-pressure deficit, increase in lung ventilation rate, and reduced importance of dry heat transfer at higher T e. In rufous hummingbirds ( Selasphorus rufus; 3.3 g) REWL during forward flight at 6 and 10 m/s was less than half the value for hovering. The proportion of total dissipated heat (TDH) accounted for by REWL during hovering at T e > 40 °C was < 40% in most species. During forward flight in S. rufus the proportion of TDH accounted for by REWL was ~ 35% less than for hovering. REWL in hummingbirds is a relatively small component of the water budget compared with other bird species (< 20%) so cutaneous evaporative water loss and dry heat transfer must contribute significantly to thermal balance in hummingbirds.

Water management in nectar-feeding birds

nectarivory is widespread in birds, but best known in three unrelated families on different continents: the American hummingbirds, Australasian honeyeaters, and the sunbirds of Africa and Asia. Bird pollinators require substantial nectar rewards and their flowers produce copious nectar, which is,

Rates of Cutaneous Evaporative Water Loss of Native Fijian Frogs

Rates of evaporative water loss (EWL) and cutaneous resistance (Rc) to water flux were measured for the two native frog species from the Fiji Islands: the Fiji Ground Frog, Platymantis vitianus, and the Fiji Tree Frog, Platymantis vitiensis (Anura: Ranidae). Surface area specific evaporative water loss was similar between the species: 10.2 (± 1.7) and 9.0 (± 1.7) mg cm−2 h−1 for P. vitiensis and P. vitianus, respectively. The total resistance (Rt) to water loss did not differ from estimates of similarly sized and shaped agar models (representative of a free water surface) for either species; thus, native Fijian frogs are “typical” anurans that have negligible cutaneous resistance (Rc) to EWL. Platymantis vitiensis is apparently able to exploit an arboreal habit, despite the concomitant increase in desiccating conditions, because of the humid environment of the Fiji Islands and by living near streams and rivers.

Comparative Analysis of Cutaneous Evaporative Water Loss in Frogs Demonstrates Correlation with Ecological Habits

Most frog species show little resistance to evaporative water loss (EWL), but some arboreal species are known to have very high resistances. We measured EWL and cutaneous resistance to evaporation (Rc) in 25 species of frogs from northern Australia, including 17 species in the family Hylidae, six species in the Myobatrachidae, and one each in the Bufonidae and the Microhylidae. These species display a variety of ecological habits, including aquatic, terrestrial, and arboreal specialisations, with the complete range of habits displayed within just the one hylid genus, Litoria. The 25 species measured in this study have resistances that range from Rc=0 to 63.1. These include low values indistinguishable from a free water surface to high values typical of "waterproof" anuran species. There was a strong correlation between ecological habit and Rc, even taking phylogenetic relationships into account; arboreal species had the highest resistance, aquatic species tended to have little or no resistance, and terrestrial species tended to have resistance between those of arboreal and aquatic frogs. For one species, Litoria rubella, we found no significant changes in EWL along a 1,500-km aridity gradient. This study represents the strongest evidence to date of a link between ecological habits and cutaneous resistance to water loss among species of frogs.

Standard Evaporative Water Loss and Metabolism of Juvenile Varanus mertensi (Squamata: Varanidae)

Evaporative water loss (EWL) varies considerably both within and between reptile species (Dmi'el, 1985; Eynan and Dmi'el, 1993; Lahav and Dmi'el, 1996) and can also be affected by acclimation (Hillman et al., 1979; Kobayashi et al., 1983). EWL data are available for a wide variety of reptiles from various habitats (see review by Mautz, 1982) but are limited for goannas under controlled conditions (Warburg, 1965; Green, 1969; Thompson and Withers, 1997a). For lizards, such as goannas, body water is lost by evaporation from the skin (including the eyes) and the respiratory passages and as a mixture of urine and feces (and in some species from nasal salt glands; Green, 1969; Saint Girons et al., 1981). The rate of cutaneous evaporative water loss (CEWL) is largely determined by the exposed surface area of the skin and eyes, the resistance of these surfaces to evaporation, and the difference in water vapor concentrations across the surfaces. The respiratory evaporative water loss (REWL) is determined mainly by the lung ventilation rate and body temperature but may be reduced by nasal countercurrent heat and water exchange (Mautz, 1980). Environmental conditions such as ambient temperature, relative humidity, and wind velocity also affect EWL (Thorpe and Kontogiannis, 1977; Mautz, 1980, 1982). EWL is generally higher for mesic reptiles than arid species (Snyder, 1971; Dmi'el, 1972; Shoemaker and Nagy, 1977), and we would expect the EWL of a semiaquatic goanna to be higher than for comparable terrestrial species. However, there are no data for EWL of semiaquatic goannas under controlled conditions. The semiaquatic goanna Varanus mertensi has a much higher field water turnover (WTR) than predicted for lizards from arid and tropical zones (Christian et al., 1996), but it is not clear whether their high WTR is associated with a high cutaneous EWL, respiratory EWL, water excretion, or a combination thereof. This study examines the standard EWL (EWL,td) and standard metabolic rate (VO2std) for juvenile V mertensi. The primary objectives were to measure total evaporative water loss and calculate resistance to water evaporation under standardized laboratory conditions (TEWLtd, rstd), and to partition TEWLStd and rstd into its respiratory and cutaneous components. Thompson and Withers (1997a) report TEWL,td and resistance to total water loss (rtota) for a number of terrestrial and arboreal Western Australian

