Energy Intake Rate Research Articles

Net rate of energy intake predicts reach-level steelhead (Oncorhynchus mykiss) densities in diverse basins from a large monitoring program

Substantial research effort has been devoted to understanding stream-dwelling salmonids’ use of summer rearing and growth habitat, with a subset of studies focusing on foraging position selection and the energetic trade-offs of differential habitat use. To date, however, cost–benefit analyses for most foraging model studies have focused on small sampling areas such as individual habitat units. To address this knowledge gap, we applied a mechanistic foraging model to 22 stream reaches (100–400 m) from two watersheds within the Columbia River Basin. We found a strong, positive correlation (R2= 0.61, p < 0.001) between predicted carrying capacities and observed fish densities. Predicted proportion of suitable habitat was weakly correlated with observed fish density (R2= 0.18, p = 0.051), but the mean net rate of energy intake prediction in sampling reaches was not a significant predictor of observed fish biomass. Our results suggest spatial configuration of habitat, in addition to quantity and quality, is an important determinant of habitat use. Further, carrying capacity predicted by the model shows promise as a habitat metric. We also evaluated the feasibility of applying this data-intensive modeling approach in a large-scale monitoring program to examine habitat quality and quantity. Though the approach can be computationally expensive, we feel the model’s ability to integrate physical habitat metrics (e.g., depth, velocity) with important biological considerations like food availability and temperature is a benefit that far outweighs associated costs. We feel this modeling approach has great potential as a tool to help understand habitat use in drift-feeding fishes.

Trade-offs between energy maximization and parental care in a central place forager, the sea otter

Between 1999 and 2014, 126 archival time–depth recorders (TDRs) were used to examine the foraging behavior of southern sea otters (Enhydra lutris nereis) off the coast of California, in both resource-abundant (recently occupied, low sea otter density) and resource-limited (long-occupied, high sea otter density) locations. Following predictions of foraging theory, sea otters generally behaved as energy rate maximizers. Males and females without pups employed similar foraging strategies to optimize rates of energy intake in resource-limited habitats, with some exceptions. Both groups increased overall foraging effort and made deeper, longer and more energetically costly dives as resources became limited, but males were more likely than females without pups to utilize extreme dive profiles. In contrast, females caring for young pups (≤10 weeks) prioritized parental care over energy optimization. The relative importance of parental care versus energy optimization for adult females with pups appeared to reflect developmental changes as dependent young matured. Indeed, contrary to females during the initial stages of lactation, females with large pups approaching weaning once again prioritized optimizing energy intake. The increasing prioritization of energy optimization over the course of lactation was possible due to the physiological development of pups and likely driven by the energetic deficit incurred by females early in lactation. Our results suggest that regardless of resource availability, females at the end of lactation approach a species-specific ceiling for percent time foraging and that reproductive females in the central portion of the current southern sea otter range are disproportionately affected by resource limitation.

Causes, mechanisms, and consequences of contest competition among female mountain gorillas in Rwanda

Socioecological models predict that contest competition will arise when high quality foods can be usurped or monopolized, leading to more favorable energy balances and higher reproductive success for high-ranking females. Gorillas are interesting species for studying such predictions due to the variety of ecological conditions that they experience in different locations. Using data from 23 female mountain gorillas in 3 social groups in the Virunga Massif, we examined food characteristics that may influence contest competition (food site residence times [FSRT]), proximate mechanisms of such competition (aggression and avoidance), and potential consequences of competition (rank-related differences in energy intake rates, travel expenditures, and activity budgets). The average FSRT of each female was significantly correlated with dominance rank, which suggests that high-ranking females may have greater access to foods that are easier to usurp (as predicted with contest competition). High-ranking females were significantly more aggressive than low-ranking females, and both aggression and avoidance were significantly higher while feeding than during other activities. Contrary to predictions for contest competition, however, rank was not significantly correlated with energy intake rates nor with the proportion of time spent traveling versus feeding. Thus, we did not find any energetic benefits to explain why high-ranking females had significantly higher reproductive success in earlier studies. We propose several alternative explanations and discuss the potential complications of assessing contest competition in species with weak dominance relationships.

The Effect of Digestive Capacity on the Intake Rate of Toxic and Non-Toxic Prey in an Ecological Context.

