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.     

Patch choice as a function of procurement cost and encounter rate.

The effects of patch encounter rate on patch choice and meal patterns were studied in rats foraging in a laboratory environment offering two patch types that were encountered sequentially and randomly. The cost of procuring access to one patch was greater than the other. Patches were either encountered equally often or the high-cost patch was encountered more frequently. As expected, rats exploited the low-cost patch on almost 100% of encounters and exploited the high-cost patch on a percentage of encounters that was inversely proportional to its cost. Meal size was the same at both patches. Surprisingly, when low-cost patches were rare, the rats did not increase their use of high-cost patches. This resulted in spending more time and energy searching for patches and a higher average cost per meal. The rats responded to this increased cost by reducing the frequency and increasing the size of meals at both patches and thereby limited total daily foraging cost and conserved total intake.     

Procurement time as a determinant of meal frequency and meal duration.

Foraging involves the expenditure of both time and effort in the acquisition of food; animals typically modify their meal patterns so as to reduce these expenditures or costs. The contribution of time, as compared with effort, to the overall cost perceived by an animal is not known. We investigated the effect of foraging time as a cost independent of effort by measuring the meal patterns of rats living in a laboratory foraging simulation in which they earned all their daily intake. They pressed a bar once to initiate an interval (procurement interval) leading to the presentation of a large cup of food from which they could eat a meal of any size. As the length of the interval increased from 1 s to 46 hr, meal frequency decreased regularly. Meal size increased in a compensatory fashion, and total daily intake was conserved through an interval of 23 hr. The changes in meal frequency occurred because of changes in the rat's latency to bar press after each meal. The functions relating meal frequency and size to the procurement interval were of the same shape as those seen when cost is the completion of a bar-press requirement, which entails the expenditure of both effort and time. When the bar-press requirement was increased to 10, meal frequency was reduced, but time and effort did not appear to simply add together in the rat's perception of cost. These data reveal that time is preceived to be a cost by rats foraging in this laboratory environment. These results suggest that the time parameters of foraging are different from those of consumption.     

The economics of the law of effect.

A corollary of the law of effect predicts that the larger the reinforcement, the greater the rate of responding. However, an animal must eat more small portions than large portions to obtain the same daily intake, and one would predict, therefore, that when eating smaller portions an efficient animal would eat less (conserving time and energy) and/or respond faster (conserving time). The latter of these predictions was supported by the present experiments with free-feeding rats for which portion size (pellet size or duration of feeder presentation) and portion price within meals were varied. Response rate was a function of the unit price (responses/g) of food: Rats responded faster when portions were smaller or when prices were higher. Meal size and frequency were relatively unaffected by unit price, but were influenced by the price of meal initiation. The results are discussed in relation to the economic differences between traditional operant and free-feeding paradigms and to both traditional and more recent formulations of the law of effect.     

The relationship between feeding rate and patch choice.

Rats in a laboratory foraging simulation searched for sequential opportunities to feed in two patches that differed in the rate at which food pellets were delivered (controlled by fixed-interval schedules) and in the size of the pellets. The profitability of feeding in each patch was calculated in terms of time (grams per minute) and in terms of effort (grams per bar press). These values were the result of the imposed fixed interval, the size of the pellets, and the rate at which the rats pressed the bar in each condition. The rats ate more food and larger meals, but not more frequent meals, at the patch offering the higher rate of food consumption, calculated as grams per minute. The relative intake at any patch was a function of the relative rate of intake during meals at that patch compared to the other patch. Rats respond to explicit manipulations of feeding time in the same manner as they respond to manipulations of feeding effort.     

Meal patterns following changes in procurement cost for rats with fornix transection

The purpose of the present study was to explore the effects of a varied procurement cost on the foraging behavior of rats with fornix transection. An operant analog of foraging requirements was used to examine the feeding patterns of the animals under free feeding, low procurement cost (FR5), and high procurement cost (FR80) situations, in an environment with minimal sensory distraction. It was found that animals with fornix transection did not differ from control rats in general consumption. Both groups were also able to adapt their feeding behavior to the varied procurement cost. As the procurement cost increased, the number of meals consumed decreased while the meal duration increased. The meal patterns themselves were different for the fornix transected animals and the control group. Animals with fornix transections ate more meals over the course of a day than did control animals; their meals were of a longer duration, and their intermeal intervals were shorter than those of control animals. During the course of a meal, the rats with fornix transections took a larger number of breaks, during which they drank, explored, or engaged in activities other than eating. These differences in the feeding patterns were seen across all procurement cost levels. The data support the possibility of hippocampal involvement in behavioral organization or sequencing.     

Changes in feeding and foraging patterns as an antipredator defensive strategy: a laboratory simulation using aversive stimulation in a closed economy.

