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During the past 60 years, mammalian hibernation (i.e., seasonal torpor) has been interpreted as a physiological adaptation for energy economy. However, direct field comparisons of energy expenditure and torpor use in hibernating and active free-ranging animals are scarce. Here, we followed the complete hibernation cycle of a fat-storing hibernator, the marsupial

In Chile, the manipulation for scientific purposes of terrestrial vertebrates from natural populations is conducted previous authorization of the Agricultural and Livestock Service and various Bioethics Committees. Obtaining such authorizations is becoming increasingly complex. The procedures do not fit the reality of scientific work, and they seem to be based on unjustified assessments of the effects of animal handling on natural populations. The aim of this commentary is to initiate a discussion in order to establish a norm of scientific manipulation of terrestrial vertebrates adjusted to biological reality and that does not interfere with scientific research.

Environmental conditions experienced by developing animals have an impact on the development and maturity of the immune system. Specifically, the diet experienced during early development influences the maintenance and function of the immune system in young and adult animals. It is well known that exposure to low-protein diets during early development are related to an attenuation of immunocompetence in adulthood. While this functional linkage has been widely studied in altricial models' mammals, it has been little explored how the nutritional history modulates the immune function in precocial animals. We evaluated the effect of dietary protein consumed during early development on the immune function and the oxidative costs in the precocial Caviomorph rodent Octodon degus, or degu. We evaluated components of the acute phase response (APR) and oxidative parameters before and after immune challenge. We found that after the immune challenge, the juveniles on the low-protein dietary treatment exhibited an attenuation of body temperature but showed higher levels of lipid peroxidation than juvenile degus on the high-protein diet. We did not find a significant effect of the interaction between diet and immune challenge on body mass, levels of inflammatory proteins, nor in the total antioxidant capacity. Our results suggest that some components of the immune function and the oxidative status in the degu can be modulated by diet during development. However, the modulation would depend on the immune variables analyzed, and the characteristics of the immune system of precocial rodents.

Evidence is growing on the ability of anurans to make immediate call adjustments in response to the propagation properties of the environment. However, our understanding of such flexibility is typically based on dichotomous stimuli (i.e. presence/absence), while its condition-dependence has been little explored. We experimentally studied the ability of adjustment and its condition dependence of advertisement calls of Hypsiboas pulchellus in response to different levels of attenuation. Individuals modified most call parameters analyzed, although the direction of adjustments was contingent on individual identity, level of attenuation and trait category (i.e. spectral or temporal). Under low attenuation conditions males lengthened some call elements, partially supporting the hypothesis of plasticity oriented towards maintaining signal transmission, but this trend was not present in the high attenuation treatment. Also, under both attenuation treatments males gave higher frequency notes, as expected from an energy-saving response to changes in social environment or reducing attractiveness due to risk perception. Across treatments individuals in better condition tended to exhibit higher plasticity. These results indicate that call structure could depend on the integration of different cues such as habitat propagation characteristics, acoustic competition for females, and predation risk, while the ability to behave flexibly depends on current body reserves.

Phenotypic plasticity is defined as the ability of an organism to produce different phenotypes in response to changes in internal or external environmental conditions. Experimental modification of animal diets has been widely used to study phenotypic plasticity in physiological and life-history traits. Here we compared the data from a previous experiment, conducted in a harvestman species, that was aimed to evaluate the effect of diet quality on maintenance costs (standard metabolic rate), growth rate, internal organ size (visceral mass) and fecundity (clutch mass and egg number), with new data obtained from field (mark—recapture) specimens. We assumed that of the two experimental diets tested before, animals in the field would probably consume the one of intermediate quality (i.e. have a generalist omnivorous diet), and then, we predicted that field animals should exhibit intermediate values for both physiological and life-history traits. We found that field animals and animals consuming a high quality diet showed a greater growth rate than animals consuming a poor quality diet. In addition, animals consuming a high quality diet showed a larger clutch mass than both field animals and animals consuming a poor quality diet, which, in turn, was related to higher maintenance costs. Our results illustrate how animals adopt different life history strategies according to the quality of the diet that is available, which is correlated with phenotypic adjustments at the anatomical and physiological levels.

We described and compared the diets of two sympatric Andean camelids, during the humid season (austral summer) in a site of Northern Chile, in presence of domestic livestock. Results indicate that: 1) grasses and shrubs are the main component in the diet of both camelids, 2) shrubs were more consumed by V. vicugna ; 3) V. vicugna and L. guanicoe used the same trophic resources but in different proportions; 4) in mountain environments, wetlands exploitation by wild camelids seems restricted by domestic cattle, which would cause the displacement of Vicunas and Guanacos to suboptimal habitat for feeding. K EYWORDS : Guanaco, Vicuna, livestock, diet, herbivory. RESUMEN Describimos y comparamos la dieta de dos poblaciones simpatricas de camelidos silvestres, durante la estacion humeda (verano) en un sector del norte de Chile en presencia de ganado domesticado. Nuestros resultados indican que: 1) gramineas y arbustos son el principal componente de la dieta de ambos camelidos; 2) arbustos fueron consumidos principalmente por

Endotherms maintain high and constant body temperatures through the production and maintenance of metabolic heat. Defining the evolutionary history of these thermal adaptations and the selective factors responsible for the evolution of endothermy despite its high metabolic costs have been elusive and controversial topics in evolutionary biology. In this sense, several models have been proposed to explain the evolution of endothermy. Among them, the parental care model explains the increase in resting metabolic rate (RMR) by the action of natural selection favoring parental care. Thus, a positive relationship between parental care behavior and RMR is predicted. However, there appears to be no or little previous work experimentally testing this relationship. In the study presented here, RMR was increased through l-tyrosine injections and parental care behavior was measured. This treatment allowed us to test the relationship between RMR level and parental care behavior in a dwarf hamster. It was found that increased RMR enhanced male parental care. Specifically, male latency time, or the time until contacting and picking up their pups, decreased when RMR increased. This study demonstrates the positive relationship between RMR and the allocation of resources to parental care. This study supports the main assumption of Kotejas's parental care model and accepts Koteja's proposed explanation for the evolution of endothermy as a plausible hypothesis.

