Ambient temperature does not affect fuelling rate in absence of digestive constraints in long-distance migrant shorebird fuelling up in captivity

Pre-flight fuelling rates in free-living red knots Calidris canutus, a specialized long-distance migrating shorebird species, are positively correlated with latitude and negatively with temperature. The single published hypothesis to explain these relationships is the heat load hypothesis that states that in warm climates red knots may overheat during fuelling. To limit endogenous heat production (measurable as basal metabolic rate BMR), birds would minimize the growth of digestive organs at a time they need. This hypothesis makes the implicit assumption that BMR is mainly driven by digestive organ size variation during pre-flight fuelling. To test the validity of this assumption, we fed captive knots with trout pellet food, a diet previously shown to quickly lead to atrophied digestive organs, during a fuelling episode. Birds were exposed to two thermal treatments (6 and 24°C) previously shown to generate different fuelling rates in knots. We made two predictions. First, easily digested trout pellet food rather than hard-shelled prey removes the heat contribution of the gut and would therefore eliminate an ambient temperature effect on fuelling rate. Second, if digestive organs were the main contributors to variations in BMR but did not change in size during fuelling, we would expect no or little change in BMR in birds fed ad libitum with trout pellets. We show that cold-acclimated birds maintained higher body mass and food intake (8 and 51%) than warm-acclimated birds. Air temperature had no effect on fuelling rate, timing of fuelling, timing of peak body mass or BMR. During fuelling, average body mass increased by 32% while average BMR increased by 15% at peak of mass and 26% by the end of the experiment. Our results show that the small digestive organs characteristic of a trout pellet diet did not prevent BMR from increasing during premigratory fuelling. Our results are not consistent with the heat load hypothesis as currently formulated.     

Shorebirds' seasonal adjustments in thermogenic capacity are reflected by changes in body mass: how preprogrammed and instantaneous acclimation work together

Phenotypic flexibility in shorebirds has been studied mainly in the context of adjustments to migration and to quality of food; little is known on how birds adjust their phenotype to harsh winter conditions. We showed earlier that red knot (Calidris canutus islandica) can acclimate to cold by elevating body mass. This goes together with larger pectoral muscles, i.e., greater shivering machinery, and thus, better thermogenic capacity. Here, we present results of a yearlong experiment with indoor captive knots to determine whether this strategy is part of their natural seasonal phenotypic cycle. We maintained birds under three thermal regimes: constant cold (5 °C), constant thermoneutrality (25 °C) and natural seasonal variation between these extremes (9-22 °C). Each month we measured variables related to the birds' endurance to cold and physiological maintenance [body mass, thickness of pectoral muscles, summit metabolic rate (M(sum)), food intake, gizzard size, basal metabolic rate (BMR)]. Birds from all treatments expressed synchronized and comparable variation in body mass in spite of thermal treatments, with a 17-18% increase between the warmest and coldest months of the year; which appeared regulated by an endogenous driver. In addition, birds living in the cold exhibited a 10% higher average body mass than did those maintained at thermoneutrality. Thickness of the pectoral muscle tracked changes in body mass in all treatments and likely contributed to greater capacity for shivering in heavier birds. Consequently, M(sum) was 13% higher in cold-acclimated birds compared to those experiencing no thermoregulation costs. However, our data also suggest that part of maximal heat production comes from nonshivering processes. Birds facing cold conditions ate up to 25% more food than did birds under thermoneutral conditions, yet did not develop larger gizzards. Seasonal variation in BMR followed changes in body mass, probably reflecting changes in mass of metabolically active tissues. Just as cold-exposed birds, red knots in the variable treatment increased body mass in winter, thereby improving cold endurance. During summer, however, they maintained a lower body mass and thermogenic capacity compared to cold-exposed birds, similar to individuals kept at thermoneutrality. We conclude that red knots acclimate to seasonal variations in ambient temperature by modulating body mass, combining a preprogrammed increase in mass during winter with a capacity for fine-tuning body mass and thermogenic capacity to temperature variations.     