Periodic arousals in hibernating mammals: is evaporative water loss involved?

1. Using existing data on the rate of cutaneous and pulmonary evaporative water loss (EWL) for hibernating Little Brown Bats (Myotis lucifugus) and on the duration of torpor bouts, body temperature (Tb) and oxygen uptake (VO2) of Golden‐mantled Ground Squirrels (Spermophilus saturatus), the rate of EWL was modelled for ground squirrels hibernating at ambient temperatures (Ta) of –2, 2, 4 and 8 °C. 2. Total EWL showed a curvilinear response to Ta, being lowest at 2 °C and increasing with both increasing and decreasing Ta. EWL at –2 °C was about equal to that at 4 °C. The duration of torpor bouts showed the same curvilinear response to Ta and torpor bout duration at –2 °C was similar to that at 4 °C (8·5 vs 8·3 days, respectively). 3. At Ta≥ 2 °C, where Tb of torpid S. saturatus is not metabolically defended, torpor bout duration is significantly related to Tb, VO2 and EWL, with the three variables having similar r2 values. 4. Using the regression equations generated at Ta≥ 2 °C to predict torpor bout durations at –2 °C, where Tb is metabolically defended, shows that the three variables do not have equivalent predictive abilities. Tb and VO2 predicted torpor bout durations of 15·2 and –40·4 days, respectively, compared with observed durations of 8·5 days at –2 °C. EWL predicted torpor bout durations of 8·4 days or only 0·1 days less than that observed at –2 °C. 5. The relation between torpor bout duration and total EWL was insensitive to major variations in cutaneous EWL. Over Ta ranging from –2 °C to 8 °C, a stepwise multiple regression including Tb, VO2 and EWL as independent variables identified EWL as the only variable significantly correlated with torpor bout duration. 6. Our analyses suggest that torpor bout duration may be influenced by EWL, indicating that animals may need to obtain free water when they arouse. An analysis of the structural and temperature characteristics of ground squirrel hibernacula suggests that they may function as a biological condensing tower. Throughout winter, water should evaporate from the warm lower levels and condense in the colder upper regions and so free water may be available to animals when they arouse.

Respiratory and cutaneous evaporative water loss at high environmental temperatures in a small bird

We measured rates of respiratory and cutaneous evaporative water loss as a function of air temperature in a small desert bird, the verdin Auriparus flaviceps. Birds were placed in a two-compartment metabolic chamber that separately collected water evaporated from the bird's head and body. Cutaneous and respiratory evaporative water loss, as well as CO2 production, were measured in resting birds at 2 &deg;C intervals between 30 and 50 &deg;C. Metabolic rate was lowest at 38 &deg;C (19 mW g-1) and increased to 28 mW g-1 at 50 &deg;C. At the lowest air temperature, 30 &deg;C, resting metabolic rate was 34 mW g-1. As air temperature increased from 30 to 50 &deg;C, cutaneous water loss increased from 3.3 to 10.3 mg g-1 h-1 and respiratory water loss increased from 2.1-64.1 mg g-1 h-1. At moderate air temperatures (30-36 &deg;C), water loss was divided almost evenly between respiratory and cutaneous components. As air temperature increased, however, verdins became heavily dependent on respiratory evaporation for heat dissipation. Evaporative water loss data for other species at high air temperatures suggest that partitioning of water loss may follow two different patterns. Evaporative heat dissipation may depend primarily on either cutaneous or respiratory modes of evaporative heat transfer. The physiological mechanisms and functional significance of these contrasting patterns of evaporative heat loss remain unknown.

