 All of these evolutionary transitions have occurred in lineages that had a socially monogamous or solitary breeding system, suggesting that strong kinship ties are an essential factor the evolutionary history of cooperative breeding.Additionally, polytocy, or the birth of multiple offspring per birthing episode, is a highly correlated evolutionary determinant of cooperative breeding in mammals.These two factors, social monogamy and polytocy, are not evolutionary associated, suggesting that they independent mechanisms leading to the evolution of cooperative breeding in mammals.