Interest is growing among psychobiologists and behavioral ecologists in the role of sibling relations in shaping individual development and life histories. In litters of domestic rabbits Oryctolagus cuniculus the heaviest pups at birth are more likely to survive the critical first postnatal week, they compete more effectively with littermates for milk and well-insulated positions in the litter huddle, and are the heaviest at weaning. Here we report that high birth weight pups are also better able to maintain body equilibrium. Testing pups' ability to maintain equilibrium when placed on a 15 degrees ramp for 2 min each day during the first postnatal week, we found that pups showed a continual daily improvement in their ability to maintain balance while moving on the ramp, rarely lost balance by postnatal day 8, and that heavier pups could maintain balance better and earlier than their lighter littermates. Better ability to maintain body equilibrium, however achieved, may help explain heavier pups' advantage in competing for vital resources such as milk and in gaining access to better-insulated positions in the litter huddle. It also provides further support for the usefulness of birth weight, not only as an absolute measure but also relative to the weight of other littermates, as a predictor of different developmental trajectories, behavioral and physiological, among same-age siblings in this mammal.