Optimal foraging and beyond: how starlings cope with changes in food availability

Heat-induced maternal effects shape avian eggshell traits and embryo development and phenotype at high incubation temperatures

Phenotypic plasticity is an important avenue by which organisms may persist in the face of rapid environmental change. Environmental cues experienced by the mother can also influence the phenotype of offspring, a form of plasticity called maternal effects. Maternal effects can adaptively prepare offspring for the environmental conditions they will likely experience; however, their ability to buffer offspring against environmental stressors as embryos is understudied. Using captive zebra finches, we performed a maternal-offspring environmental match-mismatch experiment utilizing a 2 × 2 × 2 factorial design. Mothers were exposed to a mild heat conditioning (38°C) or control (22°C) treatment as juveniles, an acute high heat (42°C) or control (22°C) treatment as adults, then paired for breeding. The eggs produced by those females were incubated at a hyperthermic (38.5°C) or optimal temperature (37.2°C). We found that when mothers were exposed to a mild heat conditioning as juveniles, their embryos exhibited reduced water loss, longer development times, and produced hatchlings with heavier pectoralis muscles when incubated at high incubation temperatures, compared to embryos from control mothers. Mothers exposed to both the mild heat conditioning as juveniles and a high heat stressor as adults produced eggs with a higher density of shell pores and embryos with lower heart rates during development. However, there was a cost when there was a mismatch between maternal and embryo environment. Embryos from these conditioned and heat-stressed mothers had reduced survival at control incubation temperatures, indicating the importance of offspring environment when interpreting potential adaptive effects.

On the use of body mass measures in severity assessment in laboratory passerine birds

Criteria for assessing the severity of scientific procedures in laboratory rodents include the loss of body mass. However, guidance is limited for passerine birds and application of criteria developed for mammals risks poor welfare decisions. Here, I ask whether, and how, body mass criteria could be incorporated into laboratory welfare assessment of passerines. Passerine birds strategically adjust their body mass to minimise combined mortality risk from starvation and predation. A systematic literature review found that strategic mass changes can be sizeable (sometimes > 10%) even over short timescales. Many aspects of a bird's current or past environment, including husbandry and experimental procedures, may alter perceived starvation or predation risks and thus drive strategic mass change via evolved mechanisms. Therefore, body mass criteria used for rodents may be too stringent for passerines, potentially leading to over-estimated severity. Strategic mass changes might obscure those stemming from experimental interventions yet could also offer insights into whether birds perceive an intervention or altered husbandry as a threat. Mass criteria for severity assessment should be species- and context-specific in order to balance needs for refinement and reduction. To guide the development of appropriate criteria, a future research priority is for greater data collection and sharing based on standardised routine monitoring of mass variation under a representative range of husbandry conditions and procedures.

Seasonal and ontological variation in diet and age-related differences in prey choice, by an insectivorous songbird

The diet of an individual animal is subject to change over time, both in response to short-term food fluctuations and over longer time scales as an individual ages and meets different challenges over its life cycle. A metabarcoding approach was used to elucidate the diet of different life stages of a migratory songbird, the Eurasian reed warbler (Acrocephalus scirpaceus) over the 2017 summer breeding season in Somerset, the United Kingdom. The feces of adult, juvenile, and nestling warblers were screened for invertebrate DNA, enabling the identification of prey species. Dietary analysis was coupled with monitoring of Diptera in the field using yellow sticky traps. Seasonal changes in warbler diet were subtle, whereas age class had a greater influence on overall diet composition. Age classes showed high dietary overlap, but significant dietary differences were mediated through the selection of prey; (i) from different taxonomic groups, (ii) with different habitat origins (aquatic vs. terrestrial), and (iii) of different average approximate sizes. Our results highlight the value of metabarcoding data for enhancing ecological studies of insectivores in dynamic environments.

Food supply and individual quality influence seabird energy expenditure and reproductive success

Breeding animals trade off maximizing energy output to increase their number of offspring with conserving energy to ensure their own survival, leading to an energetic ceiling influenced by external, environmental factors or by internal, physiological factors. We examined whether internal or external factors limited energy expenditure by supplementally feeding breeding black-legged kittiwakes varying in individual quality, based on earlier work that defined late breeders as low-quality and early breeders as high-quality individuals. We tested whether energy expenditure increased when food availability decreased in both low- and high-quality birds; we predicted this would only occur in high-quality individuals capable of sustaining high levels of energy expenditure. Here, we find that food-supplemented birds expended less energy than control birds because they spent more time at the colony. However, foraging trips of food-supplemented birds were only slightly shorter than control birds, implying that food-supplemented birds were limited by food availability at sea similarly to control birds. Late breeders expended less energy, suggesting that low-quality individuals may not intake the energy necessary for sustaining high-energy output. Food-supplemented birds had more offspring than control birds, but offspring number did not influence energy expenditure, supporting the idea that the birds reached an energy ceiling. Males and lighter birds expended more energy, possibly compensating for relatively higher energy intake. Chick-rearing birds were working near their maximum, with highest levels of expenditure for early-laying (high-quality) individuals foraging at sea. Due to fluctuating marine environments, kittiwakes may be forced to change their foraging behaviors to maintain the balance between reproduction and survival.

Allostatic Load in Gambel’s White Crowned Sparrow, Zonotrichia leucophrys gambelii: Relationships With Glucocorticoids

Regulation of energetic expenditure in a changing environment, considered here as allostatic load, is central to organism-environment interactions. The value of responses that modify behavior or physiology in coping strategies is often measured in terms of energetic benefits. In this study, the total energetic cost incurred by Gambel’s white-crowned sparrows, Zonotrichia leucophrys gambelii, was assessed using heart-rate transmitters. The use of heart rate was validated as a proxy for metabolic rate via flow-through respirometry. Applying heart rate as an indicator of allostatic load, we confirmed that ambient temperature under wintering conditions influences allostatic load. However, baseline corticosterone, proposed to mediate physiological responses to variation in allostatic load, does not appear to vary with heart rate or temperature in captivity, or with temperature under ambient conditions in the field. The relationship between allostatic load and plasma corticosterone levels was also investigated by manipulating feeding effort for captive Gambel’s white-crowned sparrows using a sand-excavation challenge that approximated a type of foraging work that these birds normally perform in the wild. This experiment was designed to test the hypothesis that experimentally increased allostatic load induces elevation in baseline corticosteroids. We did not find support for this hypothesis. We suggest that the adrenocortical response to increased allostatic load may be limited to overload or environmental conditions that meaningfully threaten energy imbalance, indicating new targets for further research.

