Reciprocity

Biological Parallel

Vampire bats share blood meals with roost-mates who failed to feed, creating reciprocal debts tracked over years. A bat that receives but never reciprocates gets excluded from future aid—cheaters are detected and punished. Reciprocity isn't ethics; it's a biological contract enforced by memory and social exclusion. The impulse to return favors is ancient debt accounting, and the discomfort of unpaid obligations is the collection notice.