The effects of the risk of electric shock on the meal patterns of rats living in an operant chamber were investigated. Rats could obtain food by working on a response lever that provided reinforcement according to chained fixed-ratio continuous reinforcement schedules that allowed the animals control over meal size. Using a two-compartment operant chamber with a safe nesting area and manipulanda area with a grid floor, shock could be correlated with responding on the schedule. Shocks (less than or equal to 1.25 per hour) were scheduled to occur randomly throughout the day, independent of the rat's behavior. Shock caused a reorganization of meal patterns such that the animals took less frequent but larger meals. This pattern reduced the time the animals spent at risk without compromising caloric balance. Similar changes in feeding pattern were obtained in both hooded and albino rats. Exposure to shock in a separate chamber did not produce these behavioral modifications. The magnitude of shock-induced alterations of meal patterns was greater with chained fixed-ratio 90 continuous reinforcement than with chained fixed-ratio 10 continuous reinforcement. Additionally, the rats seemed to be able to reduce food intake but increase caloric efficiency, such that the reduced food intake did not have deleterious effects on maintenance of body weight. These behavioral modifications reduced the number of shocks received from that which would have been expected if meal pattern changes had not occurred. We suggest that this technique may provide a useful laboratory simulation of the impact that the risk of predation has on foraging behavior.     

Drinking in a patchy environment: the effect of the price of water.

Rats in a laboratory foraging paradigm searched for sequential opportunities to drink in two water patches that differed in the bar-press price of each "sip" (20 licks) of water within a bout of drinking (Experiment 1) or the price and size (10, 20, or 40 licks) of each sip (Experiment 2). Total daily water intake was not affected by these variables. The rats responded faster at the patch where water was more costly. However, they accepted fewer opportunities to drink, and thus had fewer drinking bouts, and drinking bouts were smaller at the more costly patch than at the other patch. This resulted in the rats consuming a smaller proportion of their daily water from the more costly patch. The size of the differences in bout frequency and size between the patches appears to be based on the relative cost of water at the patches. The profitability of each patch was calculated in terms of the return (in milliliters) on either effort (bar presses) or time spent there. Although both measures were correlated with the relative total intake, bout size, and acceptance of opportunities at each patch, the time-based profitability was the better predictor of these intake measures. The rats did not minimize bar-press output; however, their choice between the patches and their bout sizes within patches varied in a way that reduced costs compared to what would have been expended drinking randomly. These data accord well with similar findings for choices among patches of food, suggesting that foraging for water and food occurs on the basis of comparable benefit-cost functions: In each case, the amount consumed is related to the time spent consuming.     

Slower eating rate reduces the food intake of men, but not women: implications for behavioral weight control

This study tested a central tenet of behavioral weight control: does slower eating rate reduce food intake and positively affect appetite ratings. This study used direct behavioral observation of participants' (n=48; 25BMI35) eating behavior using Universal Eating Monitors. Food intake and ratings of "desire to eat" (collected each minute during meals) were represented as a function of time. Based on an acclimation meal, participants' eating rate was modified by instructing them to eat a bite of food when prompted by a computer, which generated three types of meals: (1) baseline (eating rate was the same as the acclimation meal), (2) reduced-rate (eating rate was reduced by 50%), and (3) combined-rate (eating rate at the beginning of the meal was identical to that of the acclimation meal, subsequently eating rate was reduced by 50%). All meals were ad libitum. Slower eating rate during the reduced-rate and combined-rate meals resulted in less food intake compared to the baseline meal for men, but not women. Ratings of desire to eat, normalized for the amount of food consumed, were lower during the combined-rate meal for men and women. Thus, this basic premise of behavioral weight control was supported for men, but not for women.     

The timing of meals

In most individuals, food intake occurs as discrete bouts or meals, and little attention has been paid to the factors that normally determine when meals will occur when food is freely available. On the basis of experiments using rats, the authors suggest that when there are no constraints on obtaining food and few competing activities, 3 levels of interacting controls normally dictate when meals will start. The first is the genetically determined circadian activity pattern on which nocturnal animals tend to initiate most meals in the dark. The second is the regularly occurring changing of the light cycle: These changes provide temporal anchors. The third relates to the size of the preceding meal, such that larger meals cause a longer delay until the onset of the next meal. Superimposed on these 3 are factors related to learning, convenience, and opportunity.     

