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. \n\nExperiments 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. \n\nThe 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.

资源持有者通常会在与入侵者的争斗中获胜，这一现象被称为所有者优势（Owner Advantage，OA）。乔氏招潮蟹（Uca mjoebergi）是典型的招潮蟹物种，具有高度群居性与领域性，栖息于潮间带泥滩的混合性别种群中。其洞穴可用于领域防御、交配行为，以及作为躲避捕食者与环境胁迫的庇护所。领域持有者与为争夺领地及洞穴所有权而展开争斗的游荡雄蟹之间的打斗现象十分常见。

本实验于2006年9月28日至12月30日在澳大利亚北领地达尔文市以北5公里的东点保护区内的拉德米勒溪下游河段开展。本实验旨在探究乔氏招潮蟹的所有者优势是否与争斗动机相关。实验设计基于如下前提：退潮时，领域持有者因已对领地投入资源，其争斗动机被认为强于游荡雄蟹；而随着涨潮，双方为赢得争斗与领地的动机均会增强，因此二者间的动机不对称性大概率会降低。

实验结果显示，随着涨潮，获胜个体为领域持有者而非游荡雄蟹，且体型差异同样对胜负结果存在显著影响。该研究结果颇具学术意义：这是首次直接检验动机在争斗中的作用，且其结论与现有经典理论直接相悖。