Evolution of innate behavioral strategies through competitive population dynamics

Many organism behaviors are innate or instinctual and have been “hard-coded” through evolution. Current approaches to understanding these behaviors model evolution as an optimization problem in which the traits of organisms are assumed to optimize an objective function representing evolutionary fitness. Here, we use a mechanistic birth-death dynamics approach to study the evolution of innate behavioral strategies in a simulated population of organisms. In particular, we performed agent-based stochastic simulations and mean-field analyses of organisms exploring random environments and competing with each other to find locations with plentiful resources. We find that when organism density is low, the mean-field model allows us to derive an effective objective function, predicting how the most competitive phenotypes depend on the exploration-exploitation trade-off between the scarcity of high-resource sites and the increase in birth rate those sites offer organisms. However, increasing organism density alters the most competitive behavioral strategies and precludes the derivation of a well-defined objective function. Moreover, there exists a range of densities for which the coexistence of many phenotypes persists for evolutionarily long times.

The innate, or instinctual, behavioral strategies that populations of organisms employ to navigate their environments and fend for survival are shaped over epochs of evolutionary selection, in contrast to individual behaviors that can change within an individual’s lifetime based on experience and sensory input. Understanding how the interplay between organism and their environment shapes which behavior strategies emerge as the most successful for a population’s survival is a major problem in mathematical biology. Often, evolution is modeled as an optimization process that selects for behaviors that optimize the “fitness” of organisms in their environment. However, the fundamental evolutionary events are stochastic birth and death events, and the most successful organisms that emerge under these dynamics are not always those predicted by fitness-based approaches. In this work, we use agent-based stochastic simulations and mean-field approximations of a mechanistic population dynamics model to investigate the evolution of a population’s innate foraging strategies. In particular, we investigate when an emergent fitness function can be derived and how competition between individuals for resources alters the most successful behavioral strategies and precludes the derivation of a simple fitness function that predicts the most successful behavioral strategies.

The Mystery of Energy Compensation

AbstractThe received wisdom on how activity affects energy expenditure is that the more activity is undertaken, the more calories will have been burned by the end of the day. Yet traditional hunter-gatherers, who lead physically hard lives, burn no more calories each day than Western populations living in labor-saving environments. Indeed, there is now a wealth of data, both for humans and other animals, demonstrating that long-term lifestyle changes involving increases in exercise or other physical activities do not result in commensurate increases in daily energy expenditure (DEE). This is because humans and other animals exhibit a degree of energy compensation at the organismal level, ameliorating some of the increases in DEE that would occur from the increased activity by decreasing the energy expended on other biological processes. And energy compensation can be sizable, reaching many hundreds of calories in humans. But the processes that are downregulated in the long-term to achieve energy compensation are far from clear, particularly in humans-we do not know how energy compensation is achieved. My review here of the literature on relevant exercise intervention studies, for both humans and other species, indicates conflict regarding the role, if any, of basal metabolic rate (BMR) or low-level activity such as fidgeting play, particularly once changes in body composition are factored out. In situations where BMR and low-level activity are not major components of energy compensation, what then drives it? I discuss how changes in mitochondrial efficiency and changes in circadian fluctuations in BMR may contribute to our understanding of energy management. Currently unexplored, these mechanisms and others may provide important insights into the mystery of how energy compensation is achieved.

Exposure to food insecurity increases energy storage and reduces somatic maintenance in European starlings (Sturnus vulgaris)

Birds exposed to food insecurity-defined as temporally variable access to food-respond adaptively by storing more energy. To do this, they may reduce energy allocation to other functions such as somatic maintenance and repair. To investigate this trade-off, we exposed juvenile European starlings (Sturnus vulgaris, n = 69) to 19 weeks of either uninterrupted food availability or a regime where food was unpredictably unavailable for a 5-h period on 5 days each week. Our measures of energy storage were mass and fat scores. Our measures of somatic maintenance were the growth rate of a plucked feather, and erythrocyte telomere length (TL), measured by analysis of the terminal restriction fragment. The insecure birds were heavier than the controls, by an amount that varied over time. They also had higher fat scores. We found no evidence that they consumed more food overall, though our food consumption data were incomplete. Plucked feathers regrew more slowly in the insecure birds. TL was reduced in the insecure birds, specifically, in the longer percentiles of the within-individual TL distribution. We conclude that increased energy storage in response to food insecurity is achieved at the expense of investment in somatic maintenance and repair.

Using metabolic energy to quantify the subjective value of physical effort

Economists have known for centuries that to understand an individual's decisions, we must consider not only the objective value of the goal at stake, but its subjective value as well. However, achieving that goal ultimately requires expenditure of effort. Surprisingly, despite the ubiquitous role of effort in decision-making and movement, we currently do not understand how effort is subjectively valued in daily movements. Part of the difficulty arises from the lack of an objective measure of effort. Here, we use a physiological approach to address this knowledge gap. We quantified objective effort costs by measuring metabolic cost via expired gas analysis as participants performed a reaching task against increasing resistance. We then used neuroeconomic methods to quantify each individual's subjective valuation of effort. Rather than the diminishing sensitivity observed in reward valuation, effort was valued objectively, on average. This is significantly less than the near-quadratic sensitivity to effort observed previously in force-based motor tasks. Moreover, there was significant inter-individual variability with many participants undervaluing or overvaluing effort. These findings demonstrate that in contrast with monetary decisions in which subjective value exhibits diminishing marginal returns, effort costs are valued more objectively in low-effort reaching movements common in daily life.

