Rats (Rattus norvegicus) modulate eating speed and vigilance to optimize food consumption: effects of cover, circadian rhythm, food deprivation, and individual differences.

The eating behavior of rats (Rattus norvegicus) given food pellets of specified size was examined as a function of environmental, circadian, and experiential influences. Eating times were shorter in lighted, exposed environments than in dark, covered environments, even though in novel, exposed conditions the rats made many scanning movements as they ate. Eating time also varied as a function of the circadian cycle in that eating times were shorter in the night portion of the day-night cycle. Finally, eating times decreased if rats were food deprived, and deprivation had a small but enduring influence. Within the tests there were differences in the eating times of individual rats that were not attributable to the experimental manipulations. That rats can optimize food intake by varying eating speed is discussed in relation to physiological regulation of feeding and to optimal foraging theory.     

Social interaction modifies learned aversions, sodium appetite, and both palatability and handling-time induced dietary preference in rats (Rattus norvegicus)

In a series of four experiments, I examined the extent to which socially transmitted diet preference could counteract the effects of a learned aversion (Experiment 1), a palatability-based diet preference (Experiment 2), a polyethylene glycol 20,000-induced sodium appetite (Experiment 3), and a handling-time induced dietary preference (Experiment 4). I found that rats poisoned after eating a novel diet ate very substantial amounts of the averted diet following interaction with conspecifics that had eaten the averted diet. Following interaction with conspecifics that had eaten an unpalatable diet, rats offered a choice between palatable and unpalatable diets ate more than twice as much unpalatable diet as did controls lacking social experience. Sodium-deficient rats offered a choice between sodium-enriched and sodium-adequate diets ate less than half as much sodium-enriched diet, following interaction with conspecifics that had eaten sodium-adequate diet as did control rats lacking social experience. Rats offered a choice between isocaloric, roughly equipalatable foods with long and short handling times (e.g., sunflower seeds with and without shells) chose the food having the longer handling time after interacting with conspecifics eating that food. These findings suggest that social influence is a major factor in guiding diet selection by rats.     

Lack of interference in long-term memory for socially learned food preferences in rats (Rattus norvegicus)

Outside the laboratory, rats (Rattus norvegicus) are likely both to interact with several conspecifics that have eaten various foods and to eat a variety of foods themselves before they encounter any particular food for which they have a socially enhanced preference. Here the authors examine the stability of rats' socially learned food preferences following 6 days of potentially disruptive ingestive experiences. The authors found that 6 days of (a) eating unfamiliar foods, (b) interacting with demonstrators that had eaten unfamiliar foods, or (c) both eating unfamiliar foods and interacting with demonstrators that had eaten those foods had no measurable effect on rats' socially learned food preferences. The stability of socially enhanced food preferences over time and despite potentially disruptive experiences is consistent with the view that social learning about foods is an important determinant of the food choices of free-living Norway rats.     

Responses of observer rats (Rattus norvegicus) to complex, diet-related signals emitted by demonstrator rats

We explored the effects of complex, food-identifying signals emitted by demonstrator Long-Evans rats (Rattus norvegicus) on food preferences of their observers. In Experiments 1 and 2, observers identified each of 2 or 3 foods their demonstrators had eaten before interacting with observers. In Experiment 3, individual observers interacted with groups of demonstrators. Some of these demonstrators had eaten one food, some another. Observers then chose between the two foods. The greater the proportion of demonstrators in a group that had eaten a diet, the greater the proportion of that diet the observers ate. In Experiment 4, each observer interacted over several weeks with a series of demonstrators and preferred each of the foods its demonstrators had eaten. In sum, the food preferences of observers were affected by several different types of complex, food-identifying signals like those one might expect rats to encounter outside the laboratory.     

Food wrenching and dodging: Eating time estimates influence dodge probability and amplitude

Eating rats dodge conspecifics, who are trying to steal their food, by making a somewhat stereotyped 180° turn and step movement to block the robber's approach. This experiment examined the effect of food features on the form and vigor of the dodging movement. Dodge probability and its amplitude were measured as a function of food size, elapsed eating time, food hardness, and the way in which food theft was attempted. Under all conditions dodge probability and its amplitude were directly related to the time necessary to complete eating. Thus, rats estimated the time required to complete eating and adjusted the size of their evasive movements to gain this time. The results show that although dodging has the appearance of a fixed-action pattern, it is influenced by cognitive processes that may be similar to those that direct other aspects of foraging behavior. © 1994 Wiley-Liss, Inc.     

