In colonies of the queenless ant Pristomyrmex punctatus, workers cooperate and cheaters free-ride. The cheaters are larger, do less work, and reproduce more. Left unchecked, they consume the cooperative workforce — the tragedy of the commons, enacted in miniature.
Das Bairagya and Chakraborty (arXiv:2603.00457) show that territorial hostility prevents the collapse. When colonies are connected by migration, cooperators and defectors move between them at different rates — asymmetric migration. In this metapopulation, hostility toward immigrants doesn't just reduce inflow. It selectively filters: because defectors and cooperators migrate at different rates, territorial aggression disproportionately blocks the more mobile type. If defectors migrate faster, hostility culls defectors. The commons survives.
The model validates an observed behavior: these ants show more hostility toward immigrants from nearby colonies than distant ones. The nearby colonies share more migrants, so the threat of defector infiltration is higher. The aggression scales with the actual risk. This isn't irrational xenophobia. It's a calibrated immune response.
The result generalizes beyond ants. Any metapopulation of cooperators and defectors with asymmetric migration rates can sustain cooperation through hostility. The aggression that looks destructive from the individual interaction is constructive at the population level. The fence is what keeps the commons common.