Rates Of Evaporative Water Loss Research Articles

A nectar-feeding mammal avoids body fluid disturbances by varying renal function

To maintain water and electrolyte balance, nectar-feeding vertebrates oscillate between two extremes: avoiding overhydration when feeding and preventing dehydration during fasts. Several studies have examined how birds resolve this osmoregulatory dilemma, but no data are available for nectar-feeding mammals. In this article, we 1) estimated the ability of Pallas's long-tongued bats (Glossophaga soricina; Phyllostomidae) to dilute and concentrate urine and 2) examined how water intake affected the processes that these bats use to maintain water balance. Total urine osmolality in water- and salt-loaded bats ranged between 31 +/- 37 mosmol/kgH(2)O (n = 6) and 578 +/- 56 mosmol/kgH(2)O (n = 2), respectively. Fractional water absorption in the gastrointestinal tract was not affected by water intake rate. As a result, water flux, body water turnover, and renal water load all increased with increasing water intake. Despite these relationships, glomerular filtration rate (GFR) was not responsive to water loading. To eliminate excess water, Pallas's long-tongued bats increased water excretion rate by reducing fractional renal water reabsorption. We also found that rates of total evaporative water loss increased with increasing water intake. During their natural daytime fast, mean GFR in Pallas's long-tongued bats was 0.37 ml/h (n = 10). This is approximately 90% lower than the GFR we measured in fed bats. Our findings 1) suggest that Pallas's long-tongued bats do not have an exceptional urine-diluting or -concentrating ability and 2) demonstrate that the bats eliminate excess ingested water by reducing renal water reabsorption and limit urinary water loss during fasting periods by reducing GFR.

The spatial ecology of cane toads (Bufo marinus) in tropical Australia: Why do metamorph toads stay near the water?

Abstract Cane toads (Bufo marinus) have invaded large areas of Australia, killing many native predators as they have done so. The metamorph stage of the life cycle – the first terrestrial phase, immediately after transformation from the tadpole – is critical for ecological impact (because these animals are small enough to be prey for many native predators) and for potential control of toad populations (because small body size renders metamorphs vulnerable to desiccation). To quantify the spatial and temporal distribution of metamorph toads, and the biotic and abiotic factors that might affect their distributions, we surveyed toad breeding sites in Australia's wet‐dry tropics (Adelaide River floodplain, NT) in both the wet season and the dry season. Metamorphs were concentrated close to the water's edge during the dry season, especially at midday when desiccation rates were highest. During the wet season, metamorphs were widely dispersed through the landscape. Our surveys indicate that abiotic factors (risk of desiccation) are most favourable for metamorph toads close to the pond edge, but biotic factors (food supply, and risk of competition and cannibalism) are most favourable away from the water. Operative temperatures were spatially homogeneous and sublethal, and so are unlikely to influence metamorph distribution. Desiccation risk fluctuated on a diel cycle as well as seasonally. We predict that metamorph toads benefit from dispersing as soon as desiccation risk allows them to do so, and hence the distribution of metamorph toads will shift dynamically in response to weather‐mediated changes in rates of evaporative water loss.

Is the strategy for cold hardiness in insects determined by their water balance? A study on two closely related families of beetles: Cerambycidae and Chrysomelidae

The strategy for cold-hardiness and water balance features of two closely related families of Coleoptera, Cerambycidae and Chrysomelidae, were investigated. Cerambycids were freeze-avoiding with low supercooling points, whereas chrysomelids froze at high temperatures and were tolerant to freezing. Hence, the two families have adopted different strategies for cold-hardiness. Due to their low trans-cuticular water permeability, the cerambycids have low rates of evaporative water loss. Chrysomelids have much higher trans-cuticular water permeability, but freezing brings their body fluids in vapour pressure equilibrium with ice and prevents evaporative water loss. The differences in cold-hardiness strategies and rates of water loss are likely to reflect the water content of the diets of the two families. Cerambycids feed on dry wood with low water content, causing a restrictive water balance. Chrysomelids feed on leaves with high water content and may use evaporation through the cuticle as a route of water excretion. Haemolymph ice nucleators help chrysomelids to freeze at a high temperature and thus to maximize the period they spend in the water saving frozen state. The diet-related differences in water balance may be the reason why the two families have developed different strategies for cold-hardiness.