Food insecurity increases energetic efficiency, not food consumption: an exploratory study in European starlings

Food insecurity—defined as limited or unpredictable access to nutritionally adequate food—is associated with higher body mass in humans and birds. It is widely assumed that food insecurity-induced fattening is caused by increased food consumption, but there is little evidence supporting this in any species. We developed a novel technology for measuring foraging, food intake and body mass in small groups of aviary-housed European starlings (Sturnus vulgaris). Across four exploratory experiments, we demonstrate that birds responded to 1–2 weeks of food insecurity by increasing their body mass despite eating less. Food-insecure birds therefore increased their energetic efficiency, calculated as the body mass maintained per unit of food consumed. Mass gain was greater in birds that were lighter at baseline and in birds that faced greater competition for access to food. Whilst there was variation between experiments in mass gain and food consumption under food insecurity, energetic efficiency always increased. Bomb calorimetry of guano showed reduced energy density under food insecurity, suggesting that the energy assimilated from food increased. Behavioural observations of roosting showed inconsistent evidence for reduced physical activity under food insecurity. Increased energetic efficiency continued for 1–2 weeks after food security was reinstated, indicating an asymmetry in the speed of the response to food insecurity and the recovery from it. Future work to understand the mechanisms underlying food insecurity-induced mass gain should focus on the biological changes mediating increased energetic efficiency rather than increased energy consumption.

Constrained optimal foraging by marine bacterioplankton on particulate organic matter

Patch use theory predicts that organisms foraging in a heterogeneous resource landscape balance their residence time on a patch yielding diminishing returns with the sum of the metabolic, predation, and opportunity costs of foraging. By combining single-cell tracking with mathematical modeling, we show that bacteria foraging on seascapes of organic particles switch between attached and planktonic lifestyles and tune the time spent on particles to increase individual fitness. As predicted by patch use theory, bacteria remain longer on particles of higher quality and in poor environments, or when search times for fresh particles are longer. These results show that patch use theory can be a valuable framework to understand not only animals, but also microorganisms, and ultimately their ecosystem-level consequences.

Optimal foraging theory provides a framework to understand how organisms balance the benefits of harvesting resources within a patch with the sum of the metabolic, predation, and missed opportunity costs of foraging. Here, we show that, after accounting for the limited environmental information available to microorganisms, optimal foraging theory and, in particular, patch use theory also applies to the behavior of marine bacteria in particle seascapes. Combining modeling and experiments, we find that the marine bacterium Vibrio ordalii optimizes nutrient uptake by rapidly switching between attached and planktonic lifestyles, departing particles when their nutrient concentration is more than hundredfold higher than background. In accordance with predictions from patch use theory, single-cell tracking reveals that bacteria spend less time on nutrient-poor particles and on particles within environments that are rich or in which the travel time between particles is smaller, indicating that bacteria tune the nutrient concentration at detachment to increase their fitness. A mathematical model shows that the observed behavioral switching between exploitation and dispersal is consistent with foraging optimality under limited information, namely, the ability to assess the harvest rate of nutrients leaking from particles by molecular diffusion. This work demonstrates how fundamental principles in behavioral ecology traditionally applied to animals can hold right down to the scale of microorganisms and highlights the exquisite adaptations of marine bacterial foraging. The present study thus provides a blueprint for a mechanistic understanding of bacterial uptake of dissolved organic matter and bacterial production in the ocean—processes that are fundamental to the global carbon cycle.

The effect of learning on heart rate and behavior of European starlings (Sturnus vulgaris)

Wild-caught European starlings (Sturnus vulgaris) were exposed to a learning task to determine whether heart rate (HR) and behavior responses to the learning activated the sympathetic nervous system. Birds learned to discriminate between images of opposite convexity (concave and convex) based on shading cues in a closed economy (food only available through task completion). Once learned, the task was changed in three ways: (a) manipulating the angle and shape of the image; (b) altering the availability of the task; and (c) reversing the positive stimulus. HR, used as an index of catecholamine release, was measured during each change to determine whether having to alter previously established behaviors to learn new behaviors elicited a sympathetic response. Starlings decreased their HR during the initial discrimination training and did not alter their HR when presented with modified images or when the positive stimulus was reversed. However, HR increased when the task became unavailable and decreased upon its return, suggesting that preventing task performance was perceived as stressful. Birds also modified their behavior when tasks were changed. The number of trials per minute decreased during the reversal treatment, as did the success rate, suggesting that starlings may try to conserve energy when access to food diminishes. This is also supported by the decrease in perch hops per minute when the task was unavailable and the subsequent increase upon its return. Overall, these results suggest that learning per se does not activate the sympathetic nervous system and, therefore, is not a stressor for wild birds.

Food-Insecure Women Eat a Less Diverse Diet in a More Temporally Variable Way: Evidence from the US National Health and Nutrition Examination Survey, 2013-4

Food insecurity is associated with high body weight amongst women, but not men, in high-income countries. Previous research using food recalls suggests that the total energy intake of food-insecure women is not elevated, though macronutrient composition may differ from that of food-secure women. There is limited evidence on temporal patterns of food consumption. Here, we used food recalls from women in the 2013-4 cycle of the National Health and Nutrition Examination Survey (NHANES, n = 2798) to characterise temporal patterns of food consumption in relation to food insecurity. Compared to the food-secure, food-insecure women had more variable time gaps between eating; ate a smaller and less variable number of distinct foods at a time; were more variable from day to day in their time of first consumption; were more variable from day to day in the number of times they ate; and consumed relatively more carbohydrate, less protein, and less fibre. However, their overall energy intake was no higher. Food-insecure women had higher BMIs (2.25 kg/m²), and around 15% of the BMI difference between food-insecure and food-secure women was accounted for by their more variable time gaps between eating, their lower diversity of foods, and their lower fibre consumption. Food insecurity is associated with measureable differences in the temporal pattern of food consumption, and some of these differences shed light on how food-insecure women come to have higher body weights.