Role of fatty acid oxidation in control of meal pattern

To characterize the role of fatty acid oxidation in the control of food intake, we investigated the effect of 2-mercaptoacetate, which inhibits fatty acid oxidation, on meal patterns and cumulative food intake in rats. Rats were fed either a medium fat (MF, 18% fat) or a low fat (LF, 3.3% fat) diet. Mercaptoacetate (400 mumole/kg body wt), intraperitoneally injected in the middle of the bright or at the onset of the dark phase of the diurnal lighting cycle, increased cumulative food intake in MF rats by shortening the latency to eat after injection and the duration of the subsequent intermeal interval (IMI) without affecting the size of the first meal. Mercaptoacetate, injected in the middle of the bright phase, reduced the latency to eat but did not affect the duration of the subsequent IMI or cumulative food intake in LF rats. A higher dose of mercaptoacetate (600 mumole/kg body wt), initially increased and later decreased cumulative food intake in MF rats. The initial increase in food intake was due to shorter IMIs; the subsequent decrease in food intake was due to smaller meals after mercaptoacetate injection than after control injection. The results indicate that a drop in fatty acid oxidation caused by mercaptoacetate triggers a meal. This implicates fatty acid oxidation in the maintenance of postprandial satiety.     

The meal patterns of the oestrous cycle and their motivational significance

Food intake was monitored continuously throughout the oestrous cycle of the rat by operant methods. On the night of oestrus the size of meals eaten was reduced and the average intermeal interval was shorter; and even after meals of the same size, oestrous animals returned to eat again more quickly than dioestrous animals. These results suggest that the way in which ovarian oestrogens reduce food intake is by intensifying processes responsible for the short-term satiation of hunger without affecting the motivational processes responsible for its arousal. Signs of motivational arousal at oestrus could thus be the result of a self-imposed nutritional deprivation, rather than a direct effect of ovarian hormones on sexual receptivity.     

Unlearned appetite controls: watersnakes (Nerodia) take smaller meals when they have the choice

Studies of appetite in mammals emphasize that meal size is learned, but lactation and parental care constrain testing of naive individuals. Neonatal reptiles, in contrast, are self-sufficient foragers. The authors examined the effect of prey size on meal size in primivorous (at first feeding) northern watersnakes (Nerodia sipedon). When offered an excess of small prey (2%-20% of snake mass), neonates ate significantly smaller meals (M = 23.5% of snake mass) than when offered a single huge item (range = 32%-55%). The authors conclude that (a) the taking of smaller meals is not a learned effect, (b) there may be a satiety threshold for meal size rather than a target, (c) oropharyngeal stimuli may provide satiety cues, and (d) huge meals may have fitness costs.     

Effects of simultaneous oral-intragastric ingestion on meal patterns and satiety in humans

Simultaneous oral-intragastric ingestion of a liquid diet was compared with oral ingestion for 15-21 days in four males or normal weight, 18-21 years old. All the subjects are significantly more (M= 222 cm3 or kcal [930 kJ] per meal, p less than .025) during simultaneous compared with oral lunches and dinners. Subjects reduced the size and frequency of breakfasts and snacks, which were always ingested orally, however, and did not gain weight. In a second experiment, the same four subjects ate significantly more (M-177 cm3 more, p less than .001 at lunch when the rate of ingestion was 200 cm3/min compared with 100 cm3/min regardless of route (oral, intragastric or simultaneous). They also ate significantly more (M=135 cm3 more, p less than 0.25) during simultaneous meals compared with oral or intragastric meals ingested at the same rate. Thus, rate of ingestion and oral stimulation are important determinants of meal size in humans.     

Ontogeny of meal patterning in rats and its recapitulation during recovery from lateral hypothalamic lesions.

The development of feeding patterns was investigated in weanling rats and in rats with lateral hypothalamic lesions. From 16 to 25 days of age, the weanlings demonstrated a preprandial intake pattern, i.e., a positive correlation between meal size and time since the preceding meal. This subsequently declined while the postprandial relationship (correlation with time until subsequent meal) began to emerge such that by 30-35 days of age a full adult pattern was observed. Rats recovering from lateral hypothalamic lesions, for a brief period, also demonstrated a preprandial intake pattern. The postprandial relationship was abolished by the lesions. These results suggest that the development of adult meal patterning results from maturation of lateral hypothalamic mechanisms governing meal initiation.     

Resource-Allocation Strategies: A Verbal Protocol Analysis

The current study examined the strategies used by people to solve resource-allocation problems. Verbal protocols were recorded as participants provided meal choices for seven consecutive days with limited resources available to spend on meals and with daily constraints imposed on meal consumption. None of the participants incorporated the established mathematical procedures (i.e., Linear Programming) to arrive at the optimum number of meals possible in a week. However, the strategies they did use enabled them to achieve meal totals on average at 94% of this optimal amount. A few participants attempted to first solve the maximum meals possible in a week before scheduling this solution across the seven days (solve-and-schedule strategy), but the majority of participants simply selected meals on a day-to-day basis while checking resource availability each day to allow for full resource consumption (consume-and-check strategy). The findings of this study provide a preliminary step toward understanding how people make intuitive resource-allocation decisions.     