High temperature resistance of the terrestrial red mite (Balaustium sp.) as a product of suppressed heat induced water permeability

An impressive capacity to thrive at temperatures as high as 50°C is reported for adults of the velvet mite, Balaustium sp. near putmani (Acari: Erythraeidae) from Ohio, USA. This mite also has the ability to recover from injury due to heat shock, demonstrating a 2/3 survival rate after treatment at 52°C. Exposure to 54°C was lethal and could not be protected against by conditioning at a sublethal temperature. These mites exhibit a 50-52°C threshold where evaporative water loss rate accelerates sharply, an inflection point that we have designated as the lethal permeability temperature (LPT); mortality is attributed to rapid water loss at temperatures above LPT. When data are plotted on an Arrhenius plot as in the case of critical transition temperature (CTT) however, this inflection point (LPT) disappears, thus implying that this mite resists an increase in cuticular lipid fluidity as the temperature rises.

Role of Substrate Cues in Habitat Selection by Recently Metamorphosed Bufo Terrestris and Scaphiopus Holbrookii

Amphibians exhibit high rates of evaporative water loss that can affect their distribution, movements, and patterns of habitat use. Forest clearcutting alters habitat and results in environmental changes such as canopy removal and leaf litter loss that may promote drier microclimates in harvested clearcuts. Subsequently, clearcutting has been shown to generally reduce amphibian abundances and richness. We investigated the role of substrate cues in habitat differentiation between clearcuts and forests in juvenile Southern Toads (Bufo terrestris) and Eastern Spadefoot Toads (Scaphiopus holbrookii) in laboratory experiments. Neither B. terrestris nor S. holbrookii exhibited a preference for a single substrate when offered the choice between forest soil and clearcut soil. However, S. holbrookii significantly preferred forest substrate over clearcut substrate when forest litter was added to the forest soil. The affinity for forest litter exhibited by juvenile S. holbrookii suggests that the availability of suitable microhabitats may be an important determinant of S. holbrookii distributions and may explain previously reported habitat associations in field studies.

Income breeding allows an aquatic snake Seminatrix pygaea to reproduce normally following prolonged drought‐induced aestivation

1. Capital breeding is an ideal reproductive strategy for many ectotherms because it provides a disassociation between feeding and reproduction, a necessary requirement for animals that become anorexic during pregnancy. Among ectotherms, some viviparous snakes (e.g. Viperidae) exemplify the capital breeding strategy because many species (i) do not feed during pregnancy due to behavioural conflicts between reproduction and foraging, and (ii) take more than one season to accumulate sufficient energetic stores for reproduction. 2. Isolated wetlands often exhibit extreme annual fluctuations in environmental conditions with prolonged droughts periodically leaving wetlands completely dry and devoid of prey. Following droughts, however, wetlands can be extremely productive, rendering prey resources virtually unlimited for some species. 3. This study examines drought survival strategy and reproductive ecology of a small aquatic snake Seminatrix pygaea (Cope) in an isolated wetland. Seminatrix pygaea are atypical from most sympatric snake species in that (i) their small body size, reliance on aquatic prey, and high rates of evaporative water loss make them ill-suited to overland movement, and (ii) they may not be subject to costs typically associated with feeding during pregnancy. 4. We hypothesized that S. pygaea would survive periodic multiyear droughts by aestivating within the dried wetland, a survival strategy heretofore undocumented in snakes. Further, we hypothesized that if S. pygaea rely on 'typical' snake reproductive strategies of 'adaptive anorexia' and capital breeding, reproductive output would be reduced in the first wet year following drought. 5. By encircling a 10-ha wetland with a continuous drift fence before it refilled we were able to demonstrate that S. pygaea were present within the dried wetland prior to the onset of spring rains that refilled the wetland in 2003. Our results suggest that S. pygaea are capable of surviving multiyear droughts by aestivating within the dried wetland. 6. Despite having presumably depleted energy reserves during the drought, S. pygaea reproduced with the same frequency and fecundity during the first season following refilling of the wetland as in pre-drought years. 7. The ability of S. pygaea to rebound rapidly from the stresses of prolonged drought is due in part to their reproductive ecology. Seminatrix pygaea readily feed throughout pregnancy and consequently can rapidly translate high prey abundances into reproductive output through income breeding.

