Procurement time as a determinant of meal frequency and meal duration

Meal patterns of mice under systematically varying approach and unit costs for food in a closed economy

Several field and experimental studies have investigated the behavioral economics of food intake. In the laboratory, operant behavior has been used to emulate cost and to generate demand functions that express the relationship between the price of food and amount consumed. There have been few such studies of motivated food seeking and intake in mice, and none has reported demand functions. Using albino (CD1) male mice, the present study compares food intake and meal patterns across a series of ratio cost schedules. The first experiment examined unit price. A closed economy was used in which the mice were in the test chambers for 23 h/day and earned all of their food via either a nose poke or lever press response under fixed (FUP5, FUP10, FUP25, FUP50), variable (VUP10, VUP20, VUP50), and progressive (PUP1.25, PUP1.5, PUP1.75) unit prices. Mice were run for 4 days at each cost. There were no consistent differences between the first and last day indicating that behavioral adjustments to schedule changes occurred rapidly. When averaged across all price schedules, mice in the nose poke group consumed more food than their lever press counterparts but the overall shapes of the demand curves did not differ between the two operant responses, with intake decreasing as price increased. The number of meals taken per day differed between two meal-defining criteria that we applied, and there were some differences between the types of unit price schedule. In the second experiment, approach cost in the form of nose poke responses was required to activate a response device (lever) on which a fixed unit price for food was in force. These approach and unit costs were varied systematically. Meal number decreased, and meal size increased, with increasing approach cost even though nose pokes accounted for only a small fraction of the total response activity. Thus, meal patterns in mice are sensitive to approach cost while total amount consumed is more sensitive to unit price. These data are discussed in terms of the concept of foraging cost as either a unitary or a multidimensional variable.

Galanin: a potential role in mesolimbic dopamine-mediated instrumental behavior

The involvement of the neuropeptide galanin in the consumption of the primary "commodities" of food and water is well established. However, the present review describes anatomical and behavioral evidence that suggests that galanin may also modulate ascending mesolimbic dopamine function and thereby play an inhibitory role in the systems by which instrumental behavior is energized toward acquiring primary commodities. General anatomical frameworks for this interaction are presented and future studies that could evaluate it are discussed.

CCK(1) receptor is essential for normal meal patterning in mice fed high fat diet

Cholecystokinin (CCK), released by lipid in the intestine, initiates satiety by acting at cholecystokinin type 1 receptors (CCK(1)Rs) located on vagal afferent nerve terminals located in the wall of the gastrointestinal tract. In the present study, we determined the role of the CCK(1)R in the short term effects of a high fat diet on daily food intake and meal patterns using mice in which the CCK(1)R gene is deleted. CCK(1)R(-/-) and CCK(1)R(+/+) mice were fed isocaloric high fat (HF) or low fat (LF) diets ad libitum for 18 h each day and meal size, meal frequency, intermeal interval, and meal duration were determined. Daily food intake was unaltered by diet in the CCK(1)R(-/-) compared to CCK(1)R(+/+) mice. However, meal size was larger in the CCK(1)R(-/-) mice compared to CCK(1)R(+/+) mice when fed a HF diet, with a concomitant decrease in meal frequency. Meal duration was increased in mice fed HF diet regardless of phenotype. In addition, CCK(1)R(-/-) mice fed a HF diet had a 75% decrease in the time to 1st meal compared to CCK(1)R(+/+) mice following a 6 h fast. These data suggest that lack of the CCK(1)R results in diminished satiation, causing altered meal patterns including larger, less frequent meals when fed a high fat diet. These results suggest that the CCK(1)R is involved in regulating caloric intake on a meal to meal basis, but that other factors are responsible for regulation of daily food intake.

Feeding behavior, obesity, and neuroeconomics

For the past 50 years, the most prevalent theoretical models for regulation of food intake have been based in the physiological concept of energy homeostasis. However, several authors have noted that the simplest form of homeostasis, stability, does not accurately reflect the actual state of affairs and most notably the recent upward trend in body mass index observed in the majority of affluent nations. The present review argues that processes of natural selection have more likely made us first and foremost behavioral opportunists that are adapted to uncertain environments, and that physiological homeostasis is subservient to that reality. Examples are presented from a variety of laboratory studies indicating that food intake is a function of the effort and/or time required to procure that food, and that economic decision-making is central to understanding how much and when organisms eat. The discipline of behavioral economics has developed concepts that are useful for this enterprise, and some of these are presented. Lastly, we present demonstrations in which genetic or physiologic investigations using environmental complexity will lead to more realistic ideas about how to understand and treat idiopathic human obesity. The fact is that humans are eating more and gaining weight in favorable food environments in exactly the way predicted from some of these models, and this has implications for the appropriate way to treat obesity.

