Does gut function limit hummingbird food intake?

Many nectar-feeding bird species decrease food intake when sugar concentration in food is increased. This feeding response can be explained by two alternative hypotheses: compensatory feeding and physiological constraint. The compensatory feeding hypothesis predicts that if birds vary intake to maintain a constant energy intake to match energy expenditures, then they should increase intake when expenditures are increased. Broad-tailed hummingbirds were presented with sucrose solutions at four concentrations (292, 584, 876, and 1,168 mmol L(-1)) and exposed to two environmental temperatures (10 degrees and 22 degrees C). Birds decreased volumetric food intake in response to sugar concentration. However, when they were exposed to a relatively sudden drop in environmental temperature and, hence, to an acute increase in thermoregulatory energy expenditures, they did not increase their rate of energy consumption and lost mass. These results support the existence of a physiological constraint on feeding intake. A simple chemical reactor model based on intestinal morphology and in vitro measurements of sucrose hydrolysis predicted observed intake rates closely. This model suggests that intestinal sucrose hydrolysis rates were near maximal and, thus, may have imposed limits to sugar assimilation. Although sugar assimilation was high (95%), the proportions of excreted sucrose, glucose, and fructose found in excreta differed significantly. The monosaccharides glucose and fructose were about eight and three times more abundant than sucrose, respectively. Broad-tailed hummingbirds are small high-altitude endotherms that face unpredictable weather and the energetic expense of premigratory fattening. Digestive processes have the potential to impose severe challenges to their energy budgets.     

Gastrin releasing peptide-29 evokes feeding responses in the rat

In mammals, gastrin releasing peptide (GRP) 10 and 27 reduce food intake. In the current work, we test the hypothesis that GRP-29, the large molecular form of GRP in the rat, also evokes feeding responses consistent with a possible role in satiety. Here, we measured three feeding responses, size of first meal, intermeal interval (IMI, time between first and second meal) and satiety ratio (SR, satiation period for every unit of food consumed in the first meal), in overnight food deprived rats following GRP-10, 27 or 29 (0, 0.3, 1.0, 2.1, 4.1, 10.3, 17.2 nmol/kg) intraperitoneally and presentation of a 10% sucrose test diet. GRP-29 and GRP-27 reduced the size of the first meal, prolonged IMI and increased SR, but GRP-10 failed to exhibit similar feeding responses. The order of potency was GRP-29 = GRP-27 > GRP-10. The current data support a role for GRP-29 in the short-term regulation of food intake.     

Temporal patterning of feeding by hummingbirds

For five species of hummingbirds in the laboratory, time between meals was related to energy intake on the first meal and rate of energy expenditure between meals. Field observations gave similar results. Average meal sizes were similar at one intake rate independent of food caloric density; females averaged longer bouts than males. When rate of intake was approximately halved, meal duration approximately doubled and volume intake remained similar. We postulate that feeding is initiated when crop contents reach a lower threshold and that feeding is terminated after ingestion of an optimal volume determined by the added weight of the meal.     

Temporal control of feeding behaviour and its association with gastrointestinal function

This paper summarises knowledge about temporal control of ad libitum feeding in poultry, from minute to minute, hour to hour and day to day, and about how it relates to aspects of gastrointestinal function. Evidence is presented of only loose control over initiation and termination of spontaneous meals, and it is proposed that degrees of hunger and satiety determine probabilities of feeding starting and stopping. Voluntary regulation of food intake can be considered in terms of adjustments in mean meal size, meal frequency or both. Short-term variation is associated more with meal frequency and longer-term changes more with meal size. Short-term adjustments appear to depend more on alimentary control and longer-term adjustments more on metabolic control (not considered here). Long-term changes affecting meal size are associated with changes in capacity of parts of the alimentary tract. Food can accumulate in the crop and gizzard, and meal initiation and termination are associated with varying degrees of emptying and filling of these diverticula during most of the day. Later in the day there is usually a conditioned change to cumulative filling of the crop (and gizzard) with food that is digested overnight. Possible roles of osmo-/chemoreceptors and gut peptides are discussed.     

