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The costs of escalated territorial defence have not been well-documented, although in theoretical studies they are often assumed to be high. We manipulated territorial defence in red squirrels (Tamiasciurus hudsonicus) to assess the costs of territorial disputes in terms of energy, reproductive status, and predation. Feeders containing sunflower seeds were used to manipulate foraging behaviour and induce territorial disputes. Experimental squirrels displayed dramatically higher rates of territorial behaviours in response to feeders, and while there was no difference in survival between experimental and control squirrels, in 4 of the 17 disputes territory owners lost their territories. All 3 females that did so suffered reproductive costs. Territorial disputes were won by the heavier squirrels in all 12 cases where contestants were of different masses, but where the contestants were of similar masses (4 of 5 cases), territory owners were more likely to win.