We studied the putative effect of early life experience on the physiological flexibility of metabolic and osmoregulatory traits in the leaf-eared mouse, Phyllotis darwini, an altricial rodent inhabiting seasonal Mediterranean environments. Adult individuals were collected in central Chile and maintained in breeding pairs. Pups were isolated after weaning and acclimated to different temperatures (cold or warm) and water availability (unrestricted and restricted) until adulthood. Subsequently, individuals were re-acclimated to the opposite treatment. Rodents reared in the warm and subjected to water restriction had lower basal metabolic rate (BMR), total evaporative water loss (TEWL) and body mass (Mb) compared with those developing in the cold treatment; nevertheless, individuals subjected to warm temperatures had greater relative medullary thickness (RMT) and urine concentrating ability (UCA). Cold-reared rodents re-acclimated to warm conditions exhibited physiological flexibility of metabolic traits; however, their osmoregulatory attributes did not vary. Conversely, warm-reared rodents re-acclimated to cold had reduced RMT and UCA, but the metabolic traits of these individuals did not change. These results suggest a trade-off between metabolic performance and renal capabilities that might hinder physiological acclimation. Our results support the hypothesis of ontogenetic dependence of short-term acclimation in osmoregulatory and metabolic traits in P. darwini. Fil: Cavieres Parada, Grisel Beatriz. Universidad Católica de Chile; Chile. Pontificia Universidad Católica de Chile; Chile. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Provincia de Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Universidad Nacional de Cuyo. Instituto Argentino de Investigaciones de las Zonas Áridas; Argentina Fil: Nuñez Villegas, Monica. Universidad de Chile; Chile Fil: Bozinovic, Francisco. Universidad Católica de Chile; Chile Fil: Sabatino, Pablo. Universidad de Chile; Chile

Defense against natural aggressors, such as bacterial infections, requires both energy and an immune-cellular response. However, the question as to how these two components are interconnected in small endotherms by means of the host diet remains only poorly understood. Here, we tested in laboratory mice whether dietary proteins and carbohydrates can modulate the interplay between energy expenditure, food intake and the innate and adaptive immune response when confronting a bacterial challenge (Bacillus Calmette-Guérin, BCG). We observed that mice fed with a high protein diet (HP) developed a better immune response associated to increased numbers of circulating monocytes. In addition, HP diet directly influenced the peripheral blood proportions of both T and B lymphocytes even before the BCG challenge. Interestingly, animals that developed this type of immune response after BCG challenge showed an increased rate of metabolism and food consumption before being challenged. Thus, HP diet induced in non-challenged animals a similar energy expenditure and food intake described by BCG-treated mice. These data suggest that a high amount of proteins in diet can modify the energetic and nutrient dynamic in the host causing a better immune reaction against a microbial challenge.

Due to the higher energy requirements of birds during winter, it is predicted that the activities of metabolic enzymes (e.g., citrate synthase, CS and cytochrome C oxidase, COX) should increase in tandem with increases in rates of energy expenditure (e.g., basal metabolic rate, BMR). However, there is mixed support for the hypothesis of enzymatic acclimatization. Furthermore, there is little information about the effect of ambient temperature on energetics and tissue enzyme activity levels in passerines inhabiting seasonal Mediterranean environments. In this study we evaluated the interplay between BMR and enzyme activities of freshly caught individuals of the passerine Zonotrichia capensis in winter and summer in a Mediterranean environment from central Chile, and also in warm (30°C) and cold (15°C) lab-acclimated birds. The results revealed a lack of seasonal variation in BMR, thermal conductance and in the activity of CS and COX. However, we found higher BMR and lower thermal conductance in cold-acclimated than in warm-acclimated birds. Also, total CS activity was higher in the flight muscles of cold-acclimated than in warm-acclimated birds. We found also a significant correlation between BMR and total CS activity in pectoral muscle. Although some authors have suggested that BMR mainly depends on the metabolic intensity and mass of internal organs, our results revealed that skeletal muscles such as the flight muscles may also affect BMR. Finally, differences in the acclimation and acclimatization responses in Rufous-collared sparrows suggest that large-scale oscillations in the physical environment might maintain the capacity for flexibility in thermogenic traits through generations.

Climate change poses one of the greatest threats to biodiversity. Most analyses of the impacts have focused on changes in mean temperature, but increasing variance will also impact organisms and populations. We assessed the combined effects of the mean and the variance of temperature on thermal tolerances-i.e., critical thermal maxima, critical thermal minima, scope of thermal tolerance, and survival in Drosophila melanogaster. Our six experimental climatic scenarios were: constant mean with zero variance or constant variance or increasing variance; changing mean with zero variance or constant variance or increasing variance. Our key result was that environments with changing thermal variance reduce the scope of thermal tolerance and survival. Heat tolerance seems to be conserved, but cold tolerance decreases significantly with mean low as well as changing environmental temperatures. Flies acclimated to scenarios of changing variance-with either constant or changing mean temperatures-exhibited significantly lower survival rate. Our results imply that changing and constant variances would be just as important in future scenarios of climate change under greenhouse warming as increases in mean annual temperature. To develop more realistic predictions about the biological impacts of climate change, such interactions between the mean and variance of environmental temperature should be considered.

We analyzed the carbon and nitrogen isotopic values of the muscle, liver, and crop contents ("diet") of 132 individuals of 16 species of Chilean birds. The nitrogen content of diet was tightly correlated with the fraction of gut contents represented by insects relative to plant material. The carbon and nitrogen isotopic values of diet, liver, and muscle were all linearly correlated, implying high temporal consistency in the isotopic value of the diet of these birds. However, δ(15)N was not significantly related with the percentage of insects in diet. These results cast doubt on the applicability of the use of (15)N enrichment to diagnose trophic level in, at least some, terrestrial ecosystems. However, the residuals of the relationship relating the isotopic value of bird tissues with those of their diet were weakly negatively correlated with insect intake. We hypothesize that this negative correlation stems from the higher quality of protein found in insects relative to that of plant materials. Finally, our data corroborated a perplexing and controversial negative relationship between tissue to diet isotopic discrimination and the isotopic value of diet. We suggest that this relationship is an example of the commonly observed regression to the mean effect that plagues many scientific studies.

