In Cooperative breeding, individuals contribute towards the care and upbringing of offspring which are not their own. It's found in many species of birds, mammals, fish, and insects.*

The behaviour has puzzled evolutionary biologists for many decades, given that individuals apparently sacrifice some of their own breeding prospects in order to increase the potential of the species as a whole.

A behaviour is cooperative if it provides a benefit to another individual and if it has evolved at least partially because of this benefit]. Such behaviours pose a problem to evolutionary theory because — all else being equal — they would reduce the relative fitness of the performer of that behaviour and hence be selected against.

Source : Current Biology, Volume 17, issue 16, pp. 661-672,

Many explanations have been put forward, notably from William Hamilton in 1981 (ref. ^{[ paywalled ]} ) - but, to date, there is still no general agreement on any particular theory, or which combinations of factors might contribute to the behaviour.

Further reading : Life histories and the evolution of cooperative breeding in mammals Proc Biol Sci. 279(1744): 4065–4070.

*Note : There are many examples of human cultures which have (or have had) very co-operative child-rearing strategies.