Abdominal vagotomy disrupts food-related drinking in the rat

Rats with complete subdiaphragmatic bilateral transection of the abdominal vagus (Vgx-C) showed disordered food-related drinking when drinking water in temporal association with a meal of dry food after 5-hr food deprivation and when drinking water in association with a liquid meal after 24-hr food deprivation. The Vgx-C rats drank after significantly longer latencies and drank significantly less water in 1 hr than did sham-vagotomized (Sham) rats after eating the same size meal (solid or liquid) as Shams. Rats with incomplete vagal transection (Vgx-I) ate and drank like Shams. Water intake of Sham and Vgx-I rats correlated positively with the meal size of solid food, but the water intake of Vgx-C rats did not. The failure of Vgx-C rats to drink water normally when food was ingested was not due to failure of a food stimulus to reach the intestine, because Vgx-C and Sham rats emptied equivalent volumes of liquid food from the stomach into the intestine within 10 min of food entering the stomach. These results indicate that the abdominal vagus is an important neurological substrate for food-related drinking in the rat.     

Active regulation to be lean by rats with ventromedial hypothalamic lesions.

Adult female Long-Evans rats with direct-current electrolytic or radio-frequency thermocoagulatory lesions of the ventromedial hypothalamus (VMH) lived on pellet fragments or a powdered chow containing as much as 1.2% quinine sulfate by weight or lived in Skinner boxes with 45-mg Noyes pellets delivered contingent on fixed ratios (FR) of lever pressing up to FR 128. As had been found previously for intact rats, the body weights maintained by VMH rats were determined by the percentage of quinine sulfate in or the contingency of reinforcement of the food on which they lived. Even when the rats lived on highly adulterated or response-contingent food and were lean, they ate more of that food when the ambient temperature was reduced and less of that food during several weeks of forced feeding of eggnog by gavage. Weight maintenance in the cold and caloric compensation during forced feeding were as precise for VMH rats eating highly adulterated chow or Noyes pellets contingent on high fixed ratios of lever pressing as for VMH rats eating typical laboratory chow ad lib, even though the former rats at the time maintained weights no greater than intact rats and the latter rats at the time were grossly obese. Furthermore, regulation in the cold or during forced feeding was in general only a little less precise for the rats with lesions than for intact rats. It may be as characteristic of VMH rats that they eat to become lean and remain lean as that they eat to be obese. The diets of both intact and VMH rats in some manner determine the adiposities that rats will defend against caloric challenge.     

Imitation of mother’s inappropriate food preference in weanling kittens

Experiments were performed on four mother cats and their 22 kittens from 7 litters. Each mother was trained to eat banana or mashed potato in the presence of meat pellets in order to get a hypothalamic stimulation reward. Eighteen of twenty-two weanling kittens which accompanied the mother during the session imitated the mother in eating banana or potato and ignored meat pellets. The imitation of the mother started not earlier than at 35 days of age and in most kittens (10 of 18) began between 49 and 56 days of age. The kittens which ate banana or potato with the mother also ate that food when tested alone after weaning at 9 to 27 weeks of age, either during 10 min sessions in the experimental compartment or during 24 hr tests in the home compartment. When banana was offered to 9 control 13–23 week old kittens (of three other mothers) who never before had eaten that food or had seen their mother eat it, 8 of them did not consume banana during three 24 hr tests. Some consumption, however, occurred in the ninth kitten during the third of these tests. It has been concluded that weanling kittens in the period of transition from maternal to independent feeding tend to imitate the mother in food preferences, even if the food chosen by the mother is unusual for their species. The food preferences acquired through the imitation of the mother may influence the food preferences at the later stages of life.     