Global climate change poses one of the greatest threats to biodiversity. Most analyses of the potential biological impacts have focused on changes in mean temperature, but changes in thermal variance will also impact organisms and populations. We assessed the combined effects of the mean and variance of temperature on thermal tolerances, organismal survival, and population growth in Drosophila melanogaster. Because the performance of ectotherms relates nonlinearly to temperature, we predicted that responses to thermal variation (±0° or ±5°C) would depend on the mean temperature (17° or 24°C). Consistent with our prediction, thermal variation enhanced the rate of population growth (r(max)) at a low mean temperature but depressed this rate at a high mean temperature. The interactive effect on fitness occurred despite the fact that flies improved their heat and cold tolerances through acclimation to thermal conditions. Flies exposed to a high mean and a high variance of temperature recovered from heat coma faster and survived heat exposure better than did flies that developed at other conditions. Relatively high survival following heat exposure was associated with low survival following cold exposure. Recovery from chill coma was affected primarily by the mean temperature; flies acclimated to a low mean temperature recovered much faster than did flies acclimated to a high mean temperature. To develop more realistic predictions about the biological impacts of climate change, one must consider the interactions between the mean environmental temperature and the variance of environmental temperature.

A common assumption in behavioral ecology is that the valuation of a resource by consumers depends on the energetic value of the resource itself. Nevertheless, the value of a resource may be relative to the condition of the organism, which is in turn related to the abiotic conditions such as ambient temperature. We developed a theoretical model—incorporating these untested assumptions—to predict a functional relationship between territorial aggression and ambient temperature for individuals sensitive to daily variations in energy availability. We evaluated our theoretical predictions against a field experiment carried out with the hummingbird Sephanoides sephaniodes. The model predicted a quadratic relation between aggression intensity and ambient temperature. Field data were better explained by a quadratic equation than a linear function, suggesting the existence of lower and upper thresholds of temperature which determine the intensity of territorial defense. Ambient temperature affects energy expenditure for thermoregulation, and therefore, it fixes the benefit level that must be produced by the territory to pay the costs of its defense. Our findings strongly suggest that abiotic conditions can change an animal evaluation of the yield of a resource and in turn influence the behavioral strategy which it adopts.

Hummingbirds (family Trochilidae) represent an extreme outcome in vertebrate physiological design and are the only birds capable of sustained hovering. The giant hummingbird (Patagona gigas) is the largest trochilid, with a mass of ~20 g, and is found over an altitudinal range from 0 to 4,500 m above sea level. We report here measurements of daily, basal, and hovering rates of oxygen consumption in the giant hummingbird; compare these values with data from smaller hummingbirds; and assess overall metabolic and allometric limits to trochilid body size. The sustained metabolic scope (i.e., the ratio of daily energy expenditure to basal metabolic rate) in the giant hummingbird is higher than that in smaller hummingbirds but lies below a proposed theoretical maximum value for endotherms. Scaling exponents in the allometric relationships for different modes of energetic expenditure were comparable, suggesting that the giant hummingbird, although a clear outlier in terms of body size, does not obviously deviate from metabolic relationships derived from other trochilid taxa.

Biogeographic analyses of passerine birds demonstrate that suboscines are numerically dominant in South America, whereas oscines are dominant elsewhere. This suggests that oscines generally outcompete suboscines and that suboscine dominance likely persists in South America because of its long isolation from other continents, where oscines have diversified. One hypothesis for oscine competitive superiority is that oscines possess higher metabolic capacities than suboscines, and this favors oscines in most habitats. We tested this hypothesis by comparing summit metabolic rates (M(sum), maximum thermoregulatory metabolic rate) between oscines and suboscines using conventional and phylogenetically informed statistical approaches. We predicted that if the metabolic-capacity hypothesis is valid, then oscines should have higher M(sum) than suboscines. Both conventional and phylogenetically informed ANCOVA on regressions of log M(sum) against log mass showed that oscines had higher M(sum) than suboscines: least squares mean M(sum) was 74% greater for oscines. Moreover, conventional and phylogenetically informed multiple regressions identified log mass, winter-range temperature, and clade (oscines vs. suboscines) as significant effectors of log M(sum). Thus, oscines have generally higher M(sum) than suboscines, which is consistent with the metabolic-capacity hypothesis and suggests that metabolic capacity is one factor influencing the evolution of broad biogeographical patterns in passerines.

We assessed how individual foraging preferences and cognitive performance affect foraging bout interval and the flexibility of foraging behavior in the nectarivorous Green-backed Firecrown (Sephanoides sephaniodes). Our field experiment evaluated the ability of these hummingbirds to recall nectar-renewal rates in two groups of artificial flowers with the same nectar concentration in the absence of visual cues. In a second experiment, we assessed their ability to remember differences in nectar quality combined with different nectar-renewal intervals, given artificial flowers with identical visual cues. Our results indicate that Green-backed Firecrowns adjusted their foraging intervals according to nectar-renewal rates and, furthermore, that birds were able to recall nectar concentration as well as nectar-renewal rate. Individual differences in memory performance resulted in differences in energy intake. These results strongly suggest that individual preferences and individual cognitive performanc...

A population of black-necked swans (Cygnus melanocoryphus) residing in a perturbed habitat revealed a low body mass, malnutrition, and hyperferremia during 2005; the swans main dietary item, Egeria densa, was lost during an environmental crisis which occurred in 2004. The objective of this study was to monitor the diet and nutritional status of this population during 2006, as well as to verify how the consumption of sediment, as part of their new diet, may explain the mineral disorders observed in these birds. Results revealed that swans increased their body mass and had an adequate protein, lipid, and iron metabolism, in spite of the fact that they maintained the same new diet (sediment and roots) during 2005-2006. In addition, transferrine saturation was indicative of the high endogenous iron load in birds which agrees with the high iron load of their environment. On the other hand, the consumption of the Cayumapu River sediment in the diet (25%) did not affect the body mass nor the nutritional and hepatic function in domestic geese over a 45-day period.

