Owner Advantage in the fiddler crab, Uca mjoebergi. Do owners win more contests than floaters because they have a mechanical advantage?

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 due to a mechanical advanatage. Possible advantages come from the ability of the owner to utilise the burrow when fighting. For example, owners can retreat into their burrow and block the entrance using the 'flat-claw' defence. In addition to this, fiddler crabs may also gain advantage by using the burrow shaft during fights, producing greater leverage for their enlarged claw. This was tested by pitching fights between prior owners and resident owners with either their burrow blocked or open. Owners with unblocked burrows won 88% of contests, and those with blocked burrows won 38% of contests. This is a significance difference and infers that owners have a strong mechanical advantage.