Social facilitation of eating novel food in tufted capuchin monkeys (Cebus apella): input provided by group members and responses affected in the observer

Learning about food palatability from watching what conspecifics eat might be one of the advantages of group living. A previous study investigated whether group members' presence or eating activity account for social facilitation of eating of foods never previously tasted. Capuchins encountered novel colored foods when (1) alone (Alone condition) or (2) with group members visible in the nearby cage (Group-present condition) or (3) with group members present and eating a familiar food that had not been colored (Group+food condition). Social facilitation of eating occurred when group members were eating, despite the difference in color between the familiar food eaten by them and the novel food presented to the experimental subject. To clarify what subjects learnt from group members when social facilitation occurred, we further analyze here the data from the previous study. The number of visual exposures to the colored novel food (as a group member) correlated with increased consumption of that novel food when encountered later (as experimental subject). In contrast, the number of times that an individual fed on the familiar food (as a group member) did not decrease its consumption of novel food (as experimental subject). Therefore, capuchins (1) habituated to the colors of the novel foods, and (2) did not take into account that seeing group members eating a food does not provide information about the palatability of a differently colored food. Since social facilitation of eating occurs when foods do not match in color, at least in capuchins, social facilitation of eating should not be considered as a way of learning about a safe diet, but rather as a way of overcoming neophobia. Accepted after revision: 18 August 2001 Electronic Publication     

What Causes Humans to Begin and End A Meal? A Role for Memory for What Has Been Eaten, as Evidenced by A Study of Multiple Meal Eating in Amnesic Patients

Although many factors have been proposed and studied as causes of onset and termination of meals by humans, little attention has been paid to memory for what has previously been eaten. We propose that a principal determinant of meal onset and cessation in humans is memory of when a last meal was eaten and how much was consumed. Knowledge that one has just eaten a culturally defined complete meal may be sufficient grounds for refusal of further food. This hypothesis was tested by studying two densely amnesic patients who had almost no explicit memory for events that occurred more than a minute ago, and who, in particular, usually failed to remember that they had just eaten a meal. Both patients (on three occasions each) readily consumed a second lunch when it was offered 10 to 30 min after completion of the first meal, and usually began to consume a third meal when it was offered 10 to 30 min after completion of the second meal. These findings suggest that memory for what has recently been eaten is a substantial contributor to the onset or cessation of eating of a meal.     

Resistance to satiation: Reinforcing effects of food and eating under satiation

It has been shown previously that rats which have learned a response when hungry will continue to make that response when tested satiated, a phenomenon labeled resistance to satiation. Here we showed that rats which were previously trained hungry will learn a new response for the opportunity to consume pellets in a new situation when tested satiated. In four experiments various groups received each of the components of the training given when rats learn an instrumental response when hungry. Rats were placed in the goalbox of a straight alley and given food pellets when hungry or were hungry only in their home cages prior to running a straight alley in the satiated test in Experiment 1. In Experiments 2, 3, and 4 learning of a differential conditioning problem for pellets in S+ (nonreward in S?) was measured in the satiated test. Groups given pellets in their home cages when hungry with or without alley exposure learned to run more rapidly in S+ than in S? in the satiated test phase. The tendency to eat pellets in the apparatus and the reinforcing effect of eating the pellets was larger for rats which ate the pellets when hungry in their home cage than for rats which ate the pellets when satiated in their home cage. Being hungry in the home cage with no pellets was not sufficient to produce eating or running for pellets in the satiated test, indicating that any inherent reinforcing effect of the pellets is not sufficient to produce eating or running, and that incomplete satiation cannot account for the learning. These data indicate that a reinforcing effect of eating pellets under satiation is an important determiner of resistance to satiation.     

Protein deficiency magnifies social influence on the food choices of Norway rats (Rattus norvegicus)

The food choices of protein-deprived juvenile rats were more profoundly affected by interaction with conspecifics than were the food choices of protein-replete juvenile rats. When choosing among four different-flavored, protein-deficient diets, protein-deprived rats ate significantly more of the diet eaten by a conspecific demonstrator than did protein-replete rats. These data suggest that the food choices of the relatively less successful members of a population are most affected by social interaction. Consequently, the mean effect of social interaction on diet selection in a population of Norway rats is likely to be positive.     