Thermoregulation in a large bird, the emu (Dromaius novaehollandiae)

The emu is a large, flightless bird native to Australia. Its habitats range from the high snow country to the arid interior of the continent. Our experiments show that the emu maintains a constant body temperature within the ambient temperature range-5 to 45°C. The males regulate their body temperature about 0.5°C lower than the females. With falling ambient temperature the emu regulates its body temperature initially by reducing conductance and then by increasing heat production. At-5°C the cost of maintaining thermal balance is 2.6 times basal metabolic rate. By sitting down and reducing heat loss from the legs the cost of homeothermy at-5°C is reduced to 1.5 times basal metabolic rate. At high ambient temperatures the emu utilises cutaneous evaporative water loss in addition to panting. At 45°C evaporation is equal to 160% of heat production. Panting accounts for 70% of total evaporation at 45°C. The cost of utilising cutaneous evaporation for the other 30% appears to be an increase in dry conductance.

A Comparative Study of Heating Rates in Arboreal and Nonarboreal Frogs

Heating rates and time constants of heating were determined for similarly sized green tree frogs, Hyla cinerea, and southern leopard frogs, Rana sphenocephala exposed to step changes in ambient temperature from 15 to 30?C. Whereas the initial body temperatures were not significantly different (15?C), equilibration body temperatures were significantly higher in H. cinerea (26.6?C) than in R. sphenocephala (18.1?C). Rates of temperature change were 2.5 x higher in H. cinerea and are explained by the presence of reduced cutaneous evaporative heat loss in this species. The thermal time constant is inappropriate for interspecific comparisons of heating in anurans. lo , ol. 23, No. 2, pp. 141-145, 1989 ciet for the Study of Amphibians and Reptiles of eating Rates in a oreal Frogs During the nearly twenty years since reduced cutaneous evaporative water loss (CWL) was first reported in an arboreal frog (Loveridge, 1970), some degree of waterproofing has been found to be present among species belonging to at least nine tree frog genera (see Wygoda, 1988b). Because anuran hydroand thermoregulation are closely linked (Tracy, 1976; Bundy and Tracy, 1977; Beuchet et al., 1984), profound differences in thermoregulatory strategies are expected to be present between these frogs and non-waterproof frogs. However, little is known of the thermal consequences of CWL reduction because, with few exceptions (McClanahan et al., 1978; Wygoda, 1984, 1988a, b; Shoemaker et al., 1987), simultaneous water loss and body temperature measurements have not been performed. It is known that body temperatures (Tbs) of waterproof arboreal frogs approximate air temperature (Ta) up to around 40?C (Shoemaker et al., 1987), but remain constant when Ta is further increased (McClanahan et al., 1978; Shoemaker et al., 1987). TbS of non-thermally stressed moderately waterproof arboreal frogs are intermediate between Ta and the Tb of nonwaterproof frogs (Wygoda, 1984) and are less coupled to relative humidity (RH) changes than are those of non-waterproof frogs (Wygoda, 1988b). In addition to providing elevated Tbs, rel., 1987), simultaneous water loss and body e perature measurements have not b e perormed. It is known that body temperatures (Tbs) f waterproof arbo eal frogs approximate air e perature (Ta) up to around 40?C (Shoemaker t al., 1987), but remain constant whe Ta is 141 This content downloaded from 40.77.167.54 on Thu, 06 Oct 2016 04:20:27 UTC All use subject to http://about.jstor.org/terms

Humidity Acclimation and Skin Permeability in the Lizard Anolis carolinensis

Rates of cutaneous evaporative water loss (EWL) were measured in Anolis carolinensis lizards following their exposure to humid (80%-85% RH) or dry (35%-42% RH) environments. After 8 d of acclimation, lizards exposed to dry air had significantly reduced rates of EWL, whereas those exposed to humid air did not. Acclimation of EWL occurred regardless of whether lizards exposed to dry conditions were allowed to dehydrate (to 80% of initial mass) and regardless of whether lizards molted. Evaporation rates measured from isolated dorsal skin in vitro demonstrated that dry-acclimated lizards had a lower skin permeability than did humid-acclimated lizards. The differences in permeability disappeared when lipids were extracted from the skin. Our findings suggest that acclimation of integumentary permeability may result from the gradual deposition of lipids in response to a local stimulus acting at the integument.