Digestive capacity often limits food intake rate in animals. Many species can flexibly adjust digestive organ mass, enabling them to increase intake rate in times of increased energy requirement and/or scarcity of high-quality prey. However, some prey species are defended by secondary compounds, thereby forcing a toxin limitation on the forager’s intake rate, a constraint that potentially cannot be alleviated by enlarging digestive capacity. Hence, physiological flexibility may have a differential effect on intake of different prey types, and consequently on dietary preferences. We tested this effect in red knots (Calidris canutus canutus), medium-sized migratory shorebirds that feed on hard-shelled, usually mollusc, prey. Because they ingest their prey whole and crush the shell in their gizzard, the intake rate of red knots is generally constrained by digestive capacity. However, one of their main prey, the bivalve Loripes lucinalis, imposes a toxin constraint due to its symbiosis with sulphide-oxidizing bacteria. We manipulated gizzard sizes of red knots through prolonged exposure to hard-shelled or soft foods. We then measured maximum intake rates of toxic Loripes versus a non-toxic bivalve, Dosinia isocardia. We found that intake of Dosinia exponentially increased with gizzard mass, confirming earlier results with non-toxic prey, whereas intake of Loripes was independent of gizzard mass. Using linear programming, we show that this leads to markedly different expected diet preferences in red knots that try to maximize energy intake rate with a small versus a large gizzard. Intra- and inter-individual variation in digestive capacity is found in many animal species. Hence, the here proposed functional link with individual differences in foraging decisions may be general. We emphasize the potential relevance of individual variation in physiology when studying trophic interactions.

Foraging behaviour and energy budgets of sea snakes: insights from implanted data loggers

AbstractInformation on the foraging behaviour of sea snakes has the potential to clarify adaptive pathways involved in the evolutionary invasion of marine habitats by terrestrial vertebrates. However, logistical obstacles have precluded studies in this field. To obtain preliminary data of diving behaviour, we surgically implanted temperature‐depth loggers into two sympatric, amphibious, benthic foraging sea krait species from New Caledonia. Based on logger recovery from three snakes (1 Laticauda laticaudata and 2 L. saintgironsi), we obtained data on a total of 1850 dives carried out over eight foraging trips and 39.6 days at sea. Almost 99% of dives were <30 m deep. Average dive depth was 10.7 ± 7.0 m, but snakes dived as deep as 82.6 m. Maximum dive duration was ≥123.7 min. At sea, snakes dived continuously, with over 90% of surface recovery periods lasting less than 3 min in L. saintgironsi (n = 1). Periods at the surface (between successive dives) were longer at night than by day, plausibly reflecting more intense hunting activity by the snake. Locomotor speed, as measured by the rate of vertical descent or ascent during diving, was low (0.15 m s−1). In combination with other data on these species, we estimate that metabolic expenditure was about 10 times greater while the snakes were at sea than on land; and their overall field metabolic rate (70–145 kJ kg−1 day−1) was an order of magnitude less than has been reported for diving endotherms. Despite low sample sizes, our study shows that implanted data loggers can provide novel insights into sea snake biology. Our data emphasize the behavioural and ecological consequences of ectothermy for secondary marine vertebrates. In particular, they illustrate how ectothermy allows species to thrive on rates of energy intake vastly lower than are required for their endothermic competitors.

A large molluscivore bird (Common Eider, Somateria mollissima) is able to discriminate quality of blue mussels (Mytilus edulis) based on size and provenance

Molluscivore birds that forage on abundant but low-quality food have to ingest large quantities of food to achieve energy balance. Such a strategy is often associated with important digestive constraints limiting predator’s ingestion. Thus, these predators may use prey selection to ingest better-quality individuals among a generally low-quality prey population. Using captive Common Eiders (Somateria mollissima (L., 1758)) diving in a constant environment, we were able to examine their preferences for blue mussels (Mytilus edulis L., 1758) of varying qualities (different sizes or provenances). In addition, we studied the consequences prey selection had on Eiders’ energy intake rates and ingestion of flesh and shell material. Eiders selected 10–20 mm mussels and were able to discriminate and to select cultivated mussels from intertidal mussels. Prey selection allowed, in certain conditions of prey-size abundance, higher flesh and energy intake rates without increasing the ingestion of shell material. This study confirmed the energetic advantage that Eiders have when foraging in aquaculture sites, which explain the large depredation of preferred mussel sizes.

