It is evident that gestational steroid hormones, particularly estrogen and progesterone, play an important role in priming the brain in each of the vertebrate species studied, from birds to primates. In contrast, peptide hormones, such as prolactin and oxytocin, appear to have varying roles in different species. Prolactin facilitates maternal behavior in birds and rodents, but does not appear to be essential for sheep maternal behavior. Oxytocin, however, plays a role in the induction of maternal behavior in both sheep and rats, but not in strains of mice that display spontaneous maternal behavior. Little is known about the role of prolactin or oxytocin in primate parental behavior; however, prolactin levels are correlated with the onset of paternal care in marmosets.