Influence of the Pineal Gland and Melatonin on Blood Flow and Evaporative Water Loss During Heat Stress in Rats

Plasma melatonin levels of laboratory rats were elevated both during acute heat exposure (43 degrees C for 40 min) and chronic exposure (33 degrees C for 17 days) suggesting a possible correlation between melatonin and thermoregulatory mechanisms. Pinealectomy reduced the nighttime elevation in oxygen consumption and evaporative water loss. In addition, pinealectomized animals exhibited a significantly lower cutaneous evaporative water loss both at night and during the day when exposed to an acute heat exposure of 38 degrees C for 45 min. Pinealectomy elevated the blood pressure over the control group whereas melatonin infusion depressed the blood pressure without altering the cardiac output. This relationship implies an action by melatonin on the peripheral vasculature. In support of this conclusion, melatonin pretreatment tended to dampen the vasopressive effect of infused norepinephrine. These data, therefore, suggest a role of the pineal gland and melatonin in thermoregulation through an influence on the cardiovascular system and evaporative water loss.

Cutaneous gas exchange and low evaporative water loss in the frogsPhyllomedusa sauvagei andChiromantis xerampelina

1. Pulmonary and cutaneous gas exchange and cutaneous evaporative water loss (EWL) were measured in two frog species,Phyllomedusa sauvagei andChiromantis xerampelina, at 24±1.5°C. These xeric-adapted frogs are noted for their very low EWL. 2. Fractional cutaneous gas exchange varies directly with cutaneous EWL inP. sauvagei. At EWL rates typical of restingP. sauvagei, cutaneous gas exchange is of minor importance. Fractional cutaneous gas exchange is also low in restingC. xerampelina. 3. Fractional gas exchange through the skin in restingP. sauvagei andC. xerampelina is similar to that in terrestrial reptiles but only about 1/12th the fractional CO2 elimination and 1/7th the fractional O2 uptake of typical anuran amphibians. 4. Blood\(P_{{\text{CO}}_{\text{2}} } \), pH and HCO 3 − inP. sauvagei are like those in other anurans with permeable skins. We conclude thatP. sauvagei hyperventilates relative to most other anurans. 5. The reduced cutaneous gas exchange inP. sauvagei andC. xerampelina is the result of a general reduction in skin permeability. As in reptiles, the reduced permeability is of primary importance for maintaining H2O balance while the concomitant loss of cutaneous gas exchange has little functional significance.

Cutaneous Evaporation in Heat-Stressed Spotted Sandgrouse

Many species of desert birds are relatively small (<500 g), diurnally active ground dwellers that are naturally exposed to high ambient temperatures (Ta) and low relative humidities (RH). Sandgrouse (Pteroclididae), being small birds (weighing between 200 and 400 g) having a relatively high standard metabolic rate (SMR), would have to possess an effective mechanism for dissipating heat. Spotted Sandgrouse (Pterocles senegallus), like other species of the Pteroclididae, are successful desert birds (Thomas 1983, Maclean 1984). Thomas and Robin (1977) reported that in incubating female sandgrouse (P. senegallus, P. coronatus, P. alchata) neither gular-fluttering nor gaping were observed in birds exposed in the open to 40 to 50*C Ta. This strongly suggested that body and egg cooling could be achieved only ifa very efficient cutaneous evaporative water loss (CE WL) mechanism existed in in-

Low Cutaneous Evaporative Water Loss in Arboreal Frogs

Rates of cutaneous evaporative water loss were measured in seven arboreal and 11 nonarboreal anuran species under controlled environmental conditions. Areaspecific rates of cutaneous evaporative water loss of arboreal species (genera Hyla and Osteopilus) were 47% lower than those of nonarboreal species (genera Acris, Atelopus, Bufo, Gastrophryne, Rana, Scaphiopus, and Xenopus). Body and skin temperatures of arboreal species averaged 4.4 C higher than those of nonarboreal ones. Boundary layer resistance was similar in both groups of frogs and averaged 0.80 s cm⁻¹; skin resistance averaged 0.05 s cm⁻¹ in nonarboreal frogs and was 41 times higher in arboreal ones. The results of this study conflict with those of previous studies and emphasize the need for careful control of animal posture and water vapor density when measuring evaporative water loss.