Am. Nat. doi:10.1086/651588 (2010)

In a few territorial species, individuals will sometimes help a neighbour to defend its land against an intruder, but why they take on this risk of injury or death has been unclear.

Michael Jennions and his colleagues at the Australian National University in Canberra observed and analysed fights between 29 trios of males — two neighbours and an intruder — of African fiddler crab (Uca annulipes). They concluded that a male leaves his own territory to help his neighbour because it is easier to fight alongside a familiar neighbour early on than to risk having an invader become a new and more threatening neighbour.

The team also showed experimentally that males size up the two combatants when deciding whether to help. A crab was much more likely to join in the fight if he was bigger than the intruder and if the intruder was bigger than the neighbour.