The Cayumapu River's black-necked swan population in southern Chile lost its main dietary item, Egeria densa, during an environmental crisis which occurred in 2004 in the Carlos Andwanter Nature Sanctuary. The main goal of this study was to test the effect of diet on the physiologic response to this new ecologic challenge. The results revealed that the new diet of this population was composed primarily of roots and sedimentary microalgae, with chemical and energetic content similar to the diet of the control population. Nevertheless, the mean body mass of the Cayumapu River swans was 25% lower than that of control birds. In addition, the biochemical and hematologic profiles of the study population were indicative of malnutrition and a hyperferremic, hyperphosphatemic, and lymphopenic condition. Liver enzyme activities did not support that the malnutrition was a secondary consequence of liver dysfunction, as is expected under hemochromatosis or environmental toxics exposure.

A major goal of evolutionary physiology is to understand the intrinsic and the extrinsic factors that impose limitations on an animal's energy budget. Although natural selection acts upon organismal traits such as performance (e.g., burst, sustained metabolic rates), from a mechanistic perspective, organismal performance results from the integrated functioning of different levels of biological organization. Hence, a better understanding of whole-animal performance must necessarily incorporate an explicit analysis of the integration between those different levels. Although this topic has been under intense scrutiny, overall there have been very few consistent patterns. Here, we explore the phenotypic integration between organ masses and the overall energy budget under routine capacities by statistically decomposing the covariance matrix (using path analysis and canonical correlation analysis) between organ masses and thermoregulatory burst and sustained metabolisms in cold acclimated individuals of Phyllotis darwini. Our results suggest that (a) central organs associated with the processing of food (cecum and liver), residuals (kidneys) and pumping of O(2) (heart) are tightly integrated to sustained expenditure and between themselves; (b) with the exception of the heart, central energy supplying organs are weakly related to burst expenditures; (c) sustained and burst metabolisms refer to complete different strategies and (d) basal metabolic rate is not related to any of the physiological or morphological traits considered in this study. Overall, our results support the hypothesis of an economic phenotype: animals maintain their excess capacities to face those critical extreme events, but their physiology and internal morphology are tightly integrated to function under routine needs.

Developing mechanistic models to predict an organism's body temperature facilitates the study of physiological stresses caused by extreme climatic conditions the species might have faced in the past or making predictions about changes to come in the near future. Because the models combine empirical observation of different climatic variables with essential morphological attributes of the species, it is possible to examine specific aspects of predicted climatic changes. Here, we develop a model for the competitively dominant intertidal mussel Perumytilus purpuratus that estimates body temperature on the basis of meteorological and tidal data with an average difference (+/-SE) of 0.410 degrees +/- 0.0315 degrees C in comparison with a field-deployed temperature logger. Modeled body temperatures of P. purpuratus in central Chile regularly exceeded 30 degrees C in summer months, and values as high as 38 degrees C were found. These results suggest that the temperatures reached by mussels in the intertidal zone in central Chile are not sufficiently high to induce significant mortality on adults of this species; however, because body temperatures40 degrees C can be lethal for this species, sublethal effects on physiological performance warrant further investigation. Body temperatures of mussels increased sigmoidally with increasing tidal height. Body temperatures of individuals from approximately 70% of the tidal range leveled off and did not increase any further with increasing tidal height. Finally, body size played an important role in determining body temperature. A hypothetical 5-cm-long mussel (only 1 cm longer than mussels found in nature) did reach potentially lethal body temperatures, suggesting that the biophysical environment may play a role in limiting the size of this small species.

We studied the photopic spectral sensitivity in the green-backed firecrown, Sephanoides sephaniodes, a South American hummingbird, and its possible ecological relationship with preferred flowers and body colouration. Avian colour vision is in general tetrachromatic with at least four types of cones, which vary in sensitivity from the near ultraviolet (UV) to the red wavelength range. Hummingbirds represent an important family of birds, yet little is known about their eye sensitivity, especially about the role of photoreceptors and their oil droplet complements. The photopic electroretinogram shows a main sensitivity peak at 560 nm and a secondary peak in the UV, and may be explained by the presence of four single cones (λ max at ~370, 440, 508 and 560 nm), and a double cone (λ max at 560 nm) screened by oil droplets. The flowers preferred by the firecrown are those in which the red–green wavelength region predominates and have higher contrast than other flowers. The crown plumage of males is highly iridescent in the red wavelength range (peak at 650 nm) and UV; when plotted in a high-dimensional tetrachromatic space, it falls in a “red + UV” purple hue line, suggesting a potential significant communication signal for sexual differentiation.

Ecological physiology examines how animals cope with changing environmental demands. In low-productivity desert habitats, small mammals should consume low-quality, high-fiber food as a consequence of necessity rather than by choice. Diet quality of populations can differ at both spatial and temporal scales. Nevertheless, spatial and temporal variation in the digestive system has not been extensively studied in the field. We captured individuals from 4 populations of Microcavia australis and measured their digestive morphological traits. Fieldwork was carried out in 4 localities belonging to arid and semiarid regions, in dry and wet seasons. We also estimated diet quality for each population and season. We found significant effects of sex, season, and site on the size of digestive organs. The concentration of fiber and nitrogen in the plants consumed differed between populations and varied seasonally: dietary fiber was higher in the dry season and nitrogen concentration was higher in the wet season. As predicted by theory, the cecum, the organ most closely related to cellulose fermentation, was significantly larger in animals facing the lowest quality diet. The other organs also were affected by reproductive state and water requirements. Intraspecific variation in the digestive morphology of M. australis probably helps this species cope with remarkable seasonal and geographical variability.