Energetic responses to variation in food availability in the two mountain gorilla populations (Gorilla beringei beringei).

Here, we compare food availability and relate this to differences in energy intake rates, time spent feeding, and daily travel distance of gorillas in the two populations. Comparative intraspecific studies investigating spatiotemporal variation in food availability can help us understand the complex relationships between ecology, behavior, and life history in primates and are relevant to understanding hominin evolution. Differences in several variables have been documented between the two mountain gorilla populations in the Virunga Massif and Bwindi Impenetrable National Park, but few direct comparisons that link ecological conditions to feeding behavior have been made. Using similar data collection protocols we conducted vegetation sampling and nutritional analysis on important foods to estimate food availability. Detailed observations of feeding behavior were used to compute energy intake rates and daily travel distance was estimated through GPS readings. Food availability was overall lower and had greater temporal variability in Bwindi than in the Virungas. Energy intake rates and time spent feeding were similar in both populations, but energy intake rates were significantly higher in Bwindi during the period of high fruit consumption. Daily travel distances were significantly shorter in the Virungas. Overall, despite the differences in food availability, we did not find large differences in the energetics of gorillas in the two populations, although further work is needed to more precisely quantify energy expenditure and energy balance. These results emphasize that even species with high food availability can exhibit behavioral and energetic responses to variable ecological conditions, which are likely to affect growth, reproduction, and survival.

Bison distribution under conflicting foraging strategies: site fidelity vs. energy maximization.

Foraging strategies based on site fidelity and maximization of energy intake rate are two adaptive forces shaping animal behavior. Whereas these strategies can both be evolutionarily stable, they predict conflicting optimal behaviors when population abundance is in decline. In such a case, foragers employing an energy-maximizing strategy should reduce their use of low-quality patches as interference competition becomes less intense for high-quality patches. Foragers using a site fidelity strategy, however, should continue to use familiar patches. Because natural fluctuations in population abundance provide the only non-manipulative opportunity to evaluate adaptation to these evolutionary forces, few studies have examined these foraging strategies simultaneously. Using abundance and space use data from a free-ranging bison (Bison bison) population living in a meadow-forest matrix in Prince Albert National Park, Canada, we determined how individuals balance the trade-off between site fidelity and energy-maximizing patch choice strategies with respect to changes in population abundance. From 1996 to 2005, bison abundance increased from 225 to 475 and then decreased to 225 by 2013. During the period of population increase, population range size increased. This expansion involved the addition of relatively less profitable areas and patches, leading to a decrease in the mean expected profitability of the range. Yet, during the period of population decline, we detected neither a subsequent retraction in population range size nor an increase in mean expected profitability of the range. Further, patch selection models. during the population decline indicated that, as density decreased, bison portrayed stronger fidelity to previously visited meadows, but no increase in selection strength for profitable meadows. Our analysis reveals that an energy-maximizing patch choice strategy alone cannot explain the distribution ofindividuals and populations, and site fidelity is an important evolutionary force shaping animal distribution. Animals may not always forage in the richest patches available, as ecological theory would often predict, but their use of profitable patches is dependent on population dynamics and the strength of site fidelity. Our findings are likewise relevant to applied inquiries such as forecasting species range shifts and reducing human-wildlife conflicts.

Effect of food restriction on energy budget in warm-acclimated striped hamsters

The capacity of small mammals to sustain periods of food shortage largely depends on the adaptive regulation of energy budget in response to the decrease in food supply. In addition to food availability, ambient temperature (Ta) is an important factor affecting the rates of both energy intake and expenditure. To examine the effect of Ta on energy strategy and the capacity to sustain food shortage, striped hamsters were exposed to a warm condition (30°C) and were then restricted to 70% of ad libitum food intake. Body mass, energy intake and expenditure and physiological markers indicative of thermogenesis were measured. Warm exposure had no effect on body mass and digestibility, but decreased energy intake, basal metabolic rate and maximum nonshivering thermogenesis. The mitochondria protein content, cytochrome c oxidase activity and uncoupling protein 1 level of brown adipose tissue were significantly lower in hamsters at 30°C than at 21°C. Food restriction induced a significant decrease in body mass, but the decreased body mass was attenuated at 30°C relative to 21°C. This suggests that striped hamsters could not compensate for the limited food supply by decreasing daily energy expenditure at 21°C, whereas they could at 30°C. The significant reductions in the rates of metabolism and thermogenesis in warm-acclimated hamsters increase the capacity to cope with food shortage. Although, it remains uncertain whether this response represents some generalized evolutionary adaptation, the Ta-dependent adjustment in the capacity to survive food restriction may reflect that warm acclimation plays an important role in adaptive regulation of both physiology and behavior in response to the variations of food availability.