Self-reported overeating and attributions for food intake

Objective: We examined whether people’s attributions for their eating behaviour differ according to whether they believe they have eaten more, less or about the same as they normally would.

Design: Participants were served a small or large portion of pasta for lunch. Afterwards, they were asked to compare how much they ate in the study to how much they normally eat for lunch, resulting in three intake-evaluation categories: ‘ate less’, ‘ate about the same’ or ‘ate more’.

Main outcome measures: How much participants ate; the extent to which they attributed their food intake to an internal cue (i.e. hunger) and an external cue (i.e. the amount of food served).

Results: Participants served a large portion ate more than those served a small portion, but the magnitude of the portion-size effect did not vary across intake-evaluation categories. Furthermore, although participants in all groups indicated that their hunger influenced how much they ate, only those in the ‘ate more’ group indicated that the amount of food available influenced how much they ate.

Conclusion: People appear to be willing to explain their food intake in terms of an external cue only when they believe that they have eaten more than they normally would.     

Observations on the initiation of eating of new food by weanling kittens

The reaction to a new food and the initiation of its consumption were studied in daily sessions in 19 weanling kittens from four litters at 30–39 days of age. The new food consisted of canned tuna for two litters and cereal (cooked cream of wheat with 3% addition of vegetable oil and broth flavor) for the other two litters. Each litter was divided into two groups. One group was offered the new food in the presence of the mother which had been consuming it (eating cereal had to be induced in the mothers by the hypothalamic stimulation reward method). Another group of kittens received the same food in the absence of the mother. It was found that the kittens which had been offered the new food in the presence of the mother showed only a short-lasting orienting reaction and started to eat the new food on the first or second day (with a mean delay of 0.2 days). On the other hand, the kittens which had been offered the new food in the absence of the mother showed a strong orienting reaction and started to eat the new food only after several daily sessions (with a mean delay of 4.8 days). No significant difference was found between the number of days of delay in eating by kittens offered tuna and by kittens offered cereal in the absence of the mother. The initiation of consumption of the new food by these kittens was preceded by a period of playing with the plate on which the food had been placed. Additional observations showed that the fourto five-month old kittens that had eaten cereal during early training preferred that food over meat pellets; the other kittens of the same age that had eaten tuna, but no cereal, during early training refused to eat cereal when it was offered to them when they were hungry.     

The impact of manipulating personal standards on eating attitudes and behaviour

The relationship between perfectionism and eating disorders is well established and is of theoretical interest. This study used an experimental design to test the hypothesis that manipulating personal standards, a central feature of perfectionism, would influence eating attitudes and behaviour. Forty-one healthy women were randomly assigned either to a high personal standards condition (n = 18) or to a low personal standards condition for 24 h (n = 23). Measures of personal standards, perfectionism, and eating attitudes and behaviour were taken before and after the experimental manipulation. The manipulation was successful. After the manipulation, participants in the high personal standards condition ate fewer high calorie foods, made more attempts to restrict the overall amount of food eaten, and had significantly more regret after eating than those in the low personal standards condition. Other variables remained unchanged. It is concluded that experimental analyses can be of value in elucidating causal connections between perfectionism and eating attitudes and behaviour.     

"Eating lightly" and the self-presentation of femininity

In Experiment 1, male and female subjects were given an opportunity to snack as they participated in a "get-acquainted study" with a same-sex or opposite-sex partner (confederate) whose social desirability was manipulated. Consistent with the hypothesis that women may eat less when motivated to present themselves in a feminine light, female subjects ate significantly less with a desirable male partner than in the remaining three conditions. In contrast, male subjects did not eat more (or less) with a desirable woman, although they did show an overall tendency to eat less with female (vs. male) partners. In Experiment 2, female subjects snacked as they got acquainted with a desirable male partner (confederate). Before this interaction, subjects received feedback indicating that they had either very masculine or very feminine interests. In addition, subjects believed either that their male partner was aware of their gender feedback or that he was unaware. Consistent with predictions derived from Schlenker's (1982) analytic-identity theory of social conduct, subjects in the partner-aware conditions ate less when they had received masculine (vs. feminine) feedback, whereas subjects in the partner-unaware conditions ate less when they had received feminine (vs. masculine) feedback. Implications for understanding eating disorders such as anorexia and bulimia are discussed.     

