YASUDA, C.

Individual recognition can help subordinates avoid contests with lower success against familiar dominant opponents. This holds true in crustaceans, but if the dominants autotomize their weapon (chela/cheliped) before a second encounter, there is a decrease of potential fighting ability in the prior dominant and an increased chance of success in the prior subordinates than before. I examined whether familiar recognition is still effective even when subordinates encounter a familiar, but temporarily weaponless, dominant through two consecutive trials of male-male contests in the hermit crab Pagurus middendorffii. Subordinate P. middendorffii intruders avoid fights with a familiar dominant guarder. After establishing hierarchy and familiarity between subordinates and dominants possessing a weapon (major cheliped), the subordinates encountered the same dominants after experimentally induced autotomy of the weapon. Subordinate intruders actively fought with the familiar but weaponless guarders. This result might be explained by the subordinates updating information; that is, opponents were recognized the same but weaker than before. Information updating could be a useful new line of investigation into cognitive ability in invertebrates.