Risk-sensitive foraging theory elucidates risky choices made by heroin addicts

AIM

The behavioral process of risk-sensitive foraging (RSF) specifies that the level of satiation or deprivation determines choice between a constant and variable quantity of food. Specifically, RSF stipulates that organisms experiencing satiation and deprivation prefer constant (risk-averse) and variable (risk-prone) choices, respectively. The relevance of this behavioral process to risky choices of opioid-dependent patients is examined in this study.

METHODS

Thirty adult opioid-dependent out-patients made hypothetical choices between constant and variable heroin sources with equivalent means. Preferences for constant (e.g. three bags of heroin) and variable heroin sources (e.g. on average produces three bags of heroin, but vary from one to five bags of heroin on any instance) were assessed using questionnaires that manipulated the amount, delay to receipt and drug potency of heroin across approximately 20 levels. Subjects made choices between constant and variable options after hearing scripts describing the signs and symptoms of opioid satiation and opioid deprivation.

RESULTS

Consistent with the prediction of RSF, subjects purchased significantly more hypothetical heroin from a variable source when exposed to an opioid-deprived script than an opioid-satiated script. This pattern was observed across manipulations of heroin amount, delay to receipt of heroin and heroin potency. Selection of the variable option increased as a function of magnitude under the deprived conditions. The selection of the variable option generally did not increase as a function of magnitude under the satiated conditions except when delay to heroin delivery increased. As delay increased, selection of the variable option increased under the satiated script, but at a lower level observed with the deprived script.

CONCLUSIONS

These data suggest that risky choices of heroin-dependent individuals can be understood and predicted with the application of RSF theory. This research suggests that an evolutionarily old behavioral process may contribute to the risky behavior of the drug-dependent.

The currency of procurement cost

As the number of instrtumental responses required to procure access to food is increased, animals decrease the frequency of initiating meals and increase meal size, conserving total intake while limiting the increase in the overall cost of feeding. In two studies, one using wheel turns and one using bar presses as the instrumental response, we asked whether freely feeding laboratory rats measure cost according to the energy or the time they expend. In each study we varied both the price (i.e., number of wheel turns or bar presses) and the force required to make a response (i.e., torque on the wheel or weight of the bar). Price affected both procurement time (from the first to the last procurement response) and procurement work, whereas torque and bar weight affected work without altering time in most cases. Meal patterns were altered by all manipulations of price, but changes in torque and bar weight had little effect on meal patterns, except in the conditions in which they altered procurement time. These results suggest that time is a critical currency of procurement cost in rats.

The house economist and the eating paradox

An important observation of the experiments of George Collier is that animals normally prefer to maintain their body weight by eating a large number of small meals each day. However, as the effort to obtain access to food increases, the animals adapt by changing to a schedule of eating a small number of large meals each day. A strong implication of this is that there is a hidden cost to eating large meals, and this is the basis of the eating paradox that states that although food is a necessary commodity, the act of ingesting it poses certain metabolic problems for animals. Experiments on cephalic insulin secretion, conditioned insulin secretion and meal feeding are discussed to make the point that the economy demonstrated by rats in Collier's paradigm is dictated in part by predictions of the eating paradox.

Closed-economy energetics

When the behavioral cost of access to food is increased in the 24 h closed-economy procedure, the number of meals per day decreases, the amount consumed per meal increases, and the total daily food intake remains relatively constant. This "Collier function" is found widely in individuals of different species and poses interesting questions about the mechanisms by which behavioral cost achieves these effects on feeding. Data from experiments with pigeons are reviewed which show that: (a) behavioral costs that differentially alter ingestive behavior are not differentiated by energetic costs (VO(2)), (b) the effects of a rise in behavioral cost of food require several days for full expression in ingestive behavior, (c) a bimodal distribution of feeding activity across the active phase is conserved when increased costs reduce the number of meals per day, (d) when ingestive behavior fails to supply a satisfactory nutrient load, physiological and behavioral energy-saving responses occur during the inactive phase. Mechanisms responsible for behavioral-cost effects on feeding will need to make provision for development of the effects across days, conservation of within-day distribution of feeding activity, and apparent lack of differential feed back from energy expenditure related to the differential behavioral costs. A complete energetic picture of life in a closed economy will include the inactive phase of the day and consideration of the effects of ambient temperature.