Phenotypic compromise in the face of conflicting ecological demands: an example in red knots Calidris canutus

Phenotypic flexibility is a phenomenon where physiological functions in animals are reversibly adjusted in response to ecological constraints. Research usually focuses on effects of single constraints, but under natural conditions animals face a multitude of restrictions acting simultaneously, and potentially generating conflicting demands on the phenotype. We investigated the conflicting demands of low temperatures and a low quality diet on the phenotype of a shorebird, the red knot Calidris canutus . We tested the effects of switching diet from a high quality trout food to low quality hard-shelled bivalves in captive birds acclimated to temperatures reflecting natural winter conditions. Feeding on bivalves generated a digestive constraint forcing the birds to increase the height and width of their gizzard by 66% and 71%, respectively, over 30 days. The change in gizzard size was associated with an initial 15% loss of body mass and a reduction in size of the pectoral muscles by 11%. Because pectoral muscle size determines summit metabolic rate (M_sum, an indicator of cold endurance), measured M_sum declined by 9%. Therefore, although the birds were acclimated to cold, gizzard growth led to a loss of cold endurance. We propose that cold-acclimated knots facing a digestive constraint made a phenotypic compromise by giving-up cold hardiness for digestive capacity. Field studies suggest that phenotypic compromises occur in free-living red knots as well and help improve survival.     

Expanding the body mass range: associations between BMR and tissue morphology in wild type and mutant dwarf mice (<Emphasis Type="Italic">David</Emphasis> mice)

We sought to identify associations of basal metabolic rate (BMR) with morphological traits in laboratory mice. In order to expand the body mass (BM) range at the intra-strain level, and to minimize relevant genetic variation, we used male and female wild type mice (C3HeB/FeJ) and previously unpublished ENU-induced dwarf mutant littermates (David mice), covering a body mass range from 13.5 g through 32.3 g. BMR was measured at 30°C, mice were killed by means of CO₂ overdose, and body composition (fat mass and lean mass) was subsequently analyzed by dual X-ray absorptiometry (DEXA), after which mice were dissected into 12 (males) and 10 (females) components, respectively. Across the 44 individuals, 43% of the variation in the basal rates of metabolism was associated with BM. The latter explained 47% to 98% of the variability in morphology of the different tissues. Our results demonstrate that sex is a major determinant of body composition and BMR in mice: when adjusted for BM, females contained many larger organs, more fat mass, and less lean mass compared to males. This could be associated with a higher mass adjusted BMR in females. Once the dominant effects of sex and BM on BMR and tissue mass were removed, and after accounting for multiple comparisons, no further significant association between individual variation in BMR and tissue mass emerged. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Intraspecific allometry of basal metabolic rate: relations with body size, temperature, composition, and circadian phase in the kestrel, Falco tinnunculus

The relationship between body size and basal metabolic rate (BMR) in homeotherms has been treated in the literature primarily by comparison between species of mammals or birds. This paper focuses on the intraindividual changes in BMR when body mass (W) varies with different maintenance regimens. BMR varied in individual kestrels in proportion to W1.67, which is considerably steeper than the mass exponents for homomorphic change (0.667; Heusner, 1984) for interspecific comparison among all birds (0.677) or raptors (0.678), for interindividual comparison of kestrels on ad libitum maintenance regimens (0.786), and for mass proportionality (1.00). The circadian range of telemetered core temperature also varied more strongly with intraindividual than with interspecific (Aschoff, 1981a) variation in mass. This was due to reduced nocturnal core temperature at low-maintenance regimens, which was, however, insufficient to account for the excessive reduction in BMR. kidney lean mass at Carcass analysis of eight birds sacrificed revealed a disproportionate reduction in heart and kidney lean mass at low-maintenance regimens. We surmise that variation in BMR primarily reflects variation in these metabolically highly active tissues. This may account for positive correlations found between heart, kidney, and BMR residuals relative to interspecific allometric prediction, and between alpha and rho residuals, as expected on the basis of the constant excess of BMR during alpha above BMR during rho (Aschoff & Pohl, 1970a).     

