When and Why Do Territorial Coalitions Occur? Experimental Evidence from a Fiddler Crab

Abstract

Neighboring territory owners are often less aggressive toward each other than to strangers ("dear enemy" effect). There is, however, little evidence for territorial defense coalitions whereby a neighbor will temporarily leave his/her own territory, enter that of a neighbor, and cooperate in repelling a conspecific intruder. This is surprising, as theoreticians have long posited the existence of such coalitions and the circumstances under which they should evolve. Here we document territorial defense coalitions in the African fiddler crab Uca annulipes, which lives in large colonies wherein each male defends a burrow and its surrounding area against neighbors and "floaters" (burrowless males). Fights between a resident and a floater sometimes involve another male who has left his territory to fight the floater challenging his neighbor. Using simple experiments, we provide the first evidence of the rules determining when territorial coalitions form. Our results support recent models that suggest that these coalitions arise from by-product mutualism.

Models of coalition or alliance formation

“…We are most interested in coalitions whose targets belong to the same social group, as is typical for mammalian taxa. Coalitions in other taxa more likely involve neighbors allying for joint defense against an intruder, for example, in passerines (Elfström, 1997) or crustaceans (Backwell and Jennions, 2004;Detto et al, 2010).…”

The effect of claw size and wave rate on female choice in a fiddler crab

“…This decreases the likelihood that the smaller neighbour is evicted. To date, the main proposed benefits to the helper are avoiding the cost of renegotiating boundaries and/or retaining a weaker neighbour who is less likely to encroach onto the helper's territory (Backwell and Jennions 2004;Detto et al 2010). We suggest that there is a third benefit: smaller males are less likely to wave and thereby compete for females, increasing the likelihood that the helper will be chosen as a mate.…”

“…It might help to explain the function of defence coalitions by male fiddler crabs (Backwell and Jennions 2004;Detto et al 2010). In defence coalitions, males help smaller neighbours to defend their burrow.…”

Interspecific assistance: fiddler crabs help heterospecific neighbours in territory defence

“…These costs are likely to be exacerbated because successful intruders are often larger and stronger than the residents they evict (Getty 1987). Examples of this kind of cooperative territory defence come from rock pipits, Anthus petrosus (Elfströ m 1997) and two species of fiddler crab, Uca mjoebergi (Backwell & Jennions 2004) and Uca annulipes (Detto et al 2010;Milner et al 2010), in all of which male residents will leave their own territories to repel intruders on neighbours' territories. This benefits the defended neighbour.…”

“…Judicious helping behaviour has been shown in fiddler crabs, where the relative size of neighbours and intruders strongly influences the likelihood that helping occurs. Helping is most likely when the potential helper is larger than the intruder and the intruder is larger than the targeted neighbour (Detto et al 2010).…”