Foraging for high caloric anthropogenic prey is energetically costly

BACKGROUND

Several generalist species benefit from food provided by human activities. Food from anthropogenic sources is often high in caloric value and can positively influence reproductive success or survival. However, this type of resource may require specific foraging skills and habitat experience with related costs and benefits. As a result, not all individuals utilize these resources equally, with some individuals preferentially foraging in habitats where natural resources of lower energy content are predominant, possibly due to lower energy expenditure of the specific foraging strategy.

METHODS

Here we investigate whether foraging in habitats which contain high caloric resources of anthropogenic origin is energetically costlier than foraging in habitats with low caloric resources such as intertidal areas or agricultural and natural areas, for example due to increased flight costs, in a generalist seabird, the herring gull Larus argentatus. We use data from GPS trackers with tri-axial acceleration measurements that allow us to quantify time-energy budgets, representing energy expenditure during foraging trips of herring gulls for each habitat.

RESULTS

We show that the rate of energy expenditure is on average 34% higher when individuals forage for high caloric prey in marine and urban areas compared to foraging for low caloric prey in intertidal and agricultural areas. Energetic estimates suggest that if birds would feed completely on these resources, they have to gather ~ 400 kJ per day more to compensate for the higher foraging costs.

CONCLUSIONS

Energy expenditure differs among foraging habitat and may thereby influence foraging decisions of individual herring gulls. As management of anthropogenic resources changes, so too may the costs and potential benefits of foraging strategies which are strongly tied to human activities.

The Physiology of Exercise in Free-Living Vertebrates: What Can We Learn from Current Model Systems?

SYNOPSIS

Many behaviors crucial for survival and reproductive success in free-living animals, including migration, foraging, and escaping from predators, involve elevated levels of physical activity. However, although there has been considerable interest in the physiological and biomechanical mechanisms that underpin individual variation in exercise performance, to date, much work on the physiology of exercise has been conducted in laboratory settings that are often quite removed from the animal's ecology. Here we review current, laboratory-based model systems for exercise (wind or swim tunnels for migration studies in birds and fishes, manipulation of exercise associated with non-migratory activity in birds, locomotion in lizards, and wheel running in rodents) to identify common physiological markers of individual variation in exercise capacity and/or costs of increased activity. Secondly, we consider how physiological responses to exercise might be influenced by (1) the nature of the activity (i.e., voluntary or involuntary, intensity, and duration), and (2) resource acquisition and food availability, in the context of routine activities in free-living animals. Finally, we consider evidence that the physiological effects of experimentally-elevated activity directly affect components of fitness such as reproduction and survival. We suggest that developing more ecologically realistic laboratory systems, incorporating resource-acquisition, functional studies across multiple physiological systems, and a life-history framework, with reproduction and survival end-points, will help reveal the mechanisms underlying the consequences of exercise, and will complement studies in free-living animals taking advantage of new developments in wildlife-tracking.

Individual variation in metabolic reaction norms over ambient temperature causes low correlation between basal and standard metabolic rate

Basal metabolic rate (BMR) is often assumed to be indicative of the energy turnover at ambient temperatures (Ta) below the thermoneutral zone (SMR), but this assumption has remained largely untested. Using a new statistical approach, we quantified the consistency in nocturnal metabolic rate across a temperature range in zebra finches (n=3,213 measurements on 407 individuals) living permanently in eight outdoor aviaries. Foraging conditions were either benign or harsh, and body mass and mass-adjusted BMRm and SMRm were lower in individuals living in a harsh foraging environment. The correlation between SMRm at different Tas was high (r=0.91), independent of foraging environment, showing that individuals are consistently ranked according to their SMRm. However, the correlations between BMRm and SMRm were always lower (average: 0.29; range: 0<r<0.50), in particular in the benign foraging environment. Variation in metabolic response to lower Ta at least in part reflected differential body temperature (Tb) regulation: early morning Tb was lower at low Ta's, and more so in individuals with a weaker metabolic response to lower Ta's. Our findings have implications for the use of BMR in the estimation of time-energy budgets and comparative analyses: we suggest that the use of metabolic rates at ecologically relevant ambient temperatures, such as the easily tractable SMR, will be more informative than the use of BMR as a proxy for energy turnover.

Constrained Total Energy Expenditure and Metabolic Adaptation to Physical Activity in Adult Humans

Current obesity prevention strategies recommend increasing daily physical activity, assuming that increased activity will lead to corresponding increases in total energy expenditure and prevent or reverse energy imbalance and weight gain [1-3]. Such Additive total energy expenditure models are supported by exercise intervention and accelerometry studies reporting positive correlations between physical activity and total energy expenditure [4] but are challenged by ecological studies in humans and other species showing that more active populations do not have higher total energy expenditure [5-8]. Here we tested a Constrained total energy expenditure model, in which total energy expenditure increases with physical activity at low activity levels but plateaus at higher activity levels as the body adapts to maintain total energy expenditure within a narrow range. We compared total energy expenditure, measured using doubly labeled water, against physical activity, measured using accelerometry, for a large (n = 332) sample of adults living in five populations [9]. After adjusting for body size and composition, total energy expenditure was positively correlated with physical activity, but the relationship was markedly stronger over the lower range of physical activity. For subjects in the upper range of physical activity, total energy expenditure plateaued, supporting a Constrained total energy expenditure model. Body fat percentage and activity intensity appear to modulate the metabolic response to physical activity. Models of energy balance employed in public health [1-3] should be revised to better reflect the constrained nature of total energy expenditure and the complex effects of physical activity on metabolic physiology.

Intermittent Food Absence Motivates Reallocation of Locomotion and Feeding in Spotted Munia (Lonchura punctulata)

Background: Daily feeding and locomotion are interrelated behaviours. The time spent in feeding and rate of food intake depends on food availability. In low food condition, the birds would show intense movement (locomotion) for a longer time throughout the day however during abundant food supply they may chose higher activity and food intake in the morning and evening only. In the present study we hypothesized that in Spotted Munia (Lonchura punctulata), intermittent food availability during day would reallocate their interrelated behaviors, the feeding (food intake) and locomotor activity patterns.

