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Hormonal changes during non-maternal infant care have been demonstrated in many cooperatively breeding bird species, some monogamous rodents and two species of New World primates. Coevolution of hormones and social traits may have provided for the different breeding systems that occur today. Several hormones have been shown to covary with the breeding systems of vertebrates. Elevated levels of the hormone prolactin with male parenting behaviours are common to many birds, rodents and the callitrichid monkeys Callithrix jacchus and Saguinus oedipus. In birds, prolactin may be elevated in both male and female breeders during various stages of nest building, egg laying, incubating and feeding of young. Testosterone levels appear to have an inverse relationship to prolactin levels during infant care in birds and rodents, but this relationship has not been examined for primates. In cooperatively breeding birds, helpers who remain at the nest also have elevated levels of prolactin when displaying parental care behaviours. Prolactin levels are elevated in helper callitrichid monkeys during the postpartum period. Monogamous male rodents demonstrate elevated prolactin levels with parental care behaviour but, in contrast to the birds, the mechanisms mediating prolactin increase appear to differ for male and female rodents. Two factors may influence male parental behaviours and hormonal changes: stimuli from the pregnant female and stimuli from the newborn pups; whereas maternal behaviours are influenced by the maternal hormones of the female and the pup stimuli. An experiential factor may also influence male parental behaviours. Neuropeptides such as oxytocin and vasopressin appear to be involved in male rodent parental care and there may be an interaction between a series of hormones and neurosecretions and stimuli from mates and pups. Studies of Saguinus oedipus, the cotton-top tamarin, suggest that prolactin levels are responsive to stimuli from contact with infants and the level of infant care experience influences the levels of prolactin with male infant care. Father tamarins also have elevated levels of prolactin before the birth of infants suggesting that cues from the pregnant female are important. Prolactin's role in parental care may have evolved from prolactin's role in other reproductive functions. Hormonal regulation of non-maternal care may occur due to a complex interaction of many hormones and neurotransmitters. Studies described here should provide the impetus for further work on parental care hormones in a wide variety of primates.