Male parenting and response to infant stimuli in the common marmoset (Callithrix jacchus).

Callithrix jacchus infants are raised in complex family environments where most members participate in rearing the young. Many studies examining male parental behavior have focused on the carrying of infants with observations made within the family context. However, interference from family members can make it difficult to assess the father's motivation to care for infants. Our goals were to develop a testing paradigm for determining an individual's response to infant stimuli separate from family influences, compare a male's motivation to respond to an infant stimulus outside the family with his paternal behavior within the family, to compare responses to infant stimuli of parentally experienced versus inexperienced males and finally to develop a reproducible and standardized method of testing male responsiveness to infant stimulus that could serve to evaluate hormonal manipulations. Fifteen experienced common marmoset fathers were evaluated using three different measures of parental behavior: (1) instantaneous scan sampling, (2) continuous focal sampling in the family, and (3) continuous focal sampling of males presented with four infant stimuli: familiar and unfamiliar infants, familiar and unfamiliar infant vocalizations. Six parentally inexperienced males (non-fathers) served as controls. Males that carried the most in the family were typically the same males that responded most to the infant vocalization tests. Experienced fathers did not differ in their latency to enter the stimulus cage for any of the four infant stimuli response tests while inexperienced males took significantly longer to enter the stimulus cage. In addition, fathers expressed a greater frequency of infant-directed behavior than did the inexperienced males during the unfamiliar infant and unfamiliar vocalization tests. These studies show that experienced male marmosets are highly motivated to interact with infant stimuli and that there is interindividual variability in response to infant vocalizations. Testing males outside of the family allows for a clear assessment of male's interest in infant stimuli in both parentally experienced fathers and inexperienced males.