Physiology &Behavior, Vol. 19, pp. 697-699. Pergamon Press and Brain Research Publ., 1977. Printed in the U.S.A. BRIEF COMMUNICATION Motor Competence in Rats of Different Stocks Reared in Large and Small Litters J. R. GALLER Department of Child Psychiatry and Child Development, Boston University School of Medicine, Boston, MA Department of Nutrition and Food Science, Massachusetts Institute of Technology, Cambridge, MA 02139 AND G. TURKEWITZ Department of Psychology, Hunter College, City University of New York Department of Pediatrics, Albert Einstein College of Medicine, New York, NY (Received 26 January 1977) GALLER, J. R. AND G. TURKEWITZ. Motor competence in rats of different stocks reared in largeand small litters. PHYSIOL. BEHAV. 19(5) 697-699, 1977. -- The effects of sex and stock on a task of motor competence (balancing on a rod) were studied in Long-Evans, Sprague-Dawley and Wistar rats reared in large and small litters. The data indicated significant sex and stock effects but no effects of rearing condition. Although no nutritional effect was found on this task, the data suggest that stock differences should receive greater attention particularly with regard to the possibilities that such differences provide for the identification and analysis of a broad spectrum of effects. Malnutrition Stocks Motor competence DURING the past twenty years, there has been a growing concern about the effects of malnutrition on learning and adaptive functioning. Although this concern has focused on such effects in man, laboratory research aimed at identifying relevant variables and simplifying potentially confounding factors has relied heavily on studies of animals. The value of such studies in understanding the relationships between nutrition and adaptive functioning in man derives from the potential for identifying not only general relationships between nutrition and function but also the variety of relationships in different organisms. Identification of heterogeneous effects of common ante- cedent conditions may increase the possibilities for elu- cidating mechanisms whereby nutrition affects adaptive functioning. One way of assessing the homogeneity or heterogeneity of the behavioral consequences of malnutrition is to determine whether the effects found within one stock of rats are similar to those found in others. Therefore, the present study was undertaken to evaluate the effects of malnutrition and stock on a test of motor competence. METHOD Animals Three stocks of rats, two albino (Sprague-Dawley and Wistar) and one hooded (Long-Evans) were obtained from a commercial breeder. The animals were housed and tested under a reverse 12-hr day-night cycle with continuous dim illumination. Within each stock, pups born within 24 hr of each other were pooled and distributed to lactating mothers in groups if 8 (small litter) or 16 (large litter). Pups were weaned at 24 days and placed in individual wire cages. Purina Rat Chow was provided ad lib. Because prior study has shown that rearing in large litters does not uniformly produce the kinds of weight reduction that would be associated with malnutrition, animals from large litters were randomly selected from the lighter half of each litter. The sex and rearing history of the animals are presented in Table 1. Testing took place without the testers knowing the nutritional status of the animal. Experimenter naivete was not always achieved since size differences were sometimes readily apparent. Apparatus The apparatus consisted of two 4 in. × 12 in. platforms, elevated 18 in. above a tabletop. The platforms were 18 in. apart and were connected by threaded metal rods. In order to provide two levels of difficulty in the balancing task, two 697