Effects of caloric, protein, and sodium deprivation on the affiliative behavior of Norway rats (Rattus norvegicus).

The tendency of food-deprived, protein-deprived, and sodium-deprived Norway rats (Rattus norvegicus) and their respective controls to affiliate with conspecifics deprived of either food, protein, or sodium was examined. The authors found that (a) independent of internal state, focal rats offered a forced choice between protein-deprived and protein-replete target rats spent more time near replete than deprived target rats; and (b) both food-deprived and sodium-deprived focal rats offered a forced choice between food-deprived and replete target rats spent less time near fasted rats than did well-fed and sodium-replete focal rats. The data indicate that (a) rats can distinguish both food-deprived and protein-deprived rats from replete rats and (b) the deprivation states of rats can affect their willingness to affiliate with deprived conspecifics.

Food stealing by young Norway rats (Rattus norvegicus).

Six experiments were undertaken to explore factors affecting young rats' (Rattus norvegicus) frequencies of stealing food from conspecifics when identical food is available in surplus. It was found that (a) rats would walk across a bed of pellets to steal the particular pellet a peer was eating, (b) frequency of stealing within a pair did not decrease over days, (c) rats stole unfamiliar foods more frequently than familiar foods, (d) younger rats stole from older rats more frequently than older rats stole from younger ones, (e) hungry rats stole more frequently than replete rats, and (f) rats that had stolen a pellet of unfamiliar food from an anesthetized conspecific subsequently exhibited an enhanced preference for that food. Results suggest that food stealing is a mode of active seeking of information about what foods to eat.

Social influences on the amount of food eaten by Norway rats.

A naive observer Norway rat offered a choice between two foods, after it interacts with a demonstrator rat fed one of those foods, increases its preference for whichever food the demonstrator rat ate. It is not known whether interaction with a demonstrator rat would also increase the amount that an observer rat would eat if it were given access only to the food the demonstrator had eaten. In this study, each observer rat interacted with a demonstrator rat fed a food, either familiar or unfamiliar to the observer, and the observer was then offered a weighed sample of the food that the demonstrator had eaten. It was found that, during the first hour of testing, observer rats that had interacted with demonstrators fed an unfamiliar food, increased their intake of that food roughly four-fold. Observer rats that interacted with demonstrator rats fed a familiar food however, did not increase their food intake. Socially enhanced intake of unfamiliar food was seen only during the first hour that observers had access to food and was compensated for during the next 23 h of feeding. This short-term increase in observer intake of unfamiliar foods appeared to result from socially-induced motivation to ingest unfamiliar foods rather than from socially-induced reduction in neophobia.

Demonstration of a socially transmitted flavor aversion in rats? Kuan and Colwill (1997) revisited.

In each of three experiments that differed only in procedural detail, observer rats interacted with pairs of conspecific demonstrators, one fed a cocoa-flavored diet (Diet Coc) and the other a cinnamon-flavored diet (Diet Cin). Immediately after both members of a pair of demonstrators had been fed, and 5 min before they interacted with an observer or observers, one of the demonstrators was made ill by intraperitoneal injection with lithium chloride. After interacting with a pair of demonstrators for 15 min, each observer was allowed to choose between Diet Cin and Diet Coc for 22 h. In all three experiments, observer rats consumed as much Diet Cin after interacting simultaneously with both an ill demonstrator that had eaten Diet Cin and a healthy demonstrator that had eaten Diet Coc as after interacting simultaneously with both a healthy demonstrator that had eaten Diet Cin and an ill demonstrator that had eaten Diet Coc. These results raise questions about the generality of Kuan and Colwill's (1997) demonstration of socially transmitted flavor aversions in Norway rats.

Interaction with demonstrator rats changes observer rats' affective responses to flavors.

The authors examined whether exposing naive rats (observers) to recently fed conspecific demonstrator rats changed the observers' later affective responses to foods their demonstrators ate. In Experiment 1, observers learned an aversion to a flavored fluid, then interacted with demonstrators that had drunk that fluid. These observers, but not those interacting with demonstrators that had drunk water, increased their intake of the averted fluid and exhibited fewer negative responses when the averted fluid was infused into their mouths. Rats in Experiment 2 entered the arm of a T maze known to lead to banana-flavored pellets more frequently after interacting with demonstrators fed banana-flavored pellets than after interacting with demonstrators fed chow-flavored pellets. Results of both experiments indicated that interaction with demonstrator rats changed observer rats' affective responses to flavors.

Learning socially to eat more of one food than of another.

Young observer rats (Rattus norvegicus) that interacted with a conspecific demonstrator fed a diet with more cinnamon than cocoa subsequently preferred cinnamon-flavored diet to cocoa-flavored diet, whereas observer rats that interacted with a demonstrator fed a diet with more cocoa than cinnamon preferred cocoa-flavored diet to cinnamon-flavored diet. The tendency to eat more of a food when it is a major constituent, rather than a minor one, of the diet of a conspecific may be particularly useful to weaning rats as they learn to compose a nutritionally balanced diet by eating appropriate relative amounts of several different foods.

Are socially acquired behaviours irreversible?

Norway rat subjects were exposed for either 3 or 10 days to conspecific demonstrators eating a cinnamon-flavoured, protein-deficient diet. While in contact with their demonstrators, and for several days thereafter, subjects were offered a choice between the cinnamon-flavoured, protein-deficient diet that their demonstrators were eating and a less palatable, nutmeg-flavoured, protein-rich diet. While subjects were in contact with their respective demonstrators they ate little protein-rich diet; during the 7 days immediately following removal of demonstrators from the experiment, subjects learned to eat sufficient amounts of protein-rich diet to permit normal growth. The results indicate that effects of social influence on food choice are transitory. They suggest that the time scale on which animals learn individually to modify socially acquired behaviour is considerably shorter than usually considered in discussions of quantitative models of the evolution of social learning processes.

Social influences on rats' (Rattus norvegicus) preferences for flavored foods, scented nest materials, and odors associated with harborage sites: are flavored food special?

We undertook in several experiments to determine whether the enhanced preference an observer rat (Rattus norvegicus) exhibits for a food after it interacts with a demonstrator rat fed that food reflects a general enhancement of the observer's preference for objects smelling like the food its demonstrator ate or results from a change in olfactory preference specific to foods. After an observer rat interacted with a demonstrator, it exhibited an enhanced preference for either cinnamon- or cocoa-flavored food that its demonstrator had eaten, but no change in its preference for similarly scented nest materials or nest boxes. The results are not consistent with the view that social influence on food choices of rats reflects a general enhancement of rats' preferences for objects bearing scents previously experienced while interacting with conspecifics. Rather, social influences on odor preferences appear to be restricted to scented foods.

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.

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.