Metabolic State Alters Economic Decision Making under Risk in Humans

Background

Animals'' attitudes to risk are profoundly influenced by metabolic state (hunger and baseline energy stores). Specifically, animals often express a preference for risky (more variable) food sources when below a metabolic reference point (hungry), and safe (less variable) food sources when sated. Circulating hormones report the status of energy reserves and acute nutrient intake to widespread targets in the central nervous system that regulate feeding behaviour, including brain regions strongly implicated in risk and reward based decision-making in humans. Despite this, physiological influences per se have not been considered previously to influence economic decisions in humans. We hypothesised that baseline metabolic reserves and alterations in metabolic state would systematically modulate decision-making and financial risk-taking in humans.

Methodology/Principal Findings

We used a controlled feeding manipulation and assayed decision-making preferences across different metabolic states following a meal. To elicit risk-preference, we presented a sequence of 200 paired lotteries, subjects'' task being to select their preferred option from each pair. We also measured prandial suppression of circulating acyl-ghrelin (a centrally-acting orexigenic hormone signalling acute nutrient intake), and circulating leptin levels (providing an assay of energy reserves). We show both immediate and delayed effects on risky decision-making following a meal, and that these changes correlate with an individual''s baseline leptin and changes in acyl-ghrelin levels respectively.

Conclusions/Significance

We show that human risk preferences are exquisitely sensitive to current metabolic state, in a direction consistent with ecological models of feeding behaviour but not predicted by normative economic theory. These substantive effects of state changes on economic decisions perhaps reflect shared evolutionarily conserved neurobiological mechanisms. We suggest that this sensitivity in human risk-preference to current metabolic state has significant implications for both real-world economic transactions and for aberrant decision-making in eating disorders and obesity.     

Mechanical determinants of nectar-feeding energetics in butterflies: muscle mechanics,feeding geometry,and functional equivalence

Summary We develop a mechanistic model for nectar feeding in butterflies that integrates the two basic components of the feeding process: the fluid dynamics of nectar flow through the food canal and the contractile mechanics of the muscular, cibarial pump. We use the model to predict the relation between rate of energy intake during feeding and nectar concentration. We then identify nectar concentations that maximize energy intake rates (the optimal concentrations). We illustrate the model using measurements of the food canal and cibarium of Pieris butterflies. The model predicts an overall optimal range of nectar concentration of 31–39% sucrose for butterflies, which is in agreement with previously reported laboratory values. The model also predicts an interaction among the geometries of the food canal, the cibarial cavity, and the cibarial muscles, that allows us to identify the combinations of food canal, cibarium, and muscle dimensions that yield the highest rates of energy intake. Nectar-feeding is functionally equivalent in butterflies and hummingbirds: two physically different feeding mechanisms can yield identical energy intake rates. This equivalence results from a mathematical and physical similarity between quasi-steady-state fluid flow in hummingbrid tongues and the force-velocity characteristics of contracting cibarial muscle in butterflies.     

Time and energy constraints and the relationships between currencies in foraging theory

Measured foraging strategies often cluster around values thatmaximize the ratio of energy gained over energy spent whileforaging (efficiency), rather than values that would maximizethe long-term net rate of energy gain (rate). The reasons forthis are not understood. This paper focuses on time and energyconstraints while foraging to illustrate the relationship betweenefficiency and rate-maximizing strategies and develops modelsthat provide a simple framework to analyze foraging strategiesin two distinct foraging contexts. We assume that while capturingand ingesting food for their own use (which we term feeding),foragers behave so as to maximize the total net daily energeticgain. When gathering food for others or for storage (which weterm provisioning), we assume that foragers behave so as tomaximize the total daily delivery, subject to meeting theirown energetic requirements. In feeding contexts, the behaviormaximizing total net daily gain also maximizes efficiency whendaily intake is limited by the assimilation capacity. In contrast,when time available to forage sets the limit to gross intake,the behavior maximizing total net daily gain also maximizesrate. In provisioning contexts, when daily delivery is constrainedby the energy needed to power self-feeding, maximizing efficiencyensures the highest total daily delivery. When time needed torecoup energetic expenditure limits total delivery, a low self-feedingrate relative to the rate of energy expenditure favors efficientstrategies. However, as the rate of self-feeding increases,foraging behavior deviates from efficiency maximization in thedirection predicted by rate maximization. Experimental manipulationsof the rate of self-feeding in provisioning contexts could bea powerful tool to explore the relationship between rate andefficiency-maximizing behavior.     

