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The Development of a Hunger System in Young Chicks

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I. The problem is to understand hunger as a drive system in chickens and to determine how and why this drive system changes in the course of development. Specifically, this paper investigates the idea that at hatching chicks show a number of basic activities each of which is caused by relatively independent factors, but that as the chicks grow older groups of these activities come to be caused by common coordinating mechanisms (see Figure 1). Junglefowl chicks were observed at various ages from hatching to maturity under various stimulus conditions and under two levels of food deprivation. Several other kinds of experience were also varied. II. In the Basic Experiment. 10 chicks were observed from hatching to 2 months in the presence of food, sand, and an empty floor both when deprived of food for 5 hours and when not deprived. The results showed that the occurrence of each of the activities measured was related to the variables age, stimulus condition, and food deprivation in different ways (Figure 3) ; this implies that the various activities are to some extent independent of each other with respect to their causal factors. The only activity for which stimulus control changed as a function of age was pecking: on days 1 and 2 pecking occurred most to sand, but on day 3 and thereafter it occurred most to food. There were no differential changes in the control by food deprivation as a function of age for any activity. A series of experiments analyzed the factors controlling pecking. It was concluded that a newly hatched chick pecks at objects that have specific releasing characteristics. The amount of pecking is modulated by the novelty of the objects (stimulus change) and by how much pecking the chick has recently engaged in (pecking drive). By the third day, as a result of experience, the releasing value of certain stimuli changes (incentive value), and nutritional state begins to affect pecking, but only to food objects: nutritional state appears to have no effect on pecking at non-food objects during the first five days. Internal stimuli from food in the digestive tract appear to have almost no influence on pecking during the first week. III. In the Basic Experiment, pecking at food when chicks were not deprived occurred almost 70% of the time; this amount of pecking was very much higher than expected. Several experiments were performed to investigate the effects of stimulus change, experience with testing, and experience with food deprivation on pecking. It was concluded that the high rate of pecking in the food-non-deprived condition was partially due to stimulus change. In addition, neither experience with testing, nor experience with food deprivation by itself resulted in high rates of pecking; rather, the interaction of the two experience factors seemed to be responsible for the high rate. The behavior of chicks raised in small groups showed very few differences from the behavior of chicks raised in social isolation. IV. The amount of food eaten by the chicks showed a very low correlation with the amount of pecking. It was concluded that the nutritional state of the chick is probably the most important causal factor for eating. Correlations between pecking and ground scratching were calculated in several ways. The results suggested that, in the first week, both activities are causally closely associated, but, as the chicks grow older, this association between pecking and ground scratching breaks down unless it is maintained by relevant experience. Several problems involved in the calculation and interpretation of correlations were discussed. V. It is concluded that under the conditions of the present experiments, chicks do not seem to develop a hunger system of the sort found in some of the other vertebrates that have been studied. It is suggested that such a system may develop if chicks are given functional experience; that is, if the various behaviors in which chicks engage function to provide food.

Affiliations: 1: Department of Psychology, University of Toronto, Canada


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