SUMMARYThe transition from trot to gallop in quadruped mammals has been widely hypothesized to be a strategy to minimize the energetic costs of running. This view, however, has been challenged by some experimental evidence suggesting instead that this transition might be triggered by mechanical cues, and would occur when musculoskeletal stresses reach a certain critical value. All previous experiments to test those hypotheses have used relatively large species and their results, therefore, may not be applicable to small mammals. In this study we evaluated the effect of carrying loads on the locomotor energetics and gait transitions of the rodent Octodon degus running on a treadmill. Metabolic rate and cost of transport increased about 30% with a 20% increment in body mass. This increment was higher than expectations based on other mammals, where energy consumption increases in proportion to the added mass, but similar to the response of humans to loads. No abrupt change of energy consumption between gaits was observed and therefore no evidence was found to support the energetic hypothesis. The trot–gallop transition speed did not vary when subjects were experimentally loaded,suggesting that the forces applied to the musculoskeletal system do not trigger gait transition.

Through functional analyses, integrative physiology is able to link molecular biology with ecology as well as evolutionary biology and is thereby expected to provide access to the evolution of molecular, cellular, and organismic functions; the genetic basis of adaptability; and the shaping of ecological patterns. This paper compiles several exemplary studies of thermal physiology and ecology, carried out at various levels of biological organization from single genes (proteins) to ecosystems. In each of those examples, trade-offs and constraints in thermal adaptation are addressed; these trade-offs and constraints may limit species' distribution and define their level of fitness. For a more comprehensive understanding, the paper sets out to elaborate the functional and conceptual connections among these independent studies and the various organizational levels addressed. This effort illustrates the need for an overarching concept of thermal adaptation that encompasses molecular, organellar, cellular, and whole-organism information as well as the mechanistic links between fitness, ecological success, and organismal physiology. For this data, the hypothesis of oxygen- and capacity-limited thermal tolerance in animals provides such a conceptual framework and allows interpreting the mechanisms of thermal limitation of animals as relevant at the ecological level. While, ideally, evolutionary studies over multiple generations, illustrated by an example study in bacteria, are necessary to test the validity of such complex concepts and underlying hypotheses, animal physiology frequently is constrained to functional studies within one generation. Comparisons of populations in a latitudinal cline, closely related species from different climates, and ontogenetic stages from riverine clines illustrate how evolutionary information can still be gained. An understanding of temperature-dependent shifts in energy turnover, associated with adjustments in aerobic scope and performance, will result. This understanding builds on a mechanistic analysis of the width and location of thermal windows on the temperature scale and also on study of the functional properties of relevant proteins and associated gene expression mechanisms.

We study whether and how physiological demands affect foraging decisions under predation risk, by evaluating the effect of starvation on the rate of food consumption and prey-size preferences and the potential trade-off between starvation and predation risk on foraging behavior in the whelk Acanthina monodon, a gastropod inhabiting the intertidal rocky shores of central Chile. These whelks appear to adjust their foraging strategy to physiological (nutritional) demand and predation risk. Starvation reduced the effect of predation risk on the rate of food consumption by A. monodon. Thus, in the absence of the predator sea star, the rate of food consumption by starved and satiated whelks was similar. When a predator was present, starved whelks fed faster than satiated whelks. Our results indicate that foraging behaviour represents an integrated and hierarchical response to environmental conditions and the physiological conditions of the forager.

We examined the energetics of the living fossil microbiotheriid Dromiciops gliroides, a nocturnal and rare small marsupial, endemic to the northern portion of the temperate forest of southern South America. We investigated the effects of changes at ambient temperature and food restriction on the energetics and patterns of torpor. We determined whether they exhibit shallow daily torpor or deep prolonged torpor like some Australian marsupials. Thermal conductance was 92.5% of the expected value for a similarly sized eutherian and basal metabolic rate was 82.9 and 58.6% of the predicted value for standard metatherians and eutherians, respectively. Euthermic D. gliroides showed daily fluctuations in body temperature, being significantly higher during the night. Dromiciops gliroides entered torpor and aroused spontaneously. The duration of torpor bouts increased in response to decreasing ambient temperature; torpor bout duration ranged from 10 h at 20 degrees C to 120 h at 12.5 degrees C. This study is the first record of deep torpor or hibernation for a South American mammal. Torpor in this species as well as in marsupials in general appears to be an opportunistic response to unpredictable biotic and abiotic conditions.

Small day-active endotherms living in hot environments maintain heat balance and avoid overheating by behavioral adjustments of their time budgets. We examined the temporal and spatial microstructure of surface activity bouts in the degu (Octodon degus) under maximal, summertime environmental heat load in a thermally complex habitat mosaic of sun and shade. The degu is the only native small mammal regularly active during the day in central Chile. We recorded thermal environmental data and activity of individuals with video cameras to quantify time spent in open sun as compared to shade. We predicted that individuals would adjust activity ‘‘microbouts’’ (successive momentary movements in and out of shade cover) in response to increasing heat load. As heat load increased toward end of morning, the proportion of total surface time spent in shade increased and approached 100%. Likewise, direct exposures to solar radiation decreased from almost 8 min in early morning to less than 1 min by late morning. Field measurements of body temperature remained within a 24-h range of 36– 388C (mean 36.78C), suggesting an intolerance of increases above this range. The low heat tolerance of the degu seems to be a physiological limitation associated with its nocturnal family lineage. Because daytime activity is exceptional within this lineage, the degu has used behavioral adjustments to become the only regularly day-active native small mammal in its environment.