Habitat selection by wintering tufted ducks with special reference to their digestive organ and to possible segregation between neighboring populations

Habitat selection by tufted ducks (Aythya fuligula), a diving duck which swallows benthic prey organisms, was studied during winter at two neighboring lagoons (Lakes Nakaumi and Shinji, Honshu, Japan) which differ strongly in their benthic fauna and in their diving duck densities. The ducks fed overwhelmingly on the dominant bivalve found in each of the two lagoons, the mussel Musculista senhousia in L. Nakaumi and the clam Corbicula japonica in L. Shinji. In general, however, the ducks probably preferred the mussels to the clams because of: (i) their high (2.9 times) calorific content for their weight; (ii) their high digestibility; (iii) their greater accessibility; and (iv) their shorter handling time. An average tufted duck (850 g) was estimated to require 1.3 kg of mussels or 3.8 kg of clams to meet their daily energy requirements. As a result, the two wintering populations were estimated to consume 4970 t mussels and 4770 t clams during a single wintering season, amounting to some 20% of the standing clam crop. Throughout the winter the average gizzard weight (37 g), and gizzard–body mass ratio (4.2%) of the Lake Nakaumi population were half those of the Lake Shinji population (73 g, 8.1%, respectively), despite their significantly similar nutritive body condition (% body lipid > 12%). The need to maintain a specialized gizzard mass in order to be able to cope with the different prey species results in little opportunity for sampling movements of birds between lakes/prey types and as a result two subpopulations of ducks are indicated to be segregated.     

Thermogenic side effects to migratory predisposition in shorebirds

In the calidrine sandpiper red knot (Calidris canutus), the weeks preceding takeoff for long-distance migration are characterized by a rapid increase in body mass, largely made up of fat but also including a significant proportion of lean tissue. Before takeoff, the pectoral muscles are known to hypertrophy in preparation for endurance flight without any specific training. Because birds facing cold environments counterbalance heat loss through shivering thermogenesis, and since pectoral muscles represent a large proportion of avian body mass, we asked the question whether muscle hypertrophy in preparation for long-distance endurance flight would induce improvements in thermogenic capacity. We acclimated red knots to different controlled thermal environments: 26 degrees C, 5 degrees C, and variable conditions tracking outdoor temperatures. We then studied within-individual variations in body mass, pectoral muscle size (measured by ultrasound), and metabolic parameters [basal metabolic rate (BMR) and summit metabolic rate (M(sum))] throughout a 3-mo period enclosing the migratory gain and loss of mass. The gain in body mass during the fattening period was associated with increases in pectoral muscle thickness and thermogenic capacity independent of thermal acclimation. Regardless of their thermal treatment, birds showing the largest increases in body mass also exhibited the largest increases in M(sum). We conclude that migratory fattening is accompanied by thermoregulatory side effects. The gain of body mass and muscle hypertrophy improve thermogenic capacity independent of thermal acclimation in this species. Whether this represents an ecological advantage depends on the ambient temperature at the time of fattening.     

The effects of dietary linoleic acid and hydrophilic antioxidants on basal,peak, and sustained metabolism in flight‐trained European starlings

Dietary micronutrients have the ability to strongly influence animal physiology and ecology. For songbirds, dietary polyunsaturated fatty acids (PUFAs) and antioxidants are hypothesized to be particularly important micronutrients because of their influence on an individual's capacity for aerobic metabolism and recovery from extended bouts of exercise. However, the influence of specific fatty acids and hydrophilic antioxidants on whole‐animal performance remains largely untested. We used diet manipulations to directly test the effects of dietary PUFA, specifically linoleic acid (18:2n6), and anthocyanins, a hydrophilic antioxidant, on basal metabolic rate (BMR), peak metabolic rate (PMR), and rates of fat catabolism, lean catabolism, and energy expenditure during sustained flight in a wind tunnel in European starlings (Sturnus vulgaris). BMR, PMR, energy expenditure, and fat metabolism decreased and lean catabolism increased over the course of the experiment in birds fed a high (32%) 18:2n6 diet, while birds fed a low (13%) 18:2n6 diet exhibited the reverse pattern. Additionally, energy expenditure, fat catabolism, and flight duration were all subject to diet‐specific effects of whole‐body fat content. Dietary antioxidants and diet‐related differences in tissue fatty acid composition were not directly related to any measure of whole‐animal performance. Together, these results suggest that the effect of dietary 18:2n6 on performance was most likely the result of the signaling properties of 18:2n6. This implies that dietary PUFA influence the energetic capabilities of songbirds and could strongly influence songbird ecology, given their availability in terrestrial systems.     

