Came across this in the NY Times Science section's Observatory column last month:

Studies of common European hermit crabs by scientists at Queen's University Belfast show that when one crab attacks another, the defender evaluates its chances and, if it senses a lost cause, gives up without wasting any energy, in the form of glucose and glycogen, on a fight. ... The defender either holds on and outlasts the attacker or gives up, releasing its grip on the shell and allowing the attacker to evict it forcefully. ... In fights that the defender won, it used up energy holding on to its shell. In fights that ended in eviction, the defender used little or no energy.