Dynamic energy and time budgets in hummingbirds: a study in Sephanoides sephaniodes

Temperature and diet quality are two of the most important factors affecting the dynamic regulation of animal energy budgets. Because hummingbirds are very sensitive to energy stress, we used Green-backed Firecrowns (Sephanoides sephaniodes) to test the dynamics of their energy budget under different energetic challenges in chronic conditions (20 days). Experimental groups were: HQ-TNZ (high quality diet and thermoneutrality), HQ-LT (high quality diet and low temperature), LQ-TNZ (low quality diet and thermoneutrality), and LQ-LT (low quality diet and low temperature). Analysis of behavioral, morphological, and physiological variables revealed that thermal and dietary factors affect time and energy budgets independently. Hummingbirds increased energy intake during the first day of acclimation at LT, but after second day, the LQ-LT group did not maintain their energy intake and began to loose body mass. Moreover, diet quality affected digestive organs. The intestine, gizzard, liver and kidneys all increased in size when food quality was lowest, probably to obtain more food per feeding event and to more efficiently process the ingested food. Exposure to low ambient temperatures affected the most metabolically productive organs such as the heart, lungs, and muscular mass. Lower temperature increased basal and daily energy expenditure, and changed the time budget. Sephanoides sephaniodes spent more time perching when their energy balance was close to be negative. We suggest that energy budget regulation in hummingbirds does not reside exclusively in the energy output nor in the energy-input but in both pathways.     

Structure-activity relationship of linear tetrapeptides Tic-DPhe-Arg-Trp-NH2 at the human melanocortin-4 receptor and effects on feeding behaviors in rat

The melanocortin subtype-4 receptor (MC4R) has been implicated in the control of feeding behavior and body weight regulation. A series of tetrapeptides, based on Tic-DPhe-Arg-Trp-NH2-a mimic of the putative message sequence "His-Phe-Arg-Trp" and modified at the DPhe position, were prepared and pharmacologically characterized for potency and selectivity. Substitution of His with Tic gave peptides with significant increases in selectivity. The effects of the substitution pattern of DPhe were investigated and it has significant influences on potency and the level of the maximum cAMP accumulation. Intracerebroventricular administration of peptide 10 induced significant inhibition of cumulative overnight food intake and feeding duration in rats.     

Benefits for dominant red deer hinds under a competitive feeding system: food access behavior, diet and nutrient selection

Social dominance is widely known to facilitate access to food resources in many animal species such as deer. However, research has paid little attention to dominance in ad libitum access to food because it was thought not to result in any benefit for dominant individuals. In this study we assessed if, even under ad libitum conditions, social rank may allow dominant hinds to consume the preferred components of food. Forty-four red deer hinds (Cervus elaphus) were allowed to consume ad libitum meal consisting of pellets of sunflower, lucerne and orange, and seeds of cereals, corn, cotton, and carob tree. The meal was placed only in one feeder, which reduced accessibility to a few individuals simultaneously. During seven days, feeding behavior (order of access, time to first feeding bout, total time spent feeding, and time per feeding bout) were assessed during the first hour. The relative abundance of each meal component was assessed at times 0, 1 and 5 h, as well as its nutritional composition. Social rank was positively related to the amount of time spent feeding during the 1(st) h (P = 0.048). Selection indices were positively correlated with energy (P = 0.018 during the 1(st) h and P = 0.047 from 1(st) to 5(th)) and fat (only during the 1(st) h; P = 0.036), but also negatively with certain minerals. Thus, dominant hinds could select high energy meal components for longer time under an ad libitum but restricted food access setting. Selection indices showed a higher selectivity when food availability was higher (1(st) hour respect to 1(st) to 5(th)). Finally, high and low ranking hinds had longer time per feeding bout than mid ones (P = 0.011), suggesting complex behavioral feeding tactics of low ranking social ungulates.     

How closely do circulating blood glucose levels reflect feeding state in fowls?

