Long-term effects of naturally occurring within-litter differences in body weight of rats

From 10 litters of intact Fischer rats, three groups of each sex were formed on the basis of body weight at 30 days of age: Light (L), Medium and Heavy (H). Their weights were examined again at 60, 90, and 120 days. At 60 days the M animals had caught up with the H group, leaving behind the L animals which remained lighter at the end of the experiment. Starting at 63 days, the animals were given several behavioral tests. Behavioral differences between M and H groups existed at the age when weight differences between them had disappeared. The results indicate the longlasting effect of naturally occurring within-litter differences in body weight and behavior. The procedure used may help in studies of longlasting effects of early feeding.