Animals commonly compete for resources by direct aggression : for example, spiders fight for web sites(1), male red deer fight for females(2), and scorpionflies fight for prey(3). The application of game theory has considerably advanced our understanding of the evolution of such contests(4-7). A general conclusion is that, if possible, animals should assess both the relative fighting abilities and the value of resources before making tactical decisions during contests(8). These tactical decisions are assumed to be mediated by differing motivational state(7,9), but this fundamental assumption has yet to be tested. Here we test the accumulated theory by probing the motivational state of hermit crabs during fights over the ownership of gastropod shells. The test uses a stimulus, novel to the crabs, that produces a startle response, the duration of which is an independent measure of the motivation to fight. We demonstrate that motivational state differs at an early stage of the contest according to the potential gain in resource value. There was no effect of relative size of the opponent on motivational state. In these contests, relative size neither predicted the likely cost of the contest nor the probability of victory.