Effect of Water Level and Time of day on Black-Crowned Night-Heron (Nycticorax nycticorax) Foraging Behavior at an Artificial Weir: 2013–2014

Black-crowned Night-Herons (Nycticorax nycticorax) feed primarily at night throughout the year but have been reported to also feed diurnally during the breeding season. Little previous research has compared daytime and nighttime foraging activity in this species. To better understand how light level affects foraging behavior, a pilot study was conducted in 2013 at an artificial weir located in Wichita, Kansas, used by Black-crowned Night-Herons and other wading birds. Based on 24 h of observation (14 h day, 10 h night), it was determined that many activities (strike rates, capture rates, capture efficiencies, relocation rates) did not differ between day and nighttime, but birds captured significantly larger fish during the day and also experienced significantly higher rates of aggression. That study was expanded in 2014, during which water levels at the weir differed significantly from 2013. An additional 63 h of observations recorded almost four times as many birds per hour in 2014 than in 2013, both during day and night. Strike rates and capture rates both were significantly lower in 2014 than in 2013, but the aggression rate was significantly higher. Neither capture efficiency (successful strikes/total strikes) nor prey size differed between 2013 and 2014. Since many measures of foraging behavior and social interactions differed between years, it is likely that water level influenced prey detection and availability, and high levels of aggression further limited prey capture. Despite lower rates of energy intake in 2014, diurnal foraging by Black-crowned Night-Herons appears to be important in meeting increased demands of reproduction.

Is energy intake underreported in hemodialysis patients?

Underreporting of energy intake is not much studied in hemodialysis population. To evaluate the underreporting of energy intake and associated factors in hemodialysis patients. A cross-sectional study, with 344 patients stable adults, of ten hemodialysis centers in Goiânia-GO. Energy intake was assessed by six 24-hour dietary recalls and basal metabolic rate (BMR) was estimated using the Harris Benedict equation. It was considered as underreporting when the ratio between the average energy intake and basal metabolic rate was lower than1.27. For analysis of factors associated with underreporting, the Poisson regression with robust variance estimation was applied. The prevalence of underreporting was 65.7%, being more significant in individuals who are overweight and in the non dialysis days. The result of the multivariate analysis identified four factors independently associated with the underreporting: being a female (PR = 1.27, CI = 1.10 to 1.46), body mass index ≥ 25 kg/m2 (PR = 1.29, CI = 1.12 to 1.48), three meals or lower/day (PR = 1.31, CI = 1.14 to 1.51) and hemodialysis length lower than 5 years (PR = 1.19CI = 1.01 to 1.40). The population showed a high prevalence of underreporting of energy intake. Being a female, presenting overweight, lower number of meals/day and lower length time on hemodialysis were factors associated with underreporting.

Ways to be different: foraging adaptations that facilitate higher intake rates in a northerly wintering shorebird compared with a low-latitude conspecific.

At what phenotypic level do closely related subspecies that live in different environments differ with respect to food detection, ingestion and processing? This question motivated an experimental study on rock sandpipers (Calidris ptilocnemis). The species' nonbreeding range spans 20 deg of latitude, the extremes of which are inhabited by two subspecies: C. p. ptilocnemis that winters primarily in upper Cook Inlet, Alaska (61°N) and C. p. tschuktschorum that overlaps slightly with C. p. ptilocnemis but whose range extends much farther south (∼40°N). In view of the strongly contrasting energetic demands of their distinct nonbreeding distributions, we conducted experiments to assess the behavioral, physiological and sensory aspects of foraging and we used the bivalve Macoma balthica for all trials. C. p. ptilocnemis consumed a wider range of prey sizes, had higher maximum rates of energy intake, processed shell waste at higher maximum rates and handled prey more quickly. Notably, however, the two subspecies did not differ in their abilities to find buried prey. The subspecies were similar in size and had equally sized gizzards, but the more northern ptilocnemis individuals were 10-14% heavier than their same-sex tschuktschorum counterparts. The higher body mass in ptilocnemis probably resulted from hypertrophy of digestive organs (e.g. intestine, liver) related to digestion and nutrient assimilation. Given the previously established equality of the metabolic capacities of the two subspecies, we propose that the high-latitude nonbreeding range of ptilocnemis rock sandpipers is primarily facilitated by digestive (i.e. physiological) aspects of their foraging ecology rather than behavioral or sensory aspects.

