PROBING MOTIVATIONAL STATE DURING AGONISTIC ENCOUNTERS IN ANIMALS

Animals commonly compete for resources by direct aggression: for examp le, 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 c ontests(4-7). A general conclusion is that, if possible, animals shoul d assess both the relative fighting abilities and the value of resourc es 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 te st the accumulated theory by probing the motivational state of hermit crabs during fights over the ownership of gastropod shells. The test u ses a stimulus, novel to the crabs, that produces a startle response, the duration of which is an independent measure of the motivation to f ight. We demonstrate that motivational state differs at an early stage of the contest according to the potential gain in resource value. The re was no effect of relative size of the opponent on motivational stat e. In these contests, relative size neither predicted the likely cost of the contest nor the probability of victory.