Urinary prolactin is correlated with mothering and allo-mothering in squirrel monkeys

The hormone prolactin is implicated in infant care-giving by parents and allo-parents in a variety of species. Adult female squirrel monkeys (Saimiri spp.) engage in allo-mothering behavior, which includes carrying and nursing infants, but communal care of offspring has not been investigated from an endocrine standpoint in this taxon. We attempted to fill this gap by examining prolactin levels in squirrel monkeys (Saimiri sciureus) as a function of parental responsiveness. Subjects were housed at the National Institutes of Health Animal Center and assays were performed at the Wisconsin National Primate Research Center. To test for the presence of prolactin in squirrel monkey, saliva, blood and saliva were simultaneously collected from anesthetized subjects during routine health examinations. Prolactin was detectable in serum but not in saliva samples. In the core investigation, behavioral data were collected by focal animal sampling on three 1-male multi-female groups, and individually identified urine was collected non-invasively from foil containers underneath group cages on a daily basis throughout the behavioral study. Changes in urinary prolactin over time reflected changes in the reproductive state of a female who was pregnant, gave birth and lactated during the study. Mean urinary prolactin levels in non-lactating females and a male housed with infants in one group were higher than in adults from 2 groups without infants. In the group with infants, mean urinary prolactin levels in adults increased with the amount of infant contact and care-giving. The squirrel monkey may represent a new primate model for investigating the endocrinology of infant care-giving.