SUMMARYBody size is one of the most important determinants of energy metabolism in mammals. However, the usual physiological variables measured to characterize energy metabolism and heat dissipation in endotherms are strongly affected by thermal acclimation, and are also correlated among themselves. In addition to choosing the appropriate measurement of body size, these problems create additional complications when analyzing the relationships among physiological variables such as basal metabolism, non-shivering thermogenesis,thermoregulatory maximum metabolic rate and minimum thermal conductance, body size dependence, and the effect of thermal acclimation on them.We measured these variables in Phyllotis darwini, a murid rodent from central Chile, under conditions of warm and cold acclimation. In addition to standard statistical analyses to determine the effect of thermal acclimation on each variable and the body-mass-controlled correlation among them, we performed a Structural Equation Modeling analysis to evaluate the effects of three different measurements of body size (body mass, mb; body length, Lb and foot length, Lf) on energy metabolism and thermal conductance. We found that thermal acclimation changed the correlation among physiological variables. Only cold-acclimated animals supported our a priori path models, and mb appeared to be the best descriptor of body size (compared with Lb and Lf) when dealing with energy metabolism and thermal conductance. However, while mb appeared to be the strongest determinant of energy metabolism, there was an important and significant contribution of Lb (but not Lf) to thermal conductance. This study demonstrates how additional information can be drawn from physiological ecology and general organismal studies by applying Structural Equation Modeling when multiple variables are measured in the same individuals.

We analyzed the influence of environmental temperature (open versus shaded habitat) and experimental food availability on surface activity of the degu (Octodon degus), a day-active herbivorous rodent that inhabits central Chile. We manipulated food availability and compared open and shaded study plots to determine the influence of thermal conditions on aboveground activity. Degus displayed a bimodal pattern of daily activity during relatively warmer thermal conditions in the austral autumn, whereas activity was unimodal during colder conditions in winter. However, they had a unimodal activity pattern under warm conditions in the shade when food availability was artificially enhanced in autumn. We observed more animals active in the plots where food was supplemented under all conditions. Our results illustrate spatial and temporal shifts in activity of degus involving a trade-off based on avoidance of exposure to heat and the search for food.

In response to the interplay between variation in food quality and energetic demands, the foraging behaviour of captive green-backed firecrown hummingbirds Sephanoides sephaniodes was studied. Hummingbirds were exposed to two temperatures (25°C vs 15°C), two food qualities (0.5 vs 0.75 m sucrose solutions), and two costs of feeding (birds were provided with feeders with and without a perch). Food selection and consumption were measured, as well as time budgets and metabolic rate while feeding. We predicted that when given a choice, birds would minimize the cost of feeding by selecting feeders with a perch and with a high sugar concentration. However, rather than increasing energy consumption when energy availability was low and thermoregulatory demands were high, hummingbirds remained perched. They reduced feeding and spent most of their time perching. Our results identify a novel behavioural and physiological strategy in hummingbirds. These birds seem to shift their foraging behaviour depending on thermoregulatory and feeding costs. When these costs are high, rather than matching them with increased energy consumption, hummingbirds reduce energy costs by reducing activity. They seemed to adopt the following strategy: when food quality was high and thermoregulatory demands were low, they adopted a high-expense lifestyle. In contrast, when thermoregulatory costs were high, they adopted an energy conserving strategy even when food quality was high. We hypothesize that limitations imposed by physiological processes may explain why animals do not forage during all available time and why under some circumstances they choose foraging behaviours with lower rates of net energy gain.

We studied effects of habitat structure on routine travel velocities, intermittent locomotion, and vigilance by the degu (Octodon degus), a diurnal rodent of central Chile. We predicted that travel speed, pauses during locomotion, and vigilance would be greater in open (riskier) than in shrub (safer) habitats. Video recordings of marked individuals in the wild were used to measure speed and other variables of spontaneous locomotion not triggered by predatory attack or any other noticeable stimulus during nonforaging periods. Time spent vigilant while foraging was also measured. Because degus use bare-ground runways for distant movements (e.g., between burrow openings and/or food patches), data on locomotion decisions were not confounded by effects of obstructive vegetation cover and/or resource abundance. When moving across the habitat between different feeding places, degus showed an intermittent pattern of locomotion, interrupting running events with short pauses. As predicted, travel speed and the duration of pauses between locomotion bursts were significantly greater in open habitats. Further, the duration of locomotion bursts between feeding sites or between feeding sites and burrows was significantly longer in open habitats. Our assumption that pauses and velocities are independent decisions was supported by the lack of correlation between pauses and speeds during locomotion events. During foraging, degus devoted more time to vigilance in open than in shrub habitats. The static position adopted by degus during pauses, the speeds attained during movements, and the concordance between pausing behavior and vigilance across habitats suggest that pausing has an antipredatory role and is not limited to orientation and/or physiological recovery. Our results support the view that degus perceive higher predation risk in open areas and that flexible movement behavior reflects an adaptive antipredator response. Copyright 2002.

Summary 1 We studied a fluctuating population of the northern short-tailed shrew (Blarina brevicauda) in the Appalachian Plateau Province of Pennsylvania, USA, spanning 21 years of monitoring. We analysed the pattern of annual temporal variation fitting both time-series models and capture–mark–recapture (CMR) statistical models for survival and recruitment rates. 2 We determined that non-linear first-order models explain almost 80% of the variation in annual per capita population growth rates. In particular, a non-linear self-excited threshold autoregressive (SETAR) model describes the time-series data well. Average snowfall showed positive and non-linear effects on population dynamics. 3 The CMR statistical models showed that a non-linear threshold model with strong effects of population density was the best one to describe temporal variation in survival rates. On the other hand, population density or climatic variables did not explain temporal variation in recruitment rates. Survival rates were high during the study period. Weekly changes in population size attributable to new recruits entering in the population fluctuate between 21% and 0%, while the changes in population size related to survival fluctuate between 79% and 100%. 4 Two important results arise from this study. First, non-linear models with first-order feedback appear to capture the essential features of northern short-tailed shrew dynamics and demography. Secondly, climate effects represented by snowfall appear to be small and non-linear on this insectivore. The population dynamics of this shrew in the Appalachian Plateau are determined apparently by a strong non-linear first-order feedback process, which is related to survival rates. 5 This study links population dynamics and demography by detecting the underlying demographic mechanisms driving population dynamics. The feedback structure of this shrew suggests the existence of population dynamics dominated by intraspecific competitive interactions, such as aggression, solitary nesting, non-overlapping home ranges and territoriality.

