Widespread recognition of the contribution of individual differences in behavioral phenotype to evolutionary processes raises questions as to their developmental origin: when and in what contexts such differences emerge and what aspects of the developmental environment contribute to these? We studied individual differences among littermates of the domestic cat Felis silvestris catus when competing for meat at weaning, a challenging period in mammalian development. During postnatal weeks six, seven, and eight, we tested 67 weanling kittens (40 males, 27 females) from 16 litters of mixed breed cats maintained as part of a free-ranging breeding colony. Twice a week, we tested the kittens’ behavior after they were food deprived and presented together with their siblings for 2 min with a highly palatable food, a piece of raw beef. We found stable individual differences among littermates across 3 weeks of testing in latency to reach the meat, time spent eating from it, time spent monopolizing it, and number of aggressive behaviors directed toward littermates. There was no effect of sex on any of the behavioral measures. However, kittens with lower body mass at birth (and then also lower body mass at the age of testing) relative to their littermates competed more vigorously and successfully for the meat than their heavier siblings. This suggests the importance of motivational factors arising during early development in shaping individual differences in behavior such as among littermates in the present study, when competing for a biologically relevant resource.
In polytocous mammals, body mass at birth is a good predictor of growth and survival, with heavier young relative to their littermates usually obtaining a greater share of resources such as the mother’s milk. It is therefore often assumed that this advantage will translate into differences in behavior in other contexts, such that heavier littermates will gain more resources at later life stages by showing a more aggressive, “dominant” behavioral style. The findings of the present study challenge this view by demonstrating that in the domestic cat, lighter littermates were more competitive in obtaining meat at weaning. We suggest that differences in the motivational state of individuals should also be considered when accounting for the early development of individual differences in behavior, including among littermates, and may contribute to what might be broadly considered an individual’s personality or behavioral style.