Seasonal variation in body composition in an Afrotropical passerine bird: increases in pectoral muscle mass are,unexpectedly, associated with lower thermogenic capacity

Phenotypic flexibility in avian metabolic rates and body composition have been well-studied in high-latitude species, which typically increase basal metabolic rate (BMR) and summit metabolism (M_sum) when acclimatized to winter conditions. Patterns of seasonal metabolic acclimatization are more variable in lower-latitude birds that experience milder winters, with fewer studies investigating adjustments in avian organ and muscle masses in the context of metabolic flexibility in these regions. We quantified seasonal variation (summer vs winter) in the masses of organs and muscles frequently associated with changes in BMR (gizzard, intestines and liver) and M_sum (heart and pectoral muscles), in white-browed sparrow-weavers (Plocepasser mahali). We also measured pectoral muscle thickness using a portable ultrasound system to determine whether we could non-lethally estimate muscle size. A concurrent study measured seasonal changes in BMR and M_sum in the same population of sparrow-weavers, but different individuals. There was no seasonal variation in the dry masses of the gizzard, intestines or liver of sparrow-weavers, and during the same period, BMR did not vary seasonally. We found significantly higher heart (~ 18% higher) and pectoral muscle (~ 9% higher) dry mass during winter, although ultrasound measurements did not detect seasonal changes in pectoral muscle size. Despite winter increases in pectoral muscle mass, M_sum was ~ 26% lower in winter compared to summer. To the best of our knowledge, this is the first study to report an increase in avian pectoral muscle mass but a concomitant decrease in thermogenic capacity.

     

Tesaglitazar, a dual PPAR{alpha}/{gamma} agonist, ameliorates glucose and lipid intolerance in obese Zucker rats

Insulin resistance, impaired glucose tolerance, high circulating levels of free fatty acids (FFA), and postprandial hyperlipidemia are associated with the metabolic syndrome, which has been linked to increased risk of cardiovascular disease. We studied the metabolic responses to an oral glucose/triglyceride (TG) (1.7/2.0 g/kg lean body mass) load in three groups of conscious 7-h fasted Zucker rats: lean healthy controls, obese insulin-resistant/dyslipidemic controls, and obese rats treated with the dual peroxisome proliferator-activated receptor alpha/gamma agonist, tesaglitazar, 3 mumol.kg(-1).day(-1) for 4 wk. Untreated obese Zucker rats displayed marked insulin resistance, as well as glucose and lipid intolerance in response to the glucose/TG load. The 2-h postload area under the curve values were greater for glucose (+19%), insulin (+849%), FFA (+53%), and TG (+413%) compared with untreated lean controls. Treatment with tesaglitazar lowered fasting plasma glucose, improved glucose tolerance, substantially reduced fasting and postload insulin levels, and markedly lowered fasting TG and improved lipid tolerance. Fasting FFA were not affected, but postprandial FFA suppression was restored to levels seen in lean controls. Mechanisms of tesaglitazar-induced lowering of plasma TG were studied separately using the Triton WR1339 method. In anesthetized, 5-h fasted, obese Zucker rats, tesaglitazar reduced hepatic TG secretion by 47%, increased plasma TG clearance by 490%, and reduced very low-density lipoprotein (VLDL) apolipoprotein CIII content by 86%, compared with obese controls. In conclusion, the glucose/lipid tolerance test in obese Zucker rats appears to be a useful model of the metabolic syndrome that can be used to evaluate therapeutic effects on impaired postprandial glucose and lipid metabolism. The present work demonstrates that tesaglitazar ameliorates these abnormalities and enhances insulin sensitivity in this animal model.     

