Development of infant twin Japanese monkeys (Macaca fuscata) in a free-ranging group

The behavioral development of infant twin Japanese monkeys in a free-ranging group during the first 12 months of life was reported and compared with that of single born infants. There were no clear differences in mother-infant interactions toward either twin or single infants, but the twins showed a clear nipple preference. The twins spent much time in contact with and proximity to each other and as their interactions were peaceful no dominance relations between them were assumed. Each twin interacted with other infants and juveniles less frequently than did the single infants.