The effects of neighbor familiarity and size on cooperative defense of fiddler crab territories

Cooperation between neighbors in territory defense is expected when the cost of helping a neighbor is less than that of establishing new boundaries with a successful usurper of a neighboring territory. Cooperation has been documented in 3 species of fiddler crab and is understood to depend strongly on the relative sizes of participants—large residents will help smaller neighbors repel intermediate-sized intruders. Simply meeting these criteria does not, however, guarantee that helping occurs, and additional factors are likely to affect the benefits of providing help. We tested whether the likelihood that a large resident would help his smaller neighbor was affected by neighbor familiarity or the relative size of the smaller neighbor, by replacing neighbors with smaller, larger, or size-matched individuals and then simulating intrusions onto their territories. The likelihood of helping did not differ between familiar and unfamiliar neighbors of the same size, but it decreased when the replacement resident differed in size from the original resident. These results suggest that although residents do not recognize their neighbors individually, size acts as a cue to neighbor identity.