Chronic injection of pansomatostatin agonist ODT8-SST differentially modulates food intake and decreases body weight gain in lean and diet-induced obese rats

The aim of this study was to investigate the central actions of the stable pansomatostatin peptide agonist, ODT8-SST on body weight. ODT8-SST or vehicle was acutely (1μg/rat) injected or chronically infused (5μg/rat/d, 14d) intracerebroventricularly and daily food intake, body weight and composition were monitored. In lean rats, neither acute nor chronic ODT8-SST influenced daily food intake while body weight was reduced by 2.2% after acute injection and there was a 14g reduction of body weight gain after 14d compared to vehicle (p<0.01). In diet-induced obese (DIO) rats, chronic ODT8-SST increased cumulative 2-week food intake compared to vehicle (+14%, p<0.05) and also blunted body weight change (-11g, p<0.05). ODT8-SST for 14d reduced lean mass (-22g and -25g respectively, p<0.001) and total water (-19g and -22g respectively, p<0.001) in lean and DIO rats and increased fat mass in DIO (+16g, p<0.001) but not lean rats (+1g, p>0.05) compared to vehicle. In DIO rats, ODT8-SST reduced ambulatory (-27%/24h, p<0.05) and fine movements (-38%, p<0.01) which was associated with an increased positive energy balance compared to vehicle (+50g, p<0.01). Chronic central somatostatin receptor activation in lean rats reduces body weight gain and lean mass independently of food intake which is likely related to growth hormone inhibition. In DIO rats, ODT8-SST reduces lean mass but promotes food intake and fat mass, indicating differential responsiveness to somatostatin under obese conditions.     

Reinterpretation of gizzard sizes of red knots world-wide emphasises overriding importance of prey quality at migratory stopover sites

The size of digestive organs can be rapidly and reversibly adjusted to ecological circumstances, but such phenotypic flexibility comes at a cost. Here, we test how the gizzard mass of a long-distance migrant, the red knot (Calidris canutus), is adjusted to (i) local climate, (ii) prey quality and (iii) migratory fuelling demands. For eight sites around the world (both wintering and stopover sites), we assembled data on gizzard masses of free-living red knots, the quality of their prey and the local climate. Using an energetic cost-benefit approach, we predicted the gizzard size required for fastest fuelling (net rate-maximization, i.e. expected during migration) and the gizzard size required to balance daily energy budgets (satisficing, expected in wintering birds) at each site. The measured gizzards matched the net rate-maximizing predictions at stopover sites and the satisficing predictions at wintering sites. To our surprise, owing to the fact that red knots selected stopover sites with prey of particularly high quality, gizzard sizes at stopovers and at wintering sites were nevertheless similar. To quantify the benefit of minimizing size changes in the gizzard, we constructed a model incorporating the size-dependent energy costs of maintaining and carrying a gizzard. The model showed that by selecting stopovers containing high-quality prey, metabolic rates are kept at a minimum, potentially reducing the spring migratory period by a full week. By inference, red knots appear to time their stopovers so that they hit local peaks in prey quality, which occur during the reproductive seasons of the intertidal benthic invertebrates.     

Three weeks of caloric restriction alters protein metabolism in normal-weight, young men