Temporal dynamics in the foraging decisions of large herbivores

The foraging decisions involved in acquiring a meal can have an impact on an animal’s spatial distribution, as well as affect other animal species and plant communities. Thus, understanding how the foraging process varies over space and time has broad ecological implications, and optimal foraging theory can be used to identify key factors controlling foraging decisions. Optimality models are based on currencies, options and constraints. Using examples from research on free-ranging bison (Bison bison), we show how variations in these model elements can yield strong spatio-temporal variation in expected foraging decisions. First, we present a simple optimal foraging model to investigate the temporal scale of foraging decisions. On the basis of this model, we identify the foraging currency and demonstrate that such a simple model can be successful at predicting animal distribution across ecosystems. We then modify the model by changing (1) the forager’s option, from the selection of individual plants to the selection of food bites that may include more than one plant species, (2) its constraints, from being omniscient to having incomplete information of resource quality and distribution and (3) its currency, from the maximisation of energy intake rate (E) to the maximisation of the ratio between E and mortality risk (u).We also show that, where the maximisation of E fails, the maximisation of E/u can explain the circadian rhythm in the diet and movements of bison. Simple optimal foraging-theory models thus can explain changes in dietary choice of bison within a foraging patch and during the course of a day.

Raising the sugar content--orchid bees overcome the constraints of suction feeding through manipulation of nectar and pollen provisions.

Unlike most other bees, the long-tongued orchid bees ingest nectar using suction feeding. Although long tongues allow exploitation of flowers with deep spurs, the energy intake rate is optimal at 10–20% lower nectar sugar concentrations compared to that of lapping bees. This constraint might be compensated by a higher digestive throughput. Additionally, orchid bees might evaporate water from regurgitated droplets of crop contents. We found male Euglossa championi (n = 10) and Euglossa dodsoni (n = 12) to regularly regurgitate droplets of crop content to the base of their proboscis, generating a fluid film between the proximal parts of the galeae, glossa and labial palps. Rhythmic movements of the proboscis may help to increase convection. There was a significant change in sugar concentration between the initially imbibed solution and the resulting crop content (P<0.05) and the time individual bees had engaged in this liquid exposure behavior was positively correlated with the resulting crop sugar concentration. Female Euglossa townsendi and Euglossa viridissima showed the same behavior. Additionally, they manipulated their nectar-enriched pollen provisions for extensive periods of time before deposition in brood cells. The deposited pollen loads (n = 14) showed a significantly higher sugar concentration than the sugar-water available to the bees (P<0.001). Thus, both male and female euglossines show behaviors that promote evaporative water loss from nectar. We suggest that the behaviors have evolved in concert with suction feeding on dilute nectar from deep floral tubes.

Antagonism of gastric inhibitory polypeptide (GIP) by palmitoylation of GIP analogues with N- and C-terminal modifications improves obesity and metabolic control in high fat fed mice