Methods: Two groups of birds (N = 6 each) were kept individually in activity cages under 12L:12D. Group 1 (Control; C) had ad libitum food but group 2 (Treatment; T) had food for 6 hours only (2 h presence followed by 2 h absence; 2P:2A) during 12 hour light period. In the first week, group 2 received food with ‘lights on’ (TI; ZT 0–2, 4–6 and 8–10; where ZT 0= zeitgeber time 0, time of lights ON). In the following week, the food was given 2 hours after ‘lights on’ (TII; ZT 2–4, 6–8, 10–12). The food intake and locomotor activity under each condition were observed.

Results: The results showed that locomotor activity was induced during food deprivation and suppressed during food availability. Also the food deprivation led to increased food intake.

Conclusion: Our results suggest that intermittent food availability/deprivation reallocates the locomotor activity and food intake in Spotted Munia.

Asymmetry in food handling behavior of a tree-dwelling rodent (Sciurus vulgaris)

Asymmetry in motor patterns is present in a wide variety of animals. Many lateralized behaviors seem to depend on brain asymmetry, as it is the case of different tasks associated to food handling by several bird and mammal species. Here, we analyzed asymmetry in handling behavior of pine cones by red squirrels (Sciurus vulgaris). Red squirrels devote most of their daily activity to feeding, thus this species constitutes an appropriate model for studying asymmetry in food processing. We aimed to explore 1) the potential lateralization in handling of pine cones by squirrels, 2) the dominant pattern for this behavior (left- vs. right-handed), and 3) whether this pattern varies among populations and depending on the pine tree species available. Results revealed that red squirrels handle pine cones in an asymmetrical way, and that direction of asymmetry varies among populations and seems to be determined more by local influences rather than by the pine tree species.

The anterior cingulate gyrus signals the net value of others' rewards

Evaluating the costs and benefits of our own choices is central to most forms of decision-making and its mechanisms in the brain are becoming increasingly well understood. To interact successfully in social environments, it is also essential to monitor the rewards that others receive. Previous studies in nonhuman primates have found neurons in the anterior cingulate cortex (ACC) that signal the net value (benefit minus cost) of rewards that will be received oneself and also neurons that signal when a reward will be received by someone else. However, little is understood about the way in which the human brain engages in cost-benefit analyses during social interactions. Does the ACC signal the net value (the benefits minus the costs) of rewards that others will receive? Here, using fMRI, we examined activity time locked to cues that signaled the anticipated reward magnitude (benefit) to be gained and the level of effort (cost) to be incurred either by a subject themselves or by a social confederate. We investigated whether activity in the ACC covaries with the net value of rewards that someone else will receive when that person is required to exert effort for the reward. We show that, although activation in the sulcus of the ACC signaled the costs on all trials, gyral ACC (ACC(g)) activity varied parametrically only with the net value of rewards gained by others. These results suggest that the ACC(g) plays an important role in signaling cost-benefit information by signaling the value of others' rewards during social interactions.

Seasonal variation in energy expenditure is not related to activity level or water temperature in a large diving bird

There is considerable interest in understanding how the energy budget of an endotherm is modulated from a physiological and ecological point of view. In this paper, we used the heart rate method and daily heart rate (DHR), as a proxy of DEE across seasons, to test the effect of locomotion activity and water temperature on the energy budget of a large diving bird. DHR was monitored continuously in common eiders (Somateria mollissima) during seven months together with measures of time spent flying and time spent feeding. DHR varied substantially during the recording period with numerous increases and decreases that occurred across seasons although we could not find any relationship between DHR and the time spent active (feeding and flying). However, inactive heart rate (IHR) decreased as locomotion activity increases suggesting common eiders were using behavioural compensation when under a high work load. We were also unable to detect a negative relationship between water temperature and resting heart rate, a proxy of resting metabolic rate. This was unexpected based on the assumption that high thermoregulation costs would be associated with cold waters. We showed that high level of energy expenditure coincided with feather moult and warm waters, which suggest that the observed variable pattern of seasonal DEE was driven by feather growth and possibly by other productive costs. Nevertheless, our results indicate that behavioural compensation and possibly the timing of moult may be used as mechanisms to reduce seasonal variation in energy expenditure.

Bioenergetics and intestinal phenotypic flexibility in the microbiotherid marsupial (Dromiciops gliroides) from the temperate forest in South America

The microbiotherid marsupial Dromiciops gliroides inhabits the temperate forests of the Southern hemisphere, facing seasonal nutritional and energetic bottlenecks due to its apparently facultative insectivory/frugivory. In order to understand the physiological processes behind this ecological pattern, we studied the morpho-physiological changes that D. gliroides exhibits after dietary acclimation, in a sample of 21 wild-caught individuals fed over 1 month with ad libitum diet of: (1) fruit, (2) insects or (3) a mix of insects and fruit. In addition, we measured oxygen consumption (VO(2)) at resting conditions. We also performed enzyme assays (sucrase, maltase, trehalase and aminopeptidase N) and measurements of organ morphology. We found that D. gliroides cannot fulfil its nutrient requirements only from insects or fruit. It needs a mixed diet in order to maintain its body mass and energy balance. However, as a response of diet acclimation, individuals showed several-fold changes in the activities of aminopeptidase-N, maltase and sucrase (but not trehalase). This result, both the magnitude of change and the simultaneous effects on three enzymes suggests that D. gliroides could exhibit adaptive phenotypic plasticity in the activity of intestinal enzymes. This study suggests also that D. gliroides, the only living representative of the Microbiotheria order, exhibits physiological adaptations to a generalist diet.

