Schooling fish take up different positions in the group according to their aerobic abilities.

Shaun Killen at the University of Glasgow, UK, and his colleagues noted the positions of individual juvenile mullet (Liza aurata; pictured) of similar size as the fish schooled in a swim tunnel in the lab, and measured certain animals' metabolic rates and swimming abilities. When schools were swimming at high speed, fish less able to supply oxygen to their muscles ended up at the back, where they could reduce their workload. By contrast, fish with higher aerobic capacity that were better able to withstand drag forces took up positions at the front. Having fitter fish in the lead could allow schools to maximize their swimming speed.

Credit: A. REKKAS/ALAMY

Proc. R. Soc. B doi:10.1098/rspb.2011.1006 (2011)