Compromise of gastric inhibitory polypeptide (GIP) receptor signalling represents a possible therapeutic strategy for the treatment of obesity-related diabetes. This study has characterised and evaluated the C-terminally fatty acid derivatised GIP analogues, GIP(3-30)Cex-K40[Pal] and Pro3GIP(3-30)Cex-K40[Pal], as potential GIP inhibitors. Both GIP analogues lack the two N-terminal amino acids cleaved by DPP-4 and have addition of nine amino acids from the C-terminal of exendin(1-39), Cex. GIP(3-30)Cex-K40[Pal] and Pro3GIP(3-30)Cex-K40[Pal] effectively (p < 0.01 to p < 0.001) inhibited GIP-induced cAMP production and insulin secretion in vitro. In normal mice, GIP(3-30)Cex-K40[Pal] and Pro3GIP(3-30)Cex-K40[Pal] displayed a significant (p < 0.05 to p < 0.001) and prolonged inhibitory effect on GIP-induced glucose-lowering and insulin-releasing actions. When injected once daily for 21 days in obese-diabetic high fat fed mice, both GIP(3-30)Cex-K40[Pal] and Pro3GIP(3-30)Cex-K40[Pal] significantly reduced body weight (p < 0.01 to p < 0.001) and lowered circulating glucose (p < 0.001) and insulin (p < 0.01 to p < 0.001) concentrations. The observed beneficial changes were independent of effects on energy intake, locomotor activity or metabolic rate. Oral and intraperitoneal glucose tolerance were significantly (p < 0.05 to p < 0.001) improved in both treatment groups at the end of the study, despite reduced glucose-induced plasma insulin concentrations. This improvement of metabolic control was accompanied by enhanced (p < 0.05 to p < 0.01) insulin sensitivity compared with high fat controls. These data demonstrate the potential offered by GIP(3-30)Cex-K40[Pal] and Pro3GIP(3-30)Cex-K40[Pal] for the treatment of obesity-related diabetes.

Optimal management of a multispecies shorebird flyway under sea-level rise.

Every year, millions of migratory shorebirds fly through the East Asian-Australasian Flyway between their arctic breeding grounds and Australasia. This flyway includes numerous coastal wetlands in Asia and the Pacific that are used as stopover sites where birds rest and feed. Loss of a few important stopover sites through sea-level rise (SLR) could cause sudden population declines. We formulated and solved mathematically the problem of how to identify the most important stopover sites to minimize losses of bird populations across flyways by conserving land that facilitates upshore shifts of tidal flats in response to SLR. To guide conservation investment that minimizes losses of migratory bird populations during migration, we developed a spatially explicit flyway model coupled with a maximum flow algorithm. Migratory routes of 10 shorebird taxa were modeled in a graph theoretic framework by representing clusters of important wetlands as nodes and the number of birds flying between 2 nodes as edges. We also evaluated several resource allocation algorithms that required only partial information on flyway connectivity (node strategy, based on the impacts of SLR at nodes; habitat strategy, based on habitat change at sites; population strategy, based on population change at sites; and random investment). The resource allocation algorithms based on flyway information performed on average 15% better than simpler allocations based on patterns of habitat loss or local bird counts. The Yellow Sea region stood out as the most important priority for effective conservation of migratory shorebirds, but investment in this area alone will not ensure the persistence of species across the flyway. The spatial distribution of conservation investments differed enormously according to the severity of SLR and whether information about flyway connectivity was used to guide the prioritizations. With the rapid ongoing loss of coastal wetlands globally, our method provides insight into efficient conservation planning for migratory species.

Effects of Warm Water Discharge from Sanmen Nuclear Power Plant on Filtration Behavior, Calcification Rate, and Ammonia Excretion of Razor Clam, &lt;i&gt;Sinonovacula constrict&lt;/i&gt;&lt;i&gt;a&lt;/i&gt;: A Simulation Study

Warm water discharge from nuclear power plant will elevate the temperature of seawater and may subsequently have influences on important marine aquiculture bivalves. In present study, the effects of temperature elevation caused by warm water discharge on the filtration behavior, calcification rate, and ammonia excretion of a major local bivalve species, razor clam, were investigated via laboratory simulation. The results showed that the filtration rates of razor clam increased significantly with an increased temperature, which were 0.46±0.06, 0.62±0.09, and 0.85±0.08L/g·h for 28 °C, 30 °C, and 32 °C, respectively. Under the experimental temperature gradient, the pattern of calcification rates of razor clam was shown to be bell-shaped, with the highest calcification rate 2.85±0.28μmol/FWg·h at 30 °C. The ammonia excretion rates were shown to increase slightly with increased temperature, which were 0.31±0.03, 0.32±0.09, and 0.34±0.06mg/ g·h for 28 °C, 30 °C, and 32 °C, respectively. The results suggest that razor clam will increase its energy intake and metabolic rate to maintain a relatively constant growth rate under thermal stress within 4 oC range even during the summer season.

Association between skeletal muscle fat content and very-low-density lipoprotein-apolipoprotein B-100 transport in obesity: effect of weight loss.