Spatio-temporal variation in territory quality and oxidative status: a natural experiment in the Seychelles warbler (Acrocephalus sechellensis)

1. Fluctuations in the quality of the habitat in which an animal lives can have major consequences for its behaviour and physiological state. In poor-quality habitat with low food availability, metabolically intensive foraging activity is likely to result in increased generation of reactive oxygen species, while scarcity of food can lead to a weakening of exogenously derived antioxidant defences. The consequent oxidant/antioxidant imbalance may lead to elevated oxidative stress. 2. Although the link between food availability and oxidative stress has been studied in the laboratory, very little is known about this relationship in the wild. Here, we investigate the association between territory quality (measured through food availability) and oxidative stress in the Seychelles warbler (Acrocephalus sechellensis). 3. Seychelles warblers are insectivorous birds that inhabit a fixed feeding territory year round. Individuals experience profound and rapid local fluctuations in territory quality within these territories, owing to changing patterns of vegetation defoliation resulting from seasonal changes in prevailing wind direction and wind-borne salt spray. 4. As expected, oxidant generation (measured as reactive oxygen metabolites; ROMs) was higher when territory quality was low, but there was no correlation between territory quality and antioxidant capacity (OXY). The negative correlation between territory quality and ROMs was significant between individuals and approached significance within individuals, indicating that the pattern resulted from individual responses to environmental variation. 5. ROMs and OXY levels within individuals were positively correlated, but the relationship between territory quality and ROMs persisted after including OXY as a covariate, implying that oxidative stress occurs in low territory quality conditions. 6. Our results indicate that the oxidative stress balance of an individual is sensitive to relatively short-term changes in territory quality, which may have consequences for the birds' fitness.

Limited access to food and physiological trade-offs in a long-distance migrant shorebird. II. Constitutive immune function and the acute-phase response

In response to unbalanced energy budgets, animals must allocate resources among competing physiological systems to maximize fitness. Constraints can be imposed on energy availability or energy expenditure, and adjustments can be made via changes in metabolism or trade-offs with competing demands such as body-mass maintenance and immune function. This study investigates changes in constitutive immune function and the acute-phase response in shorebirds (red knots) faced with limited access time to food. We separated birds into two experimental groups receiving either 6 h or 22 h of food access and measured constitutive immune function. After 3 wk, we induced an acute-phase response, and after 1 wk of recovery, we switched the groups to the opposite food treatment and measured constitutive immune function again. We found little effect of food treatment on constitutive immune function, which suggests that even under resource limitation, a baseline level of immune function is maintained. However, birds enduring limited access to food suppressed aspects of the acute-phase response (decreased feeding and mass loss) to maintain energy intake, and they downregulated thermoregulatory adjustments to food treatment to maintain body temperature during simulated infection. Thus, under resource-limited conditions, birds save energy on the most costly aspects of immune defense.

Shifts in metabolic demands in growing altricial nestlings illustrate context-specific relationships between basal metabolic rate and body composition

Basal metabolic rate (BMR) in animals is interpreted as reflecting the size and metabolic intensity of energy-consuming tissues. However, studies investigating relationships between the mass of specific organs and interindividual variation in BMR have produced inconsistent patterns with regard to which organs have the largest impact on BMR variation. Because of the known flexibility in organ mass and metabolic intensity within individual organs, relationships between BMR and body-composition variables are bound to be context specific. Altricial nestlings are excellent models to illustrate this phenomenon because of the extreme variation in body composition occurring during growth. Using European starlings at three age classes, we studied changes in body composition together with its effect on variation in resting metabolic rate (RMR) in order to highlight the context-specific nature of these relationships. Our data suggest a transition in metabolic costs during growth in starling nestlings. During the linear phase of growth, energy is mainly consumed by tissue-synthesis processes, with fast-growing organs having a large influence on RMR. In the plateau phase of growth, the energy expenditure is transferred to functional costs, with high-intensity organs having a predominant effect on RMR variation. Our data illustrates the context-specific nature of organ mass-metabolic rate correlations, which complicates inter- and intraspecific comparisons of BMR. In the future, such comparisons must be done while taking the physiological state of the study animal into account.

Great ranging associated with greater reproductive investment in mammals

Most animals must travel to find food, incurring an unavoidable energy and time cost. Economic theory predicts, and experimental work confirms, that within species, increasing the distance traveled each day to find food has negative fitness consequences, decreasing the amount of energy invested in maintenance, repair, and reproduction. Here, we show that this relationship between daily distance traveled and reproductive success is fundamentally different between species and over evolutionary time in many lineages. Phylogenetically controlled analyses of 161 eutherian mammals indicate that, after controlling for body mass, evolutionary increases in the daily distance traveled are associated with corresponding increases in both total fertility (number of offspring per lifetime) and total offspring mass (grams of offspring per lifetime). This suggests that over evolutionary time, increasing travel distance is often part of a strategy for procuring more food energy and not necessarily a response to decreased food availability. These results have important implications for ecological comparisons among species, including assessments of habitat quality based on locomotor behavior.

Female mice respond differently to costly foraging versus food restriction

Experimental manipulation of foraging costs per food reward can be used to study the plasticity of physiological systems involved in energy metabolism. This approach is useful for understanding adaptations to natural variation in food availability. Earlier studies have shown that animals foraging on a fixed reward schedule decrease energy intake and expenditure. However, the extent to which these changes depend on decreased food intake or increased foraging costs per se has never been tested. We manipulated foraging costs per food reward in female Hsd:ICR(CD-1) laboratory mice, comparing animals faced with low (L) and high (H) foraging costs to non-foraging animals receiving a food restriction (R) matched to the intake of H animals. Mice in the H group ran as much as L mice did but ate significantly less. They concurrently reduced daily energy expenditure and resting metabolic rate, decreased the size of major metabolic organs and utilized body fat stores; mass-specific resting metabolic rate did not differ between groups. We found evidence that these alterations in energy balance may carry fitness costs. As a secondary response to our experimental treatment, H females and, eventually, some R females ceased to show signs of estrous cyclicity. Surprisingly, results of an immune challenge with keyhole limpet hemocyanin showed that primary immune response did not differ between L and H groups, and was actually higher in R mice. Our results demonstrate that high foraging costs per se--the combination of high activity and low food intake--have pronounced physiological effects in female mice.