1. Amounts of food eaten in 30 min, by fasted-refed immature hens, were not correlated with plasma glucose levels before feeding, or with increases in glucose after feeding. 2. When previously fasted birds were given 0, 5, 10, 15 or 20 g food to eat, their plasma glucose increased by similar amounts with 10, 15 and 20 g, by slightly less with 5 g, and hardly at all with 0 g. 3. When food was removed from free-feeding birds, their plasma glucose levels declined slightly in 2 hr and then remained steady. They declined markedly further overnight, but recovered to a level higher than before deprivation when 10 g food was provided. 4. Four hours darkness during normal daytime had no effect on plasma glucose in fasted-refed birds, but sight of food and presentation of an empty food pan caused slight increases in glucose in 24-hr fasted birds. 5. It is concluded that blood glucose in fowls fluctuates around several "typical" levels, but that there is also much variation between and within birds. Direct (absorbed) and indirect (thermogenic) consequences of feeding probably both contribute to fluctuations in glucose, and these seem more likely to influence regulation of food intake in the longer-term rather than the short-term.     

Development of a photosynthesis model with an emphasis on ecological applications

Summary Hummingbirds selected food in choice experiments based primarily on sugar concentration and secondarily on rate of intake and position. Sugar compositions had little effect on food choice, but the preferred sugar compositions appear to be the most common in nectars of plants visited by hummingbirds. Most amino acids in sugar water were not detected at concentrations found in nectars. Higher amino acid concentrations generally resulted in rejection. Hummingbirds did not necessarily select food in the laboratory to maximize feeding efficiency, but under natural circumstances similar choices could result in optimal feeding efficiencies. The determinants of food choice by hummingbirds provide a rationale for viewing factors important in plant competition for pollinator visits.     

Energetic cost of a meal in a frequent feeding lizard

Specific dynamic action (SDA) describes the rise in metabolism following feeding in animals and represents the energetic cost of digesting and assimilating a meal. The overall energetic cost of feeding may depend on whether or not an animal is post-absorptive at the time of feeding. The aim of this study was to compare the energetic cost of SDA due to feeding frequently compared with infrequently in the eastern water skink, Eulamprus quoyii. For similar quantities of food, repeated feeding incurred an energetic cost equal to 8.8% of the metabolizable energy of the meal (25,220 J), while single feeding incurred an energetic cost of 9.4% of the metabolizable energy of the meal (26,072 J). Experimental lizards maintained a rise in (VO2) that was on average 1.8 times greater than the (VO2) of the unfed controls over a 50-h interval as a result of feeding frequently. This prolonged rise in metabolism resulting from frequent feeding does not result in a higher energetic cost of SDA compared with that resulting from infrequent single feeding.     

Annual variation in fat storage by a migrant warbler overwintering in the Indian tropics

1. We studied fat storage in a population of greenish leaf warblers ( Phylloscopus trochiloides ) in southern India over four winters (1993–97). This species breeds in temperate regions and overwinters in India from October to April.
2. Diurnal variation in fat scores was comparable to that seen among temperate wintering passerines. Seasonal variation was slight, except for premigratory fattening. There was significant annual variation: in drier winters, which were also winters of low food supplies, fat scores were higher.
3. Energy metabolized overnight is unlikely to vary across winters, and annual variation in fat scores is considered a response to uncertainty of food intake on a given day. Fat scores varied inversely with short-term rainfall, which drives the abundance of arthropod prey.
4. Annual variation in fat scores was not accompanied by changes in total body mass. This suggests that protein reserves were being compromised by increased fat deposition in dry years and offers an explanation for the observed delay in moult during drier winters. These results suggest that strategies for maximizing short-term probability of survival translate into future fitness costs, and may shape long-term life-history strategies through simple physiological processes.     

Food Energy Expenditures by the Gypsy MothLymantria disparL. (Lepidoptera, Lymantriidae) at Different Stages of Ontogeny

Food energy expenditures by gypsy moth at different developmental stages under conditions of feeding on larch needles are described. The distribution of total food energy (%) for metabolism, the formation of exuvia (throughout ontogeny), and the formation of an adult insect is assessed. The proportion of food energy lost with excrements over the entire period of feeding is the same (58%) in females and males. The assimilated part of food energy is distributed differently: energy expenditures for the formation of exuvia and adult insects are greater in females than in males. The proportion of food energy expended for metabolism throughout the insect life span is approximately 30% in both females and males.     