The effects of prolonged caloric restriction (CR) on protein kinetics in lean subjects has not been investigated previously. The purpose of this study was to test the hypotheses that 21 days of CR in lean subjects would 1) result in significant losses of lean mass despite a suppression in leucine turnover and oxidation and 2) negatively impact exercise performance. Nine young, normal-weight men [23 +/- 5 y, 78.6 +/- 5.7 kg, peak oxygen consumption (Vo2 peak) 45.2 +/- 7.3 ml.kg(-1).min(-1), mean +/- SD] were underfed by 40% of the calories required to maintain body weight for 21 days and lost 3.8 +/- 0.3 kg body wt and 2.0 +/- 0.4 kg lean mass. Protein intake was kept at 1.2 g.kg(-1).day(-1). Leucine kinetics were measured using alpha-ketoisocaproic acid reciprocal pool model in the postabsorptive state during rest and 50 min of exercise (EX) at 50% of Vo2 peak). Body composition, basal metabolic rate (BMR), and exercise performance were measured throughout the intervention. At rest, leucine flux (approximately 131 micromol.kg(-1).h(-1)) and oxidation (R(ox); approximately 19 micromol.kg(-1).h(-1)) did not differ pre- and post-CR. During EX, leucine flux (129 +/- 6 vs. 121 +/- 6) and R(ox) (54 +/- 6 vs. 46 +/- 8) were lower after CR than they were pre-CR. Nitrogen balance was negative throughout the intervention ( approximately 3.0 g N/day), and BMR declined from 1,898 +/- 262 to 1,670 +/- 203 kcal/day. Aerobic performance (Vo2 peak, endurance cycling) was not impacted by CR, but arm flexion endurance decreased by 20%. In conclusion, 3 wk of caloric restriction reduced leucine flux and R(ox) during exercise in normal-weight young men. However, despite negative nitrogen balance and loss of lean mass, whole body exercise performance was well maintained in response to CR.     

Field metabolic rate and water turnover of the numbat (<Emphasis Type="Italic">Myrmecobius fasciatus</Emphasis>)

The numbat (Myrmecobius fasciatus) is a diurnal and exclusively termitivorous marsupial. This study examines interrelationships between diet, metabolic rate and water turnover for wild, free-living numbats. The numbats (488±20.8 g) remained in mass balance during the study. Their basal metabolic rate (BMR) was 3.6 l CO₂ day^–1, while their field metabolic rate (FMR) was 10.8±1.22 l CO₂ day^–1 (269±30.5 kJ day^–1). The ratio FMR/BMR was 3±0.3 for numbats. We suggest that the most accurate way to predict the FMR of marsupials is from the regression log FMR=0.852 log BMR+0.767; (r²=0.97). The FMR of the numbat was lower than, but not significantly different from, that of a generalised marsupial, both before (76%) and after (62–69%) correction for the significant effect of phylogeny on FMR. However the numbat's FMR is more comparable with that of other arid-habitat Australia marsupials (98–135%), for which the regression relating mass and FMR is significantly lower than for nonarid-habitat marsupials, independent of phylogeny. The field water turnover rate (FWTR) of free-living numbats (84.1 ml H₂O day^–1) was highly correlated with FMR, and was typical (89–98%) of that for an arid-habitat marsupial after phylogenetic correction. The higher than expected water economy index for the numbat (FWTR/FMR=0.3±0.03) suggests that either the numbats were drinking during the study, the water content of their diet was high, or the digestibility of their termite diet was low. Habitat and phylogenetic influences on BMR and FMR appear to have pre-adapted the numbat to a low-energy termitivorous niche.Abbreviations BMR basal metabolic rate - FMR field metabolic rate - EWL evaporative water loss - FWTR field water turnover rate - MR metabolic rate - PVR phylogenetic vector regression - RER respiratory exchange ratio - T_a ambient temperature - T_b body temperature - TBW total body water - CO₂ rate of carbon dioxide production - O₂ rate of oxygen consumption - WEI water economy index - WER water efflux rate - WIR water influx rateCommunicated by I.D. Hume     

Effects of strength training on body composition and bone mineral content in children who are obese

