  Why might a social behaviour reduce an animal's short-term fitness but boost its lifetime fitness? This could arise in cases of ‘directed reciprocation’, where the beneficiary of the behaviour returns the favour at some point in the future (cf. Sachs et al. 2004). By performing the behaviour, and suffering the short-term cost, the animal thus ensures (or raises the chance) that it will receive return benefits in the future. Similarly, in symbioses between members of different species, it may pay an organism to sacrifice resources for the benefit of a symbiont with which it has a long-term relationship, as its long-term welfare may be heavily dependent on the symbiont's welfare.  