Flightlessness and the energetic cost of wing molt in a large sea duck

Although the replacement of feathers apparently represents the major event of somatic production in the annual cycle of wild birds, knowledge about the energetics of molt has always been hampered by logistical and technical difficulties, which are exacerbated by the fact that birds are able to compensate behaviorally to buffer any variation in energy demand. During wing molt, sea ducks (Mergini) and other diving birds lose all of their wing feathers at once, leading to a period of temporary flightlessness of variable duration, a condition that considerably restricts their movements and increases the probability of predation. In the present study, we present the first results aimed at quantifying the duration of flightlessness, energy expenditure, and foraging effort during molt of a wing-propelled diving bird, the Common Eider (Somateria mollissima). Data loggers were implanted in the body cavity of 13 females to record heart rate and hydrostatic pressure (depth) every two seconds for a period of 220 days. Flight frequency and duration were assessed from elevated and constant heart rate, and the absence of flight was used to quantify the duration of flightlessness, which lasted, on average, 36 +/- 8 days (mean +/- SD). Using a period of four weeks before and four weeks after the flightless period, we found that dive depth (ranging from 1 to 2 m, on average) and daily diving time did not vary during the course of the study. Daily metabolic rate increased by 9%, and resting metabolic rate by 12% from the pre-molt period to the flightless period and remained high during the post-molt period. This study indicates that the energetic costs of replacing flight remiges in female eiders are substantial, although this is not associated with any change in foraging effort, which suggests that female Common Eiders lose mass during wing molt. Finally, estimates of energy savings associated with the total absence of flights during wing molt represent 6% of daily metabolic rate or 14% of resting metabolic rate. This finding contrasts with the classical view that little or no benefit is associated with a flightless condition. We suggest that such energy savings may have favored the evolution of temporary flightlessness in diving birds.

Behavioural and physiological responses to increased foraging effort in male mice

Free-living animals must forage for food and hence may face energetic constraints imposed by their natural environmental conditions (e.g. ambient temperature, food availability). Simulating the variation in such constraints, we have experimentally manipulated the rate of work (wheel running) mice must do to obtain their food, and studied the ensuing behavioural and physiological responses. This was done with a line of mice selectively bred for high spontaneous wheel running and a randomly bred control line that vary in the amount of baseline wheel-running activity. We first determined the maximum workload for each individual. The maximum workload animals could engage in was around 23 km d(-1) in both control and activity-selected mice, and was not associated with baseline wheel-running activity. We then kept mice at 90% of their individual maximum and measured several physiological and behavioural traits. At this high workload, mice increased wheel-running activity from an average of 10 to 20 km d(-1), and decreased food intake and body mass by approximately 20%. Mass-specific resting metabolic rate strongly decreased from 1.43 to 0.98 kJ g(-1) d(-1), whereas daily energy expenditure slightly increased from 2.09 to 2.25 kJ g(-1) d(-1). Costs of running decreased from 2.3 to 1.6 kJ km(-1) between baseline and workload conditions. At high workloads, animals were in a negative energy balance, resulting in a sharp reduction in fat mass as well as a slight decrease in dry lean mass. In addition, corticosterone levels increased, and body temperature was extremely low in some animals at high workloads. When challenged to work for food, mice thus show significant physiological and behavioural adjustments.

Assessing western gull predation on purple sea urchins in the rocky intertidal using optimal foraging theory

Purple sea urchins ( Strongylocentrotus purpuratus (Stimpson, 1857)) are abundant grazing invertebrates that can have a major impact on the rocky intertidal community. Predators can control the urchin population and indirectly reduce grazing activity. We determined the effects of western gull ( Larus occidentalis Audubon, 1839) predation on purple sea urchins in the rocky intertidal using the framework of optimal foraging theory and taking into account different prey-handling techniques. We recorded the foraging behavior of gulls, measured urchin availability, and estimated prey caloric content with bomb calorimetry. Western gulls selected purple sea urchins significantly more than other prey items (snails (genus Tegula Lesson, 1835), limpets (genus Collisella Dall, 1871), sea stars ( Pisaster giganteus (Stimpson, 1857) and Pisaster ochraceus (Brandt, 1835))). Larger urchins contained relatively more calories. Gulls foraged optimally when pecking by frequently selecting the most profitable size class. However, gulls chose smaller urchins than expected when air-dropping, which could have been influenced by group size and age. Gulls selected smaller purple sea urchins when foraging in larger groups likely owing to the risk of kleptoparasitism. Adults chose larger, and juveniles smaller, urchins when air-dropping, suggesting that juveniles are less experienced in foraging techniques. We estimated that gull predation could affect up to one third of the sea urchin populations locally, which could increase species diversity in the rocky intertidal community.

Energetic influence on gull flight strategy selection

During non-migratory flight, gulls (Larids) use a wide variety of flight strategies. We investigate the extent to which the energy balance of a bird explains flight strategy selection. We develop a model based on optimal foraging and aerodynamic theories, to calculate the ground speeds and airspeeds at which a gull is expected to flap or soar during foraging flight. The model results are compared with observed flight speeds, directions, and flight strategies of two species of gulls, the black-headed gull Larus ridibundus and the lesser black-backed gull Larus fuscus. The observations were made using a tracking radar over land in The Netherlands. The model suggests that, especially at combinations of low ground speed (approximately 5-10 m s(-1)), high air speed (approximately 20-25 m s(-1)) and low ground and air speed, gulls should favor soaring flight. At intermediate ground and air speeds the predicted net energy gain is similar for soaring and flapping. Hence the ratio of flapping to soaring may be higher than for other air and ground speed combinations. This range of speeds is broadest for black-headed gulls. The model results are supported by the observations. For example, flapping is more prevalent at speeds where the predicted net energy gain is similar for both strategies. Interestingly, combinations of air speed and flight speed that, according to the model, would result in a loss of net energy gain, were not observed. Additional factors that may influence flight strategy selection are also briefly discussed.