The purpose of this study was to test the hypothesis that strength training benefits diet-controlled obese children with respect to lean mass and bone mineral acquisition. Eighty-two Hong Kong school children (aged 10.4 +/- 1.0 years, 70 in Tanner stage 1, 12 in stage 2) who were obese/overweight were randomly assigned to receive either a balanced low-energy (900-1200 cal) diet plus strength training (n = 41) (training group) or the diet alone (n = 41) (control group). The training group attended a 75-minute strength exercise program 3 times/week for 6 weeks (phase 1), after which they were offered and 22 children opted to continue a once-weekly program for a further 28 weeks (phase 2). All children were evaluated at baseline, after 6 weeks, and at the end of the 36-week study (including an intervening 2-week introduction to phase 2). Body composition and bone mineral content were measured by dual-energy X-ray absorptiometry, and diet was assessed by food-frequency questionnaire. The results showed that the exercise programs were well accepted, with good attendance at the exercise classes. After 6 weeks, the children in the training group showed significantly larger increases in lean body mass (+ 0.8 kg [2.4%] vs. +0.3 kg [1.0%], p < 0.05) and total bone mineral content (+46.9 g [3.9%] vs. +33.6 g [2.9%], p < 0.05) than those in the control group. At the end of the study, these trends were maintained in the continued-training subgroup, though no longer reaching statistical significance. We conclude that in diet-controlled prepubertal obese/overweight children, participation in an exercise program with emphasis on strength training resulted in improved lean mass and bone mineral accrual.     

Non-invasive measure of body composition of snakes using dual-energy X-ray absorptiometry

Non-invasive techniques to measure body composition are critical for longitudinal studies of energetics and life histories and for investigating the link between body condition and physiology. Previous attempts to determine, non-invasively, the body composition of snakes have proven problematic. Therefore, we explored whether dual-energy X-ray absorptiometry (DXA) could be used to determine the body composition of snakes. We analyzed 20 adult diamondback water snakes (Nerodia rhombifer) with a DXA instrument and subsequently quantified their body composition by gravimetric and chemical extraction methods. Body composition components scaled with body mass with mass exponents between 0.88 and 1.53. DXA values for lean tissue mass, fat mass and total-body bone mineral mass were significantly correlated with observed masses of lean tissue, fat and ash from chemical analysis. Using regression models incorporating DXA values we predicted the fat-free tissue mass, lean tissue mass, fat mass, ash mass and total body water content for this sample of water snakes. A cross-validation procedure demonstrated that these models estimated fat-free tissue mass, lean tissue mass, fat mass, ash mass and total-body water content with respective errors of 2.2%, 2.3%, 16.0%, 6.6% and 3.5%. Compared to other non-invasive techniques, include body condition indices, total body electrical conductivity (TOBEC) and cyclopropane absorption, DXA can more easily and accurately be used to determine the body composition of snakes.     

Personality drives physiological adjustments and is not related to survival

The evolutionary function and maintenance of variation in animal personality is still under debate. Variation in the size of metabolic organs has recently been suggested to cause and maintain variation in personality. Here, we examine two main underlying notions: (i) that organ sizes vary consistently between individuals and cause consistent behavioural patterns, and (ii) that a more exploratory personality is associated with reduced survival. Exploratory behaviour of captive red knots (Calidris canutus, a migrant shorebird) was negatively rather than positively correlated with digestive organ (gizzard) mass, as well as with body mass. In an experiment, we reciprocally reduced and increased individual gizzard masses and found that exploration scores were unaffected. Whether or not these birds were resighted locally over the 19 months after release was negatively correlated with their exploration scores. Moreover, a long-term mark–recapture effort on free-living red knots with known gizzard masses at capture confirmed that local resighting probability (an inverse measure of exploratory behaviour) was correlated with gizzard mass without detrimental effects on survival. We conclude that personality drives physiological adjustments, rather than the other way around, and suggest that physiological adjustments mitigate the survival costs of exploratory behaviour. Our results show that we need to reconsider hypotheses explaining personality variation based on organ sizes and differential survival.     

