nature science update 9 July 1998 reported...
Adoption is common in many mammal and bird populations. While it is obvious that an adopted individual will be better off than without support, how do the adopting animals benefit? According to natural selection, such a behaviour is unlikely to spread through a species unless a benefit exists.
The most commonly-known example of adoption occurs with brood parasites like cuckoos where the inability of parents to recognise their offspring from strangers can result in parents raissing offspring much larger than they are. However not all cases of adoption can be explained in this way. Other routes to caring for offspring that are not one's own include adoption at an age when parents can recognise offspring, where parents merge broods, where a parent is replaced, and after failed breeding attempts.
Some Israeli researchers have suggested that the act of adoption could be learned by copying the behaviour from parents or other adults and therefore could spread even if it has no benefit or is disadvantageous for the parent. Another situation in which practice of adoption could spread is analogous to an arranged marriage where parents adopt an individual as a future mate for their offspring.