Body morphology appears to strongly determine the behavior of infant rats in an arena (May et al., 2006). In this article, we systematically examined the influence of body-flexibility on the group behavior of infant rats (Rattus norvegicus) by comparing one-, two-, and three-segmented simulated rats with 7- and 10-day-old rats in an arena. Simulated rats provided very different fits to 7-day-old rat pups compared with 10-day-old pups. With respect to 7-day-old pups, three-segmented rats were similar on 83% of our metrics, two-segmented rats were similar on 33% of our metrics, and one-segmented rats were similar on 66% of our metrics. Both one- and three-segmented simulated agents matched 7-day-old rats better than two-segment agents, but it is still premature to conclude which of the three simulated agents best matched the 7-day-old rats. However, no group of simulated rats was similar to 10-day-old pups on any metric. Our results suggest that morphology strongly determines behavior in groups of 7-day-old rats, but that other mechanisms, from more active intersegment control to more influential top-down control, are required to explain the group behavior of 10-day-old rats.