Seeming to be a model of