Physiological benefits as precursors of sociality: why banded geckos band

Aggregating has been widely studied in a variety of animals and found to have important benefits in terms of sociality, courtship, predator avoidance and physiology. Several species of nocturnal geckos form diurnal aggregations; however, little is known about the benefits of these groupings. We conducted a series of experiments to determine the benefit of aggregation for the desert-dwelling western banded gecko, Coleonyx variegatus. We found that banded geckos benefit from aggregation by a reduction in evaporative water loss (EWL). No social or mating benefits were detected, and geckos did not group to avoid predators. Geckos did not select diurnal retreat sites based solely on the scent of conspecifics, although they aggregated readily when conspecifics were present. Thus, C. variegatus appear to achieve physiological but not social benefits from grouping. Banded geckos belong to an ancestrally tropical lineage whose descendants invaded present-day North American deserts at a time when these regions were more mesic. This may explain their relatively high rate of EWL. Aggregating seems to be a solution to this physiological handicap. Our study also suggests a path for the evolution of social behaviour: as animals aggregate for physiological benefits, the stage is set for the evolution of more complex social interactions.

Short‐ and long‐term consequences of thermal variation in the larval environment of anurans

1. To survive adverse or unpredictable conditions in the ontogenetic environment, many organisms retain a level of phenotypic plasticity that allows them to meet the challenges of rapidly changing conditions. Larval anurans are widely known for their ability to modify behaviour, morphology and physiological processes during development, making them an ideal model system for studies of environmental effects on phenotypic traits. Although temperature is one of the most important factors influencing the growth, development and metamorphic condition of larval anurans, many studies have failed to include ecologically relevant thermal fluctuations among their treatments. 2. We compared the growth and age at metamorphosis of striped marsh frogs Limnodynastes peronii raised in a diurnally fluctuating thermal regime and a stable regime of the same mean temperature. We then assessed the long-term effects of the larval environment on the morphology and performance of post-metamorphic frogs. 3. Larval L. peronii from the fluctuating treatment were significantly longer throughout development and metamorphosed about 5 days earlier. Frogs from the fluctuating group metamorphosed at a smaller mass and in poorer condition compared with the stable group, and had proportionally shorter legs. 4. Frogs from the fluctuating group showed greater jumping performance at metamorphosis and less degradation in performance during a 10-week dormancy. Treatment differences in performance could not be explained by whole-animal morphological variation, suggesting improved contractile properties of the muscles in the fluctuating group.

Tribute to R. G. Boutilier: Skin colour and body temperature changes in baskingBokermannohyla alvarengai(Bokermann 1956)

In amphibians solar basking far from water sources is relatively uncommon since the highly permeable amphibian skin does not represent a significant barrier to the accompanying risk of losing water by evaporation. A South American frog, Bokermannohyla alvarengai (Bokermann 1956), however, spends a significant amount of the day exposed to full sun and relatively high temperatures. The means by which this frog copes with potentially high rates of evaporative water loss and high body temperatures are unknown. Thus, in this study, skin colour changes, body surface temperature, and evaporative water loss rates were examined under a mixture of field and laboratory conditions to ascertain whether changes in skin reflectivity play an important role in this animal's thermal and hydric balance. Field data demonstrated a tight correlation between the lightness of skin colour and frog temperature, with lighter frogs being captured possessing higher body temperatures. Laboratory experiments supported this relationship, revealing that frogs kept in the dark or at lower temperatures (20 degrees C) had darker skin colours, whereas frogs kept in the light or higher temperatures (30 degrees C) had skin colours of a lighter hue. Light exhibited a stronger influence on skin colour than temperature alone, suggesting that colour change is triggered by the increase in incident solar energy and in anticipation of changes in body temperature. This conclusion is corroborated by the observation that cold, darkly coloured frogs placed in the sun rapidly became lighter in colour during the initial warming up period (over the first 5 min), after which they warmed up more slowly and underwent a further, albeit slower, lightening of skin colour. Surprisingly, despite its natural disposition to bask in the sun, this species does not possess a ;waterproof' skin, since its rates of evaporative water loss were not dissimilar from many hylid species that live in arboreal or semi-aquatic environments. The natural history of B. alvarengai is largely unknown and, therefore, it is likely that the herein reported colour change and basking behaviour represent a complex interaction between thermoregulation and water balance with other ecologically relevant functions, such as crypsis.

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.