Diurnal changes in blood metabolites and their relation to plasma growth hormone and time of feeding in mithun heifers (Bos frontalis)

The aim of the present study was to investigate what, if any, diurnal changes occur in blood metabolites in relation to plasma growth hormone (GH) and feeding time among mithun (Bos frontalis), a semi-wild ruminant. Blood samples were collected at hourly intervals during a 24 h span from 6 mithun heifers (averaging 2.5 yr of age and averaging 230 kg in weight) that were fed twice a day at 11:00 and 16:00 h. Samples were assayed for plasma GH and blood metabolites, non-esterified fatty acids (NEFA), glucose, and alpha-amino nitrogen. The total sampling period was divided into a 1) postprandial (after meal) period (period I: 11:00 to 21:00 h) and 2) interprandial period (period II: 22:00 to 10:00 h) and also into night (20:00 to 05:00 h) and day (06:00 to 10:00 h) periods for statistical analysis. Plasma glucose and alpha-amino nitrogen levels increased (p<0.01), and plasma NEFA and GH decreased (p<0.01) after each meal. No diurnal rhythmicity was detected in plasma glucose or alpha-amino nitrogen levels. Interestingly, plasma NEFA and GH levels were higher (p<0.01) during the interprandial (period II) and night periods, indicating an energy deficit that occurred progressively during the interprandial period of nocturnal feed deprivation. In twice-daily-fed mithuns we conclude that: 1) plasma metabolites and GH exhibited a definite pattern of change with time of feeding; 2) concentrations of plasma NEFA were higher nocturnally due to an energy deficit and that GH levels were higher during the interprandial period after the second meal; 3) the interprandial period after the second feeding may be considered to constitute a short-term food deprivation; 4) the longer interprandial period of 19 h in this study between the second and subsequent morning meal may be changed into equally divided feedings to minimize the short-term energy deficit; and 5) blood sampling for blood metabolites in mithuns should be conducted at a fixed time of day with special emphasis on time of feeding.     

Chronic loss of melanin-concentrating hormone affects motivational aspects of feeding in the rat

Current epidemic obesity levels apply great medical and financial pressure to the strenuous economy of obesity-prone cultures, and neuropeptides involved in body weight regulation are regarded as attractive targets for a possible treatment of obesity in humans. The lateral hypothalamus and the nucleus accumbens shell (AcbSh) form a hypothalamic-limbic neuropeptide feeding circuit mediated by Melanin-Concentrating Hormone (MCH). MCH promotes feeding behavior via MCH receptor-1 (MCH1R) in the AcbSh, although this relationship has not been fully characterized. Given the AcbSh mediates reinforcing properties of food, we hypothesized that MCH modulates motivational aspects of feeding.Here we show that chronic loss of the rat MCH-precursor Pmch decreased food intake predominantly via a reduction in meal size during rat development and reduced high-fat food-reinforced operant responding in adult rats. Moreover, acute AcbSh administration of Neuropeptide-GE and Neuropeptide-EI (NEI), both additional neuropeptides derived from Pmch, or chronic intracerebroventricular infusion of NEI, did not affect feeding behavior in adult pmch(+/+) or pmch(-/-) rats. However, acute administration of MCH to the AcbSh of adult pmch(-/-) rats elevated feeding behavior towards wild type levels. Finally, adult pmch(-/-) rats showed increased ex vivo electrically evoked dopamine release and increased limbic dopamine transporter levels, indicating that chronic loss of Pmch in the rat affects the limbic dopamine system.Our findings support the MCH-MCH1R system as an amplifier of consummatory behavior, confirming this system as a possible target for the treatment of obesity. We propose that MCH-mediated signaling in the AcbSh positively mediates motivational aspects of feeding behavior. Thereby it provides a crucial signal by which hypothalamic neural circuits control energy balance and guide limbic brain areas to enhance motivational or incentive-related aspects of food consumption.     