Expanding on cross-price elasticity: Understanding tobacco product demand and substitution from the cross-price purchase task

We examine whether cigarettes serve as substitutes for electronic nicotine delivery systems (ENDS) among ENDS users and demonstrate methodological extensions of data from a cross-price purchase task to inform policies and interventions. During a clinical laboratory study, n = 19 exclusive ENDS users and n = 17 dual cigarette/ENDS users completed a cross-price purchase task with cigarettes available at a fixed price while prices of own-brand ENDS increased. We estimated cross-price elasticity using linear models to examine substitutability. We defined five additional outcomes: nonzero cross-price intensity (purchasing cigarettes if ENDS were free), constant null demand (not purchasing cigarettes at any ENDS price), cross-product crossover point (first price where participants purchased more cigarettes than ENDS), dual-demand score (percentage of prices where both products were purchased), and dual-use break point (minimum relative price to force complete substitution). The cross-price elasticity results indicated that cigarettes could serve as substitutes for ENDS among ENDS users on average, but this average effect masked substantial heterogeneity in profiles of demand (here, a measure of the drug's reinforcement potential). Policies and regulations that increase ENDS prices appear unlikely to steer most exclusive ENDS users toward cigarette use, as most would not purchase cigarettes at any ENDS price, but they could prompt some dual users to substitute cigarettes completely while others remain dual users. This heterogeneity in consumer responses suggests additional indices of cross-product demand are useful to characterize the anticipated and unanticipated effects of tobacco price policies more fully.     

‘What's New?’ at the Dinner Table: Family Dynamics During Mealtimes in Two Cultural Groups in Hawaii

This study shows systematic differences in how parents structure mealtime discourse in two cultural groups in Hawaii. Fifty-nine families were videotaped during week-day evening meals at home. Thirty of the families were middle-class Japanese-American (JA) families; 29 were middle-class Caucasian-American (CA) families. Evening meals in the Caucasian-American homes in this sample are clear-cut events. Families sit at tables and talk throughout the meal. Children are encouraged to describe their experiences, wishes, plans and perceptions. Parents cue children to focus on distinctive aspects of their realities, to talk about ‘what's new’, or unfamiliar to the audience. Evening meals in the Japanese-American homes are less focused on formal discussion. Half the families watch television while eating; others play games, cook, listen to music, talk on the phone or play with babies or pets during dinner. JA family members talk to each other less and structure their discourse differently. Rather than report on distinct, individual experiences, JA family members co-narrate and comment on what they are collectively doing. The observed differences are related to cultural differences in construals of self and other. The cultural importance of being able to clearly communicate individual realities across separate beings is seen in the CA mealtimes. Appreciation of smooth, group process and coordinated belonging is seen in the JA evening meals.     

Manipulations of start and food locations affect navigation on a foraging task

Rats were able to search multiple food cups in a foraging task and successfully return to a fixed, but not a variable, start location. Reducing the number of food cups to be searched resulted in an improvement in performance in the variable start condition. Performance was better when only one or two food cups had to be visited but was still impaired if the food was not found in the first cup searched. Variable start locations impaired performance when only one food cup had to be searched, if that cup was moved over the table. These findings suggest that there is an interaction between memory processes and the navigational processes that allows an animal to return to its start location after a foraging trip. It appears that a fixed location for the food or the start point of a foraging trip is a necessary precondition for accurate performance.     

Residence time in concurrent foraging with fixed times to prey arrival

Five pigeons were trained in a concurrent foraging procedure in which reinforcers were occasionally available after fixed times in two discriminated patches. In Part 1 of the experiment, the fixed times summed to 10 s, and were individually varied between 1 and 9 s over five conditions, with the probability of a reinforcer being delivered at the fixed times always .5. In Part 2, both fixed times were 5 s, and the probabilities of food delivery were varied over conditions, always summing to 1.0. In Parts 3 and 4, one fixed time was kept constant (Part 3, 3 s; Part 4, 7 s) while the other fixed time was varied from 1 s to 15 s. Median residence times in both patches increased with increases in the food-arrival times in either patch, but increased considerably more strongly in the patch in which the arrival time was increased. However, when arrival times were very different in the two patches, residence time in the longer arrival-time patch often decreased. Patch residence also increased with increasing probability of reinforcement, but again tended to fall when one probability was much larger than the other. A detailed analysis of residence times showed that these comprised two distributions, one around a shorter mode that remained constant with changes in arrival times, and one around a longer mode that monotonically increased with increasing arrival time. The frequency of shorter residence times appeared to be controlled by the probability of, and arrival time of, reinforcers in the alternative patch. The frequency of longer residence times was controlled directly by the arrival time of reinforcers in a patch, but not by the probability of reinforcers in a patch. The environmental variables that control both staying in a patch and exiting from a patch need to be understood in the study both of timing processes and of foraging.