Owner Advantage in the fiddler crab, Uca mjoebergi. Do owners win more contests than floaters because they are more motivated?

Resource owners generally win contests against intruders, a phenomenon referred to as the 'Owner Advantage' (OA). Uca mjoebergi is a typical fiddler crab that is highly sociable, territorial and lives in mixed sex colonies on intertidal mudflats. Burrows are used in territory defence, for mating, and as refuge from predators and environmental stresses. Fights are common between owners and floating males who battle for ownership of the territory and burrow. Experiments were conducted from 28th Sept - 30 Dec 2006 at the lower reaches of Ludmilla Creek within East Point Reserve, 5km North of Darwin, NT. This experiment was designed to determine if owner advantage in the fiddler crab Uca mjoebergi is related to motivation. This was designed on the premise that at low tide owners are thought of as being more motivated than floaters in contests because they have invested in the territory. It was thought that as the tide comes in, the motivational asymmetry is likely to be reduced as both are increasingly motivated to win the contest and the territory. The results indicate that as the tide came in owners, not floaters, were more likely to win contests, and size difference also had a significant effect on who won. The result is interesting because it is the first direct test of the role of motivation in contests, and it is a direct contradiction to established theory.

资源所有者通常会在与入侵者的竞争中获胜，这一现象被称为“所有者优势（Owner Advantage, OA）”。莫氏招潮蟹（Uca mjoebergi）是典型的招潮蟹物种，具有高度社会性与领域性，栖息于潮间带泥滩的混合性别群体中。其洞穴可用于领域防御、交配活动，同时也是躲避捕食者与环境胁迫的庇护所。领地所有者与游离雄蟹之间的争斗十分常见，二者会为争夺领地与洞穴的所有权展开搏斗。本实验于2006年9月28日至12月30日期间，在北领地达尔文市以北5公里处的东点保护区内的拉德米勒溪下游河段开展。本实验旨在探究莫氏招潮蟹的所有者优势是否与其竞争动机相关，实验设计基于如下前提：退潮时，领地所有者因已对领地投入资源，其竞争动机被认为强于游离雄蟹；而随着涨潮，双方为争夺领地与搏斗胜利的动机均会增强，此时二者的动机不对称性大概率会降低。实验结果显示，随着涨潮，获胜者为领地所有者而非游离雄蟹，且体型差异同样会对竞争胜负产生显著影响。该研究结果颇具学术意义：这是首次直接检验动机在动物竞争中的作用，且其结论与现有主流理论直接相悖。