It can be less costly to help a neighbor repel an intruder than to renegotiate boundaries with a new and potentially stronger individual. Male fiddler crabs will help a smaller neighbor fight off an intruder when the intruder is intermediate in size relative to the 2 neighbors. Fights involving neighbor coalitions are costly for an intruder because he rarely wins when a larger neighbor intervenes. It might therefore be expected that territory-seeking males will avoid fighting residents that have large neighbors. We found a strong effect of the neighbor’s size on whether or not a territory-seeking male initiated a fight with a resident male. Although territorial coalitions in the fiddler crab Uca mjoebergi are relatively uncommon, the potential for them to occur appears to impose strong selection on the fighting decisions of territory-seeking males.
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