Body wiping behaviors associated with cutaneous lipids in hylid tree frogs of Florida

Body wiping behavior, integumentary secretions and rates of evaporative water loss (EWL) were examined in six species of Florida tree frogs (Anura: Hylidae). Additionally, morphology of the integument and dermal glands were compared among these and one other Florida tree frog (Hyla andersonii), an arid-adapted tree frog (Phyllomedusa hypochondrialis), and a highly aquatic frog (Rana utricularia). An extra-epidermal layer of lipid and mucus, presumably secreted from dermal granular glands, was detected on the skin of all Florida hylid frogs examined. Distinct body wiping behaviors were observed in the hylid frogs, but these were less complex than those described previously in phyllomedusine frogs, which occupy arid habitats, secrete lipids onto their skin, and are regarded as relatively 'waterproof'. Florida hylids occupy seasonally arid habitats and appear to have reduced rates of EWL. The suite of traits we observed in these frogs have been previously documented in a rhacophorid tree frog from seasonally arid regions of India and likely represent an evolutionary convergent response to periodic dehydration stress. The presence of lipids that are spread by simple wiping behaviors to form an extra-epidermal water barrier may represent an early stage of the more advanced adaptations described in more waterproof arboreal frogs.

Morphological and Physiological Correlates of Hybrid Parthenogenesis.

Asexuality frequently evolves in association with hybridity and polyploidy. The phenotypic consequences of this association must be understood before we can fully appreciate the reason for the short-term success, and the long-term failure, of asexual genetic systems. Parthenogenetic forms within the Australian gecko complex Heteronotia binoei are triploid hybrids and overlap geographically with both of their parental taxa as well as another sexual lineage. We compared nine morphological and nine physiological traits in wild-caught adult sexual and asexual individuals as well as their captive-bred progeny across a 1,200-km latitudinal gradient. Genome dosage effects were apparent in that the parthenogens were most phenotypically similar to the parental form for which they had a double dosage. Physiological differences between parthenogens and sexuals were often in the opposite direction of that expected under heterosis. Sympatric populations of sexual and parthenogenetic H. binoei differ in ecologically significant phenotypic traits, possibly reducing niche overlap. In keeping with the Red Queen hypothesis, parthenogenetic H. binoei had more ectoparasitic mites than did sexuals in some regions. Lizards with high mite loads also had high rates of evaporative water loss, suggesting a direct link between phenotypic traits and vulnerability to parasites.

MAPPING THE FUNDAMENTAL NICHE: PHYSIOLOGY, CLIMATE, AND THE DISTRIBUTION OF A NOCTURNAL LIZARD

The fundamental niche can be viewed as the set of conditions and resources that allow a given organism to survive and reproduce in the absence of biotic interactions. Quantitative descriptions of the environmental variables with which organisms are asso- ciated are becoming common with the advent of geographic information systems (GIS). Although such descriptive approaches to the niche are useful for interpolating species distributions, they implicitly incorporate biotic interactions and therefore do not represent the fundamental niche. A mechanistic understanding of the fundamental niche, when com- bined with GIS data, can provide us with greater insight into the causes of distribution and abundance, a solid foundation for exploring the role of biotic interactions, and greater confidence in extrapolating to novel circumstances such as climate change and species introductions. We apply such a mechanistic approach to study the climatic component of the fundamental niche of a nocturnal lizard, Heteronotia binoei, across an entire continent. We combine physiological measurements of this species (thermal requirements for egg development, thermal preferences and tolerances, metabolic and evaporative water loss rates), and high-resolution climatic data for the Australian continent (air temperature, cloud cover, wind speed, humidity, and radiation), with biophysical models to calculate the cli- matic component of the fundamental niche of this lizard and map it onto the Australian landscape at high resolution. We also use this approach to predict the effects of a mild global warming on the degree-days in the soil for egg development and the potential for aboveground activity of the study organism.

Cold-hardiness and dehydration resistance of hatchling Blanding's turtles (Emydoidea blandingii): implications for overwintering in a terrestrial habitat