The role of gender and taste class in the effects of stress on eating.

Participating in a laboratory study of the effects of stress on eating, healthy, nonsmoking men and women were shown a film about industrial accidents (stress condition) or a pleasant travelogue (control condition) and had access to sweet, salty, and bland snack food. Analyses of food consumption data revealed a significant interaction between sex of subject and the stress manipulation: Stress markedly and significantly decreased food consumption by men but resulted in some increased food consumption by women. Across the three food taste categories, men consistently ate less under stress than they did in the control condition. In contrast, women ate nearly twice as much sweet food and more bland food under stress than they did in the control condition, but these effects were not statistically significant. These results indicate that the relationship between stress and eating depends on the sex of the subject and may relate to type of food available. Contrasting generalizations about stress and eating reported in the archived literature can largely be reconciled by considering these interacting variables.     

Acetylcholine release in the hippocampus and prelimbic cortex during acquisition of a socially transmitted food preference

Interference with cholinergic functions in hippocampus and prefrontal cortex impairs learning and memory for social transmission of food preference, suggesting that acetylcholine (ACh) release in the two brain regions may be important for acquiring the food preference. This experiment examined release of ACh in the hippocampus and prefrontal cortex of rats during training for social transmission of food preference. After demonstrator rats ate a food with novel flavor and odor, a social transmission of food preference group of rats was allowed to interact with the demonstrators for 30 min, while in vivo microdialysis collected samples for later measurement of ACh release with HPLC methods. A social control group observed a demonstrator that had eaten food without novel flavor and odor. An odor control group was allowed to smell but not ingest food with novel odor. Rats in the social transmission but not control groups preferred the novel food on a trial 48 h later. ACh release in prefrontal cortex, with probes that primarily sampled prelimbic cortex, did not increase during acquisition of the social transmission of food preference, suggesting that training-initiated release of ACh in prelimbic cortex is not necessary for acquisition of the food preference. In contrast, ACh release in the hippocampus increased substantially (200%) upon exposure to a rat that had eaten the novel food. Release in the hippocampus increased significantly less (25%) upon exposure to a rat that had eaten normal food and did not increase significantly in the rats exposed to the novel odor; ACh release in the social transmission group was significantly greater than that of the either of the control groups. Thus, ACh release in the hippocampus but not prelimbic cortex distinguished well the social transmission vs. control conditions, suggesting that cholinergic mechanisms in the hippocampus but not prelimbic cortex are important for acquiring a socially transmitted food preference.     

Food choice in rats (Rattus norvegicus): the effect of exposure to a poisoned conspecific

Three experiments were conducted to examine the effects of exposure to a poisoned conspecific on subsequent food aversion in rats. In Experiment 1A, rats that had been aversively conditioned to a cocoa-flavored food were exposed to a poisoned conspecific that had eaten the same food. On the subsequent choice test, the animals increased their aversion to that food. These results were reconfirmed in Experiment 1B, in which a cinnamon-flavored food was used as the stimulus. In Experiment 2, subjects were first exposed to a poisoned conspecific and then conditioned to the food which the conspecific had eaten. On the test, they showed no sign of increased aversion to that food.     

Social identification of toxic diets by Norway rats (Rattus norvegicus)

In the present experiment, a naive rat (an observer) interacted with two conspecifics (demonstrators) that had recently eaten a diet unfamiliar to the observer, ate two unfamiliar foods in succession, one of which was the food its demonstrators had eaten, suffered toxicosis, and finally, was offered a simultaneous choice between the two diets it had eaten prior to toxicosis induction. During the choice test, observers exhibited an aversion to that diet their respective demonstrators had not eaten. This result indicates that exposure of a rat to conspecifics that have eaten a diet can act, as does actual ingestion of a diet, to reduce that diet's subsequent associability with toxicosis. I discuss this finding as suggesting that interaction with conspecifics may provide an alternative to individual trial and error learning in identification of toxic foods by rats that ingest a number of novel foods in succession before becoming ill.