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.