The overwintering habits of hatchling Blanding's turtles, Emydoidea blandingii (Holbrook, 1838), are not well understood. To ascertain whether these turtles are well suited to hibernation on land, we examined susceptibility to dehydration, supercooling capacity, resistance to inoculative freezing, capacity for freeze tolerance, and physiological responses to somatic freezing in laboratory-reared, hatchling E. blandingii. Rates of evaporative water loss (mean ± SE = 4.1 ± 0.2 mg·g–1·d–1) were intermediate to rates previously reported for the hatchlings of species known to hibernate on land and in water. Supercooled hatchlings recovered from a 1-h exposure to –8 °C or a 7-d exposure to –4 °C. Additional turtles supercooled to –14.3 ± 1.2 °C (mean ± SE) before spontaneously freezing. However, when immersed in frozen soil, their capacity to supercool was severely limited by an inability to resist inoculative freezing following contact with external ice and ice nuclei. Therefore, hatchlings likely do not use supercooling as a winter survival strategy. Hatchlings tolerated a 72-h period of somatic freezing to –3.5 °C and responded to somatic freezing by increasing plasma concentrations of the putative cryoprotectants lactate and glucose. Our results suggest that hatchling E. blandingii could overwinter in moist, terrestrial hibernacula where risk of dehydration is reduced and freeze tolerance is promoted.

Aardwolf adaptations: a review

The aardwolf Proteles cristatus has developed a number of anatomical and morphological adaptations for feeding on its termite prey. These adaptations preclude the aardwolf from feeding on other, larger (vertebrate) prey, especially during the winter months, when the Ta drops below 9°C and when Trinervitermes termites are largely unavailable. Winter is therefore a stressful period for aardwolves, as evidenced by increased juvenile mortality, a 20% decline in body mass and depletion of subcutaneous fat reserves. The aardwolf uses both physiological and behavioural means to overcome this period of food stress. An increased period is spent underground in the thermally-stable environment of the den. Den sharing is more common in the winter months and social thermoregulation may result in energy savings. While inactive the aardwolf lowers its Tb (to an average of 34.1±1.6°C during the middle of the inactive period during winter) and in some individuals this results in energy savings of up to 18%. Seasonal changes in pelage density and thermal conductance allow for more passive heat transfer during summer and improved heat retention during winter. Laboratory and field measurements of metabolic rate showed that the aard-wolf has a low BMR and a low FMR (between 40 and 78% of that predicted by allometric equations). The aardwolf's metabolic rate is 11% lower during the winter months, resulting in reduced energy requirements during the period of limited food availability. Evaporative water loss rates are low, further reduced during the winter months, adaptations to survive in a desert environment and on a seasonally-unavailable food source. The aardwolf is therefore well adapted to feed on a diet of Trinervitermes termites, a food niche that has been largely unexploited by other animals, and it uses various ecological, behavioural and physiological adaptations to cope with this seasonally unavailable food source.

Adaptations to terrestrial overwintering of hatchling northern map turtles, Graptemys geographica.

We conducted a 3-year field and laboratory study of winter biology in hatchlings of the northern map turtle ( Graptemys geographica). At our study area in northern Indiana, hatchlings routinely overwintered in their natal nests, emerging after the weather warmed in spring. Winter survival was excellent despite the fact that hatchlings were exposed frequently to subfreezing temperatures (to -5.4 degrees C). In the laboratory, cold-acclimated hatchlings exhibited low rates of evaporative water loss (mean=2.0 mg g(-1) day(-1)), which would enable them to conserve body water during winter. Laboratory-reared hatchlings were intolerant of freezing at -2.5 degrees C for 24 h, conditions that are readily survived by freeze-tolerant species of turtles. Winter survival of hatchling G. geographica probably depended on their extensive capacity for supercooling (to -14.8 degrees C) and their well-developed resistance to inoculative freezing, which may occur when hatchlings contact ice and ice-nucleating agents present in nesting soil. Supercooled hatchlings survived a brief exposure to -8 degrees C. Others, held at -6 degrees C for 5 days, maintained ATP concentrations at control levels, although they did accumulate lactate and glucose, probably in response to tissue hypoxia. Therefore, anoxia tolerance, as evidenced by the viability of hatchlings exposed to N(2) gas for 8 days, may promote survival during exposure to subfreezing temperatures.

Influence of temperature on evaporative water loss and cutaneous resistance to water vapour diffusion in the orange-thighed frog (Litoria xanthomera)

We evaluated the effect of ambient temperatures between 25 and 43°C on the rate of evaporative water loss (EWL) in eight adult Litoria xanthomera (average body mass = 7.3 ± 0.6 g). Frogs were placed in a cylindrical chamber that permitted them to fully conceal their ventral surfaces using a water-conserving posture. Their EWL was 7.1 ± 0.7 mg g–1 h–1 at 25°C and reached 28.0 ± 2.5 mg g–1 h–1 at 43°C. Agar replicas of the frogs were used to evaluate boundary-layer resistances associated with the EWL measurements and, thus, to permit evaluation of cutaneous resistance to vapour diffusion (rc) in live frogs. The rc of L. xanthomera was stable over the temperature range of 25–35°C, averaging about 28 s cm–1, and then declined stepwise with ambient temperatures above 37°C. The highest rc recorded for each individual over the range of temperatures studied averaged 32.0 ± 1.2 s cm–1. The thermolabile nature of rc demonstrates a well developed thermoregulatory control of EWL in this species, a trait very similar in pattern and extent to that previously measured in the closely related Litoria chloris.