We studied morphological and functional variations in jaws of coastal and mountain populations of subterranean Spalacopus cyanus inhabiting soils with contrasting hardness. We found almost no morphological differentiation between populations in the variables we measured. However, there were clear differences in incisor resistance between them. Apparently, soil hardness did not represent a selective pressure on cururos' digging apparatus. An Andean origin of this genus could explain our results.

We studied the standard and comparative energetics of the Rufous-tailed Plancutter (Phytotoma rara), one of the smallest avian herbivores. The Rufous-tailed Plancutter had basal metabolic rate (BMR) values that so far are the highest mass-independent values observed in avian herbivores. Probably the BMR values attained by P. rara reflect its geographic distribution in temperate environments. Using a comparative analysis, we observed that herbivorous birds from temperate geographic areas tend to have higher BMR than tropical ones.

We examined the effect of food quality on ingestion, digestion, and metabolic rate during pregnancy and lactation in Octodon degus, a precocial rodent, under laboratory conditions. We also examined standard energetics during reproduction in relation to litter size and litter mass. Resting metabolic rate increased significantly during lactation, and that increase resulted from variations in food quality. The highest increase (39%) in resting metabolic rate was found in lactating females maintained on high-quality food when compared with nonreproductive females. Although food intake was always higher during lactation, the maximum intake was observed among lactating females that were given high-quality food. A significant positive correlation also was found between resting metabolic rate and food intake during early lactation, which revealed an increase in energy processing during that demanding period. Significant positive relationships also were found between resting metabolic and ingestion rate relative to litter mass and size. Allocation of energy in O. degus during lactation did not follow the mode typical of precocial rodents. In contrast, conversion efficiency of metabolizable energy into tissue growth appears to be linked to environmental quality of food.

The energetics and burrowing behaviour of the semifossorial Octodon degus (Rodentia: Octodontidae) were investigated and compared with that of more specialized fossorial rodents. An open-flow respirometry system was used to record energy expenditure of single degus inside respirometers partially filled with soft (moist) or hard (dry) soil. In addition, digging behaviour was recorded in groups of three animals inside a large terrarium under controlled conditions of food, photoperiod and temperature. In the field, the digging activity of degus was monitored, along with seasonal variations in rainfall, content of soil moisture and soil hardness. Mass-specific metabolic rate during digging was found to be higher in animals burrowing in soft soils compared to hard soil. However, animals burrowing in soft soil removed more soil per min than animals in hard soil. Thus, gram per gram, excavating in hard soil was energetically more expensive. The digging cost of semifossorial degus tends to be either similar to or above those of similarly sized, but more fossorial, rodents. In the field, heightened digging activity coincided with the occurrence of rainfall, greater content of soil moisture and relatively soft soil conditions. Degus generally use their front feet and teeth to shear the soil; disposal of accumulated debris being carried out by moving their front and hind feet backwards. We also observed the establishment of digging chains when two or three individuals burrowed at the same site. As far as digging is concerned, the behaviour of degus is similar to that of other fossorial rodents, such as African bathyergids and the more closely related South American ctenomyids.

We examined the hypothesis that a main benefit of group-living in the semifossorial rodent, Octodon degus (Rodentia: Octodontidae), is to decrease individual cost of burrow construction. We contrasted the digging behavior of groups of three same-sex, adult-sized individuals with that of solitary degus. The behavior of singles and trios was recorded inside a large terrarium partially filled with natural soil and under controlled conditions of food, light, and temperature. The observation that degus in groups do not decrease their burrowing time or frequency of digging compared with solitary diggers does not support the hypothesis that communal burrowing is a primary cause of degu sociality. On the other hand, the observation that degus in groups removed significantly more soil per capita than solitary digging degus, and that grouped individuals coordinated their digging – group members burrowed mostly in the same sites and formed digging chains –, suggests that social burrowing may potentially reduce the cost of burrow construction in the long term. We suggest that such long-term benefits will be a consequence rather than a cause of degu group-living.

We determined non-shivering thermogensis (NST) and maximum metabolic rate (MMR) as functions of thermal acclimation in a small mammal species in a seasonal environment. We studied the rodent Phyllotis xanthopygus (Muridae) living in the cool to cold climate of the high Andean Plateau of northern Chile. As expected, NST and MMR were constantly higher in cool-acclimated individuals. Nevertheless, the observed differences in shivering thermogenesis (ST) as a result of temperature acclimation (>200%) exceeded our expectations. The large contributions of ST was due to a 94% increase in MMR while a 49% of increased in NST. Thus, changes in ST in P. xanthopygus account for most of the metabolic plasticity and thermogenic capability that enables this species to cope with thermal variations in the Andean environment.

We examined the feeding behavior and nutritional ecology of one of the smallest species of avian herbivores (ca. 44 g), the Rufous-tailed Plant cutter Phytotoma rara (Phytotomidae). This species inhabits forests and scrublands, as well as crop fields and orchards in central Chile. Field and laboratory data revealed that P. rara is a herbivorous bird with marked preferences for leaves in comparison to fruits and insects. When P. rara eats a highly diluted plant material (such as lettuce), a shorter digesta retention time and a lower apparent digestibility was observed in comparison to a high-quality diet. Wet-mass daily intake of plant material was 5.4 times body mass, two times higher compared with the high-quality diet. The larger proportion of water and refractory materials in lettuce diet may explain the variation in ingestion rates. Feeding bouts were longer with lettuce diet, because birds spent more time handling and chewing previous in gesta. Chewing may increase overall digestibility through physical breakdown of cell walls, making available the highly digestible cell contents. These strategies may allow P. rara to maintain body mass balance when feeding on highly diluted plant material.

We measured basal metabolic rate, thermal conductance, body temperature, and temperature differential of three populations of the Sigmodontine-rodent Abrothrix andinus in three different habitats of the Andean range of northern Chile. Individuals from the three habitats were good thermoregulators, were capable of maintaining a high mass-independent temperature differential, and were able to survive the cold ambient temperatures during the night. We compared our data with previously published information on seasonal energetics of another population of A. andinus in the Andean range of central Chile and with species of the genus Abrothrix in a Mediterranean climate. Energetics of Abrothrix did not vary in response to different climatic conditions, which favored the hypothesis that it was a fixed character probably reflecting a common ancestor of Andean origin or radiation along the Andes Mountains.