Ectopic deposition of fat in skeletal muscle is a feature of metabolic syndrome, but its specific association with very-low-density lipoprotein (VLDL)-apolipoprotein (apo) B-100 metabolism remains unclear. We examined the association between skeletal muscle fat content and VLDL-apoB-100 kinetics in 25 obese subjects, and the responses of these variables to weight loss. The fat contents of liver, abdomen and skeletal muscle were determined by magnetic resonance imaging, and VLDL-apoB-100 kinetics were assessed using stable isotope tracers. In obese subjects who were insulin sensitive (homeostasis model assessment, HOMA, score ≤ 2.6, n = 12), skeletal muscle fat content was significantly associated with hepatic fat content (r = 0.636), energy intake (r = 0.694), plasma triglyceride (r = 0.644), apoB-100 (r = 0.529), glucose (r = 0.622), VLDL-apoB-100 concentrations (r = 0.860), VLDL-apoB-100 fractional catabolic rate (FCR; r = -0.581) and VLDL-apoB-100 secretion rate (r = 0.607). These associations were not found in obese subjects who were insulin resistant (HOMA score >2.6, n = 13). Of these 25 subjects, 10 obese subjects underwent a 16-week weight loss program. The low-fat diet achieved significant reduction (p < 0.05) in body weight, visceral and subcutaneous fat areas, liver and skeletal muscle fat, energy intake, triglyceride, insulin, HOMA score, VLDL-apoB100 concentrations and VLDL-apoB100 secretion rate. The percentage reduction of skeletal muscle fat with weight loss was significantly associated with the corresponding changes in VLDL-apoB100 concentration (r = 0.770, p = 0.009) and VLDL-apoB-100 secretion (r = 0.682, p = 0.030). Skeletal muscle fat content is associated with VLDL-apoB-100 transport. Weight loss lowers skeletal muscle fat and VLDL-apoB-100 secretion.

Digestive capacity and toxicity cause mixed diets in red knots that maximize energy intake rate.

Among energy-maximizing animals, preferences for different prey can be explained by ranking the prey according to their energetic content. However, diet choice also depends on characteristics of the predator, such as the need to ingest necessary nutrients and the constraints imposed by digestion and toxins in food. In combination, these factors can lead to mixed diets in which the energetically most profitable food is not eaten exclusively even when it is abundant. We studied diet choice in red knots (Calidris canutus canutus) feeding on mollusks at a West African wintering site. At this site, the birds fed primarily on two species of bivalves, a thick-shelled one (Dosinia isocardia) that imposed a digestive constraint and a thin-shelled one (Loripes lucinalis) that imposed a toxin constraint. The latter species is toxic due to its symbiotic association with sulfide-oxidizing bacteria. We estimated experimentally the parameters of a linear programming model that includes both digestive and toxin constraints, leading to the prediction that red knots should eat a mixture of both mollusk species to maximize energy intake. The model correctly predicted the preferences of the captive birds, which depended on the digestive quality and toxicity of their previous diet. At our study site, energy-maximizing red knots appear to select a mixed diet as a result of the simultaneous effects of digestive and toxin constraints.

The effects of a competitor on the foraging behaviour of the shore crab Carcinus maenas.

Optimal Diet Theory suggests that individuals make foraging decisions that maximise net energy intake. Many studies provide qualitative support for this, but factors such as digestive constraints, learning, predation-risk and competition can influence foraging behaviour and lead to departures from quantitative predictions. We examined the effects of intraspecific competition within a classic model of optimal diet – the common shore crab, Carcinus maenas, feeding on the mussel, Mytilus edulis. Unexpectedly, we found that breaking time (Tb), eating time (Te), and handling time (Th) all decreased significantly in the presence of a conspecific. Reduced handling time in the presence of a competitor resulted in an increased rate of energy intake, raising the question of why crabs do not always feed in such a way. We suggest that the costs of decreased shell breaking time may be increased risk of claw damage and that crabs may be trading-off the potential loss of food to a competitor with the potential to damage their claw whilst breaking the shell more rapidly. It is well documented that prey-size selection by crabs is influenced by both the risk of claw damage and competition. However, our results are the first to demonstrate similar effects on prey handling times. We suggest that crabs maximise their long-term rate of energy intake at a scale far greater than individual foraging events and that in order to minimise claw damage, they typically break shells at a rate below their maximum. In the presence of a competitor, crabs appear to become more risk-prone and handle their food more rapidly, minimising the risk of kleptoparasitism.