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

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 degrees C for periods of 2-5 h. Pregnant females maintained normothermic body temperatures (35.7+/-0.7 degrees C; mean +/- S.E.M.) independent of air temperature. In contrast, males became torpid during the majority (68%) of exposures to air temperatures <25 degrees C. The thermal neutral zone (TNZ) ranged between approximately 30 degrees C and 34 degrees 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.

An Experimental Investigation of Landscape Resistance of Forest versus Old‐Field Habitats to Emigrating Juvenile Amphibians

Abstract: We used an experimental approach to investigate the effects of landscape composition on the initial dispersal success of juvenile amphibians. Larval amphibians—spotted salamander (Ambystoma maculatum), small‐mouthed salamander (A. texanum), and American toad ( Bufo americanus )—were added to artificial pools in four dispersal arrays on forest edges. Each array consisted of a pool surrounded by a circular drift fence with pitfall traps and two 2.5 × 50 m enclosures (runs) extending into forest and old‐field habitat. Juveniles captured at the circular fences were individually marked and released into either field or forest runs. We determined initial distance, initial rate, total distance, and net distance moved by juveniles in the field versus forest from recaptures in the runs. We also conducted 24‐hour dehydration trials to compare the rates of evaporative water loss by spotted and small‐mouthed salamanders in field and forest. Initial orientation of spotted salamanders and toads was significantly biased toward forest. Orientation of small‐mouthed salamanders did not differ significantly from random expectations. The avoidance of open‐canopy habitat by juvenile American toads in particular indicates that predictions of dispersal behavior based on adult habitat use may be misleading. Spotted salamanders moved almost four times farther and toads more than three times farther into the forest than into the field, and recapture rates of both species were much lower in the field. We attribute the lower recapture rates and shorter distances moved in the field to higher mortality due to desiccation or an abundance of predators. Juvenile spotted and small‐mouthed salamanders experienced greater evaporative water loss in the field. Our data on movement behavior and dehydration rates suggest that old‐field habitats offer greater landscape resistance to dispersing juveniles of some species. Thus, forest fragmentation is likely to reduce dispersal rates between local populations of these three species, with potentially negative consequences for population persistence in altered landscapes.

Energetic Bottlenecks and Other Design Constraints in Avian Annual Cycles

The flexible phenotypes of birds and mammals often appear to represent adjustments to alleviate some energetic bottleneck or another. By increasing the size of the organs involved in digestion and assimilation of nutrients (gut and liver), an individual bird can increase its ability to process nutrients, for example to quickly store fuel for onward flight. Similarly, an increase in the exercise organs (pectoral muscles and heart) enables a bird to increase its metabolic power for sustained flight or for thermoregulation. Reflecting the stationary cost of organ maintenance, changes in the size of any part of the "metabolic machinery" will be reflected in Basal Metabolic Rate (BMR) unless changes in metabolic intensity also occur. Energetic bottlenecks appear to be set by the marginal value of organ size increases relative to particular peak requirements (including safety factors). These points are elaborated using the studies on long-distance migrating shorebirds, especially red knots Calidris canutus. Red knots encounter energy expenditure levels similar to experimentally determined ceiling levels of ca. 5 times BMR in other birds and mammals, both during the breeding season on High Arctic tundra (probably mainly a function of costs of thermoregulation) and during winter in temperate coastal wetlands (a function of the high costs of processing mollusks, prey poor in nutrients but rich in shell material and salt water). During migration, red knots phenotypically alternate between a "fueling [life-cycle] stage" and a "flight stage." Fueling red knots in tropical areas may encounter heat load problems whilst still on the ground, but high flight altitudes during migratory flights seem to take care of overheating and unacceptably high rates of evaporative water loss. The allocation principles for the flexible phenotypes of red knots and other birds, the costs of their organ flexibility and the ways in which they "organize" all the fast phenotypic changes, are yet to be discovered.