Short-term energy regulation of whitebellied sunbirds (Nectarinia talatala): effects of food concentration on feeding frequency and duration

Avian nectarivores show compensatory feeding by adjusting their volumetric intake in response to variation in nectar concentration. This study used an infrared photo-detection system to investigate the short-term feeding patterns of whitebellied sunbirds (Nectarinia talatala) consuming three different sucrose concentrations (10, 20 and 30% w/w). Sunbirds increased their feeding frequency on the most dilute diet, but there was no change in feeding duration. Thus, the increase in total time spent feeding on the dilute diet was due to the increased feeding frequency. No difference in short-term feeding patterns was found between the 20% and 30% diets. Total time spent feeding was extremely short on all diets (96-144 s in every hour). Birds maintained the same steady increase in body mass over the course of the day on all three diets. Daily rhythms in feeding patterns were evident, with longer feeding duration and lower feeding frequency in the early morning and evening than during the rest of the day. Because ingestion rates on a particular diet may vary through the day, caution must be exercised in using feeding duration as a surrogate for meal size. Individual birds varied greatly in their feeding patterns irrespective of diet concentration.

Metabolic adjustments to increasing foraging costs of starlings in a closed economy

Knowledge of the physiological consequences of variation in food availability may be essential for understanding behavioural and life history responses to such variation. To study the physiological consequences of food availability animals are generally subjected to caloric restriction or starvation, thereby reducing the upper limit to the energy budget. The relevance of this approach to free-living animals is questionable, however, because under natural conditions low food availability often results in higher foraging costs, and everything else remaining equal this results in a higher energy budget. We manipulated food availability by varying the foraging costs and studied effects on daily energy expenditure (DEE) and energy allocation of captive starlings Sturnus vulgaris. Birds in a closed economy earned their food by flying between two perches 5 m apart. The probability of a reward was set at three different levels, thereby creating a 'poor', 'intermediate' and 'rich' environment. Compared with the rich environment, birds flew 4 times more (2.3 h per day) in the poor environment, and increased DEE by 43% to 220 kJ day-1 (3.7xBMR), within the range of free-living parents rearing young. To our knowledge this is the first study to show an increase in DEE with decreasing food availability. Body mass, basal metabolic rate (BMR) and pectoral muscle size were reduced in the poor environment. Nocturnal energy expenditure was further reduced by reaching BMR earlier in the night. Calculations show that the energy demands in the poor environment could not be met with the flight costs of 20.5 W that we measured previously in a rich environment. Flight costs derived indirectly from the energy budget were lower, at 17.5 W, probably due to lower body mass. By reducing body mass by 20%, and economising during sleep, the birds achieved savings of 37% in their DEE. Without these savings, a DEE substantially higher than measured in free-living parents rearing young would be required to remain in energy balance. Surprisingly little data exist to verify whether free-living animals use the same tactics to survive periods with low food availability.

Effects of intake rate on energy expenditure, somatic repair and reproduction of zebra finches

Understanding the effect of food availability on food requirements is critical when linking food availability e.g. to reproduction or habitat selection. Decreasing intake rate (intake per unit foraging effort) can be expected to increase daily energy expenditure (DEE), due to increased foraging costs. However, all the studies we could find that have tested this hypothesis (with one exception) found DEE to be constant or decreasing when intake rate was experimentally decreased. This may be due to the design of the reward schedule, which can be fixed (e.g. 20 units effort required for each reward) or variable (e.g. each unit effort rewarded with probability 1/20). Most studies used fixed reward rates, but foraging motivation is generally higher for variable reward rates, and the only study in which animals increased DEE when intake rate decreased used variable reward rates. To assess the generality of this result, we exposed zebra finches Taeniopygia guttata to different intake rates using variable reward rates. We decreased intake rate by mixing 25 g of seeds with 0, 25 or 75 g of chaff. With increasing chaff/seed ratio the time spent foraging increased from 6% to 27%, but this was insufficient to compensate for the lower intake rate, because DEE decreased by 6.6%. Body mass was independent of chaff/seed ratio. Effects of intake rate on foraging time and DEE were stronger at lower temperatures, when DEE was higher. The decrease in DEE in adverse conditions raises the question of what prevents such behaviour in benign circumstances. We hypothesize that energy is saved at the expense of ;condition', and we tested this hypothesis in two ways. Firstly, we tested the effect of intake rate on the replacement of two plucked tail feathers (a form of somatic repair). Replacement feathers were shorter when intake rate was low, indicating an effect of intake rate on somatic repair ability. Secondly, we tested for carry-over effects of intake rate on reproduction, by giving pairs the opportunity to reproduce with access ad libitum to food after feeding on one of the three chaff/seed ratios for 6 weeks. The interval until laying the first egg increased with decreasing intake rate in the preceding 6 weeks. The effects of intake rate on somatic maintenance and reproduction may explain why birds sustained higher metabolic rates than apparently necessary, but the physiological mechanisms determining the optimal metabolic rate remain to be discovered.

A Theory of Associating Food Types with Their Postingestive Consequences

Animals often face complex and changing food environments. While such environments are challenging, an animal should make an association between a food type and its properties (such as the presence of a nutrient or toxin). We use information theory concepts, such as mutual information, to establish a theory for the development of these associations. In this theory, associations are assumed to maximize the mutual information between foods and their consequences. We show that associations are invariably imperfect. An association's accuracy increases with the length of a feeding session and the relative frequency of a food type but decreases as time delay between consumption and postingestive consequence increases. Surprisingly, the accuracy of an association is independent of the number of additional food types in the environment. The rate of information transfer between novel foods and a forager depends on the forager's diet. In light of this theory, an animal's diet may have two competing goals: first, the provision of an appropriate balance of nutrients, and second, the ability to quickly and accurately learn the properties of novel foods. We discuss the ecological and behavioral implications of making associational errors and contrast the timescale and mechanisms of our theory with those of existing theory.