Diet, phylogeny, and basal metabolic rate in phyllostomid bats

Aside from the pervasive effects of body mass, much controversy exists as to what factors account for interspecific variation in basal metabolic rates (BMR) of mammals; however, both diet and phylogeny have been strongly implicated. We examined variation in BMR within the New World bat family Phyllostomidae, which shows the largest diversity of food habits among mammalian families, including frugivorous, nectarivorous, insectivorous, carnivorous and blood-eating species. For 27 species, diet was taken from the literature and BMR was either measured on animals captured in Brazil or extracted from the literature. Conventional (nonphylogenetic) analysis of covariance (ANCOVA), with body mass as the covariate, was first used to test the effects of diet on BMR. In this analysis, which assumes that all species evolved simultaneously from a single ancestor (i.e., a "star" phylogeny), diet exerted a strong effect on mass-independent BMR: nectarivorous bats showed higher mass-independent BMR than other bats feeding on fruits, insects or blood. In phylogenetic ANCOVAs via Monte Carlo computer simulation, which assume that species are part of a branching hierarchical phylogeny, no statistically significant effect of diet on BMR was observed. Hence, results of the nonphylogenetic analysis were misleading because the critical values for testing the effect of diet were underestimated. However, in this sample of bats, diet is perfectly confounded with phylogeny, because the four dietary categories represent four separate subclades, which greatly reduces statistical power to detect a diet (= subclade) effect. But even if diet did appear to exert an influence on BMR in this sample of bats, it would not be logically possible to separate this effect from the possibility that the dietary categories differ for some other reason (i.e., another synapomorphy of one or more of the subclades). Examples such as this highlight the importance of considering phylogenetic relationships when designing new comparative studies, as well as when analyzing existing data sets. We also discuss some possible reasons why BMR may not coadapt with diet.     

Stable isotope and pen feeding trial studies confirm the value of horseshoe crab Limulus polyphemus eggs to spring migrant shorebirds in Delaware Bay

We used stable isotope (SI) methods in combination with pen feeding trials to determine the importance of eggs of the Atlantic horseshoe crab Limulus polyphemus to migratory fattening of red knots Calidris canutus rufa and ruddy turnstones Arenaria interpres morinella during spring stopover in Delaware Bay. By manifesting measurable fractionation (ca +3‰) and rapid turnover, blood plasma δ¹⁵ nitrogen proved a functional marker for SI diet tracking during the short 3-week stopover. Blood samples from free-ranging knots (3 data sets) and turnstones (1 data set) produced similar convergence of plasma δ¹⁵N signatures with increasing body mass that indicated highly similar diets. Asymptotes deviated slightly (0.3‰ to 0.7‰) from that of captive shorebirds fed a diet of only crab eggs during stopover, thus confirming a strong crab egg-shorebird linkage. The plasma δ¹⁵N crab-egg diet asymptote was enriched ca +4.5‰ and therefore readily discriminated from that of either blue mussels Mytilus edulis or coquina clams Donax variabilis , the most likely alternative prey of knots in Delaware Bay. Crab eggs were highly palatable to captive knots and turnstones which achieved rates of mass gain (3–11 g/d) comparable to that of free-ranging birds. Peak consumption rates during hyperphagic events were 23,940 and 19,360 eggs/bird/d, respectively. The empirical conversions of eggs consumed to body mass gained (5,017 eggs/g for knots and 4,320 eggs/g for turnstones) indicate the large quantities of crab eggs required for the maintenance of these shorebird populations during stopover.     

Energetic bottlenecks and other design constraints in avian annual cycles

The flexible phenotypes of birds and mammals often appear torepresent adjustments to alleviate some energetic bottleneckor another. By increasing the size of the organs involved indigestion and assimilation of nutrients (gut and liver), anindividual 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 sustainedflight or for thermoregulation. Reflecting the stationary costof 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. Energeticbottlenecks appear to be set by the marginal value of organsize increases relative to particular peak requirements (includingsafety factors). These points are elaborated using the studieson long-distance migrating shorebirds, especially red knotsCalidris canutus. Red knots encounter energy expenditure levelssimilar to experimentally determined ceiling levels of ca. 5times BMR in other birds and mammals, both during the breedingseason on High Arctic tundra (probably mainly a function ofcosts of thermoregulation) and during winter in temperate coastalwetlands (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 betweena "fueling [life-cycle] stage" and a "flight stage." Fuelingred knots in tropical areas may encounter heat load problemswhilst still on the ground, but high flight altitudes duringmigratory flights seem to take care of overheating and unacceptablyhigh rates of evaporative water loss. The allocation principlesfor the flexible phenotypes of red knots and other birds, thecosts of their organ flexibility and the ways in which they"organize" all the fast phenotypic changes, are yet to be discovered.