In evolutionary biology, reciprocal altruism is a behaviour whereby an organism acts in a manner that temporarily reduces its fitness while increasing another organism's fitness, with the expectation that the other organism will act in a similar manner at a later time.
Cleaning symbiosis: a small cleaner wrasse (Labroides dimidiatus) with advertising coloration services a big eye squirrelfish (Priacanthus hamrur) in an apparent example of reciprocal altruism.
Eurasian jay, Garrulus glandarius, gives loud alarm calls from its tree perch when it sees a predator.
Arctic terns protect their offspring extremely aggressively by very quick drop-downs from a stationary flight over their enemies. Other birds often benefit from this behavior by breeding very close to the Arctic terns.
Tit for tat is an English saying meaning "equivalent retaliation". It developed from "tip for tap", first recorded in 1558.
In Western business cultures, a handshake when meeting someone is a signal of initial cooperation.