Effort- and time-cost effects on demand curves for food by pigeons under short session closed economies

Demand curves for food were compared under the effort- and time-cost conditions using response-initiated fixed-ratio (FR) and fixed-interval (FI) schedules. For the effort-cost conditions, two pigeons were exposed to FR 3, 30, 90, and 150 for six sessions each. The time equivalent of each ratio was a FI schedule, each FI value equal to an average time from the first peck on the ratio to reinforcement (3-s access to mixed grain). The experiment was repeated in 1.5-, 3.0-, and 4.5-h closed economies in a different order for each pigeon. Food consumption as a function of time-based unit price (time equivalent per reinforcement duration) for each session length showed moderate convergence on a single demand function for the two cost conditions. When the demand function was separately fitted to each cost condition, however, the ranges of inelastic demand were generally larger in time-cost than effort-cost conditions. These curve-fitting analyses suggest that although time is a critical cost factor decreasing consumption at moderate prices, food intake under the effort-cost condition decreases more rapidly than under the time-cost condition as unit price increases. The analyses provide useful descriptions for the functional difference of costs.

Dissociation of licking and volume intake controls in rats ingesting glucose and maltodextrin

The volume of fluid that rats acquire with each lick was systematically varied across short-term tests with 12.5% glucose (Experiment 1) or 12.5% maltodextrin (Experiment 2). For glucose, rats increased the number of licks emitted as lick volume was reduced such that meal size remained remarkably stable across all (8, 4, and 2 microl) but the smallest (1 microl) lick volume conditions tested. Rats similarly compensated for lick volume reduction (8 to 4 microl) with maltodextrin by approximately doubling the number of licks emitted. Meal duration and a number of lick-microstructural parameters (initial ingestion rate, mean burst duration, terminal lick and ingestion rates, and burst duration) were not correlated with the intake outcome insofar as they varied significantly across conditions over which intake remained stable. Thus, in response to lick volume manipulation, rats demonstrated an impressive degree of behavioral flexibility in what may be regarded as a defense of meal size.

The relation between meal size and the time between meals: effects of cage complexity and food cost

Certain popular models of the regulation of food intake predict a positive correlation between the size of a meal and the preceding and/or following intermeal interval. However, the reported strength of these prandial correlations has varied widely in the literature. To determine what factors may influence the strength of these relationships, we measured the timing of and amount consumed in meals of laboratory rats as a function of 1) whether they were housed in isolation or with partial access to peers or a running wheel, 2) whether they were disturbed daily or weekly for maintenance procedures, and 3) whether food was free or contingent on operant responding. We also compared two definitions of "meal" and "intermeal interval." Strong prandial correlations were found only occasionally. Caging, food cost, and maintenance frequency did not affect the size of the correlations, although these factors did influence the rats' meal patterns. We conclude the regulation of food intake cannot be explained by models relying on a regular relationship between meal size and intermeal interval.

Prey size and prey density affect meal patterns of rats in depleting and nondepleting patches

We examined meal-ending decisions by rats feeding in depleting patches (where the rate of return decreased during a meal) and nondepleting patches (where the rate of return was constant) in a closed-economy, laboratory setting that allowed the precise measurement of feeding rates and meal patterns. The rats were free living in an environment where costs were imposed with bar-press requirements for (1) travelling to a patch to begin a meal and (2) earning prey during the ensuing meal in that patch. The prey in each patch were large or small, and dense or sparse. In both depleting and nondepleting patches, meals were larger as travel price increased. In nondepleting patches, meal size (in grams) was unaffected by the rate of return within the patch, but in depleting patches, meals were larger where the rate of return was higher. In depleting patches, the rats ate the same number of small and large prey per meal, suggesting that meals end when the rate of return reaches some level relative to a comparison value; however, the end-of-meal rate was lower than the average rate (calculated over foraging and feeding time), and thus meals were too large to maximize rate or minimize cost. No obvious rule of thumb explained meal size in both depleting and nondepleting patches. Copyright 1999 The Association for the Study of Animal Behaviour.