Effect of short-term energy intake level and exercise on oxygen consumption in men

The influence of short-term energy intake and cycle exercise on oxygen consumption in response to a 1.5 MJ test meal was investigated in ten young, adult men. On the morning after a previous day's "low-energy" intake (LE regimen) of 4.5 MJ, the mean resting oxygen consumption increased by 0.7 ml X kg-1 X min-1 after the test meal (P less than 0.025). After a "high-energy" intake (HE regimen) of 18.1 MJ, the resting measurement was unchanged (+0.4 ml X kg-1 X min-1) after the meal (n.s.). These trends are the reverse of what would be expected if oxygen consumption in response to feeding is a factor in the acute control of body weight. The mean fasting oxygen consumption during cycle exercise at 56% of VO2max (constant work) for both LE and HE prior intakes was not different at 31.1 ml X kg-1 X min-1. Oxygen consumption during exercise increased after feeding by 0.5 ml X kg-1 X min-1 on the LE regimen (n.s.) and decreased by 1.2 ml X kg-1 X min-1 on the HE regimen (n.s.). These results are also the reverse of what would be expected if oxygen consumption in response to exercise is related to short-term energy intake.     

Increased short-term food satiation and sensitivity to cholecystokinin in neurotrophin-4 knock-in mice

Neurotrophin-4 (NT-4) knockout mice exhibited decreased innervation of the small intestine by vagal intraganglionic laminar endings (IGLEs) and reduced food satiation. Recent findings suggested this innervation was increased in NT-4 knock-in (NT-4KI) mice. Therefore, to further investigate the relationship between intestinal IGLEs and satiation, meal patterns were characterized using solid and liquid diets, and cholecystokinin (CCK) effects on 30-min solid diet intake were examined in NT-4KI and wild-type mice. NT-4KI mice consuming the solid diet exhibited reduced meal size, suggesting increased satiation. However, compensation occurred through increased meal frequency, maintaining daily food intake and body weight gain similar to controls. Mutants fed the liquid diet displayed a decrease in intake rate, again implying increased satiation, but meal duration increased, which led to an increase in meal size. This was compensated for by decreased meal frequency, resulting in similar daily food intake and weight gain as controls. Importantly, these alterations in NT-4KI mice were opposite, or different, from those of NT-4 knockout mice, further supporting the hypothesis that they are specific to vagal afferent signaling. CCK suppressed short-term intake in mutants and controls, but the mutants exhibited larger suppressions at lower doses, implying they were more sensitive to CCK. Moreover, devazepide prevented this suppression, indicating this increased sensitivity was mediated by CCK-1 receptors. These results suggest that the NT-4 gene knock-in, probably involving increased intestinal IGLE innervation, altered short-term feeding, in particular by enhancing satiation and sensitivity to CCK, whereas long-term control of daily intake and body weight was unaffected.     

Role of sleep timing in caloric intake and BMI

Sleep duration has been linked to obesity and there is also an emerging literature in animals demonstrating a relationship between the timing of feeding and weight regulation. However, there is a paucity of research evaluating timing of sleep and feeding on weight regulation in humans. The goal of this study was to evaluate the role of sleep timing in dietary patterns and BMI. Participants included 52 (25 females) volunteers who completed 7 days of wrist actigraphy and food logs. Fifty-six percent were "normal sleepers" (midpoint of <5:30 AM) and 44% were "late sleepers" (midpoint of sleep ≥5:30 AM). Late sleepers had shorter sleep duration, later sleep onset and sleep offset and meal times. Late sleepers consumed more calories at dinner and after 8:00 PM, had higher fast food, full-calorie soda and lower fruit and vegetable consumption. Higher BMI was associated with shorter sleep duration, later sleep timing, caloric consumption after 8:00 PM, and fast food meals. In multivariate models, sleep timing was independently associated with calories consumed after 8:00 PM and fruit and vegetable consumption but did not predict BMI after controlling for sleep duration. Calories consumed after 8:00 PM predicted BMI after controlling for sleep timing and duration. These findings indicate that caloric intake after 8:00 PM may increase the risk of obesity, independent of sleep timing and duration. Future studies should investigate the biological and social mechanisms linking timing of sleep and feeding in order to develop novel time-based interventions for weight management.