Dietary chemistry and an animal digestive physiology should be considered in any explanation of behavioral patterns of food use, as both influence dietary preference. In the degu Octodon degus (Molina, 1782), a generalist herbivorous rodent inhabiting central Chile, authors determine the profitability of natural food-plant items by measuring digestive characteristics, such as retention time and assimilation rate while also considering the effects of food chemistry.

Reduced energy expenditure resulting from huddling in small mammals is mainly attributed to the reduced surface area/volume ratio of the huddling group. Authors propose a model to account for the reduction of the relative exposed area of grouped animals anf for the diminution of metabolic rate during huddling. Authors attempt to expain mechanistically how changes in energy savings may operate. Authors applied model to results obtained by grouping deformable bodies, and also laboratory measurements of oxygen consumption on huddling small mammals of four species.

Authors report on body mass dynamics, field growth rates and age-specific growth curves for the leaf-eared mouse Phyllotis darwini (Waterhouse, 1837). Mark-recapture methods provided data for a population of P. darwini in a semi-arid region of Chile from 1987 to 1996. There were significant effects of sex, season, and slope exposure on body mass. In addition, authors found significant effects of sex, mass class, and season on field growth rates.

Summary Mammals exposed to low temperatures increase their metabolic rate to maintain constant body temperature and thus compensate heat loss. This high and costly energetic demand can be mitigated through thermoregulatory behavior such as social grouping or huddling, which helps to decrease metabolic rate as function of the numbers of individuals grouped. Sustained low temperatures in endothermic animals produce changes over time in rates of energy expenditure, by means of phenotypic plasticity. However, the putative modulating effect that huddling exerts on the flexibility of the basal metabolic rate (BMR) due to thermal acclimation remains unknown. We determined BMR values in Octodon degus, an endemic Chilean rodent, after being acclimated either to 15°C or 30°C during 60 days, both alone and in groups of 3 and 5 individuals. At 15°C, BMR of huddling individuals was 40% lower than that of animals housed alone. Moreover, infrared thermography revealed a significant increase in local surface temperatures in huddled animals. Furthermore, individual thermal conductance was lower in individuals acclimated to 15°C than at 30°C, but no differences were observed between single and grouped animals. Our results indicate that huddling prevent an increase in BMR when animals are acclimated to cold conditions and that this effect is proportional to the number of animals grouped.

Seasonal changes in birds' energy requirements as well as in environmental food availability and quality, influence birds' capability to obtain and digest food (Karasov 1990). Increased metabolic rates, hypertrophy of the gastrointestinal tract, and increased absorption of nutrients may be viewed as responses ofwintering birds to physical and biotic seasonal habitats (Sibly 1981, Dykstra and Karasov 1992). In central Chile, the climate is seasonal with warm, dry summers and cool rainy winters; mean annual temperature is 22.1"C and mean annual minimum temperature is 7.7"C (di Castri and Hajek 1976). One of the most conspicuous resident birds of the mediterranean environments in central Chile is the Rufouscollared Sparrow Zonotrichia capensis (Emberizidae). This species is opportunistic, feeding on seeds and insects and showing seasonal dietary shifts according to food availability (L6pez-Calleja 1995). Individuals of this species also exhibit seasonal changes in metabolic rates and thermal insulation (Novoa 1993). Consequently, Z. capensis lives under marked seasonal variations in energy availability and requirements. Specifically, when nutritional demands increase during winter, birds may respond to the seasonal environment through behavioral, anatomical and physiological changes that allow the maintenance of a positive energy budget and consequently permit survival. The aim of this work is to examine the relationship between seasonal changes in diet, digestive tract morphology, and digestive efficiency ofRufous-collared Sparrows in central Chile.

In the insectivorous Chilean mouse-opossum, Thylamys elegans (Didelphidae), acclimated to eating pulp of fruits with sucrose added and live insects, we studied the activity of intestinal disaccharidases, digestive efficiency, and the effect of each diet on the energy budget. We observed activity modulation of the enzymes sucrase, maltase, and trehalase as well as changes in substrate affinities correlated with dietary carbohydrates. Although this species is able to hydrolyze and assimilate sugars such as sucrose from fruits, it is nevertheless unable to obtain sufficient energy to satisfy its euthermic daily energy requirements and to survive with an exclusive diet of fruits. This situation contrasts with that observed in individuals feeding on insects, for they were able to satisfy their maintenance energy requirements with an exclusive diet of insects. The preferences observed in the mouse-opossum for insects over fruits is explained in terms of trade-offs between factors such as digesta transit time, food digestion (including sugars hydrolysis), and food characteristics.

Complex polysaccharides such as cellulose, hemicellulose, and lignin (fiber) are important structural constituents of plants that often are difficult for small herbivorous mammals to digest. These polysaccharides may affect the efficiency with which food is digested and with which nutrients and energy are transformed and allocated. To determine how small herbivorous mammals cope with such high-fiber food, I used as a model the herbivorous, caviomorph octodontid rodent Octodon degus , the degu, an inhabitant of the semiarid and Mediterranean environments of northern and central Chile. When given a choice, degus minimized fiber intake, showing pronounced preferences for food containing low fiber. Because low-fiber items are not available in the field during the dry season, I postulated that observations of degus feeding on grass containing a high percentage (nearly 60%) of fiber during summer are more likely the consequence of necessity than of choice. I suggested that during nutritional bottlenecks, degus operate according to the principles of foraging theory and principles governing digestion. Degus seemed to compensate for the low digestibility of high-fiber food by increasing the volume of digesta in the alimentary canal as a consequence of changes in rates of food intake and, hence, increases in turnover time of digesta. The digestive responses allowed them to increase the amount of energy obtained from fiber and to satisfy their maintenance energy costs during temporal exposures to different levels of food fiber.