Proc. R. Soc. B doi:10.1098/rspb.2010.1418 (2010)

A perfect shell is worth fighting for. But although common hermit crabs (Pagurus bernhardus) with undersized accommodation have increased motivation to trade up, they may be less physically fit for duels over shells.

Hermit crabs protect their soft abdomens with discarded snail shells. As they outgrow their borrowed homes, they fight for bigger shells, sizing up their opponents' shells with their appendages and then rapping their shells against their opponents', hoping to evict the incumbent. Sharon Doake and Bob Elwood at Queen's University, Belfast, observed 130 interactions between individual crabs. Crabs squeezed into shells that were much too small were more likely to attack. However, the small shell restricted respiration, meaning that these crabs tired more quickly.