Simple and conditional visual discrimination with wheel running as reinforcement in rats

Three experiments explored whether access to wheel running is sufficient as reinforcement to establish and maintain simple and conditional visual discriminations in nondeprived rats. In Experiment 1, 2 rats learned to press a lit key to produce access to running; responding was virtually absent when the key was dark, but latencies to respond were longer than for customary food and water reinforcers. Increases in the intertrial interval did not improve the discrimination performance. In Experiment 2, 3 rats acquired a go-left/go-right discrimination with a trial-initiating response and reached an accuracy that exceeded 80%; when two keys showed a steady light, pressing the left key produced access to running whereas pressing the right key produced access to running when both keys showed blinking light. Latencies to respond to the lights shortened when the trial-initiation response was introduced and became much shorter than in Experiment 1. In Experiment 3, 1 rat acquired a conditional discrimination task (matching to sample) with steady versus blinking lights at an accuracy exceeding 80%. A trial-initiation response allowed self-paced trials as in Experiment 2. When the rat was exposed to the task for 19 successive 24-hr periods with access to food and water, the discrimination performance settled in a typical circadian pattern and peak accuracy exceeded 90%. When the trial-initiation response was under extinction, without access to running, the circadian activity pattern determined the time of spontaneous recovery. The experiments demonstrate that wheel-running reinforcement can be used to establish and maintain simple and conditional visual discriminations in nondeprived rats.

Motivational conflict among water need, palatability, and cold discomfort in rats

Rats were placed in situations pitting three motivations against each other. Two motivations, ambient temperature and need of water, were physiological drives. The third, water sweetness provided by sodium saccharin, was not considered as immediately physiological because saccharin does not provide physiological benefits for the animals; nevertheless they continued to seek the sweet taste after repeated exposure to it. Therefore, our aim was to explore whether these motivations are of the same nature for rats and, if they are, whether they are also quantitatively comparable. From the behavioral evidence we wanted to obtain information on the common currency that permits the rats to solve conflicts. Our results confirm the existence of a common currency in rats' motivations. The similarity of rats' behavior to that of humans observed in conflict situations, where maximizing the bidimensional sum of pleasure was the key to optimal behavior, allows us to suggest a role for affectivity in decision making of mammals.

Who is in charge? Animal vs experimenter control

In the 1920s Curt Richter (1927) stated that the central problem for psychology was to discover the determinants of the initiation and termination of bouts of behavior. Ignoring this challenge, experimentation in animal psychology has been dominated by the session paradigm in which animals work in brief sessions for a resource of which they have been deprived. In this open economy, no behavioral strategy of the animal can meet its demand, and the beginnings and ends of bouts are controlled by the experimenter; thus, Richter's problem cannot be addressed. In contrast, in a free-feeding, closed economy, the animal controls the initiation and termination of feeding and can regulate its intake, and bout patterns can be observed. If the paradigm is modified to simulate a habitat where resources are distributed discontinuously and the animal must work to discover and procure access to a commodity before it can be used, behavioral strategies allowing the animal to regulate its intake while tending to maximize the ratio of benefits to costs are revealed. We offer an answer to Richter's question based on a cost/benefit analysis of feeding behavior in this foraging paradigm. We show that the time and energy costs of resource acquisition and resource consumption are powerful determinants of the pattern of resource use, and that they have different and independent effects. The former costs are reduced by reducing the frequency of initiating bouts, and the latter costs, by altering the rate and amount of consumption. Further, the time window of these relations is much longer than expected from analyses in the session paradigm. We conclude that the recurrent nature of behavior is due to the discontinuous distribution of resources rather than to cycles of physiological depletion and repletion, and that the determinants of bout initiation and termination lie in the economics of the allocation of time and effort to different resources and activities.

Meal patterns of cats encountering variable food procurement cost

The meal patterns of 2 cats in a laboratory habitat with variable foraging costs were examined in a foraging paradigm in which subjects could initiate meals at any time by completing a predetermined number of bar presses (the procurement price) and then could eat any amount. From meal to meal, the procurement price either was fixed or varied among a geometric series of five prices. As the fixed price or the mean of the variable prices increased, meal frequency decreased and meal size increased; daily intake was unaffected. Within variable-price schedules, meal size was not related to the just-paid procurement price. These results suggest that cats respond to the global rather than to the local cost structure of their habitat. They appear to respond to an average of the prices encountered, initiating meals of a frequency and size appropriate to that average. This was true even when the average price was high, meals were infrequent, and thus price encounters were widely separated in time. Therefore, the time window over which the consequences of behavior can affect behavior is longer than often conceived, at least in economies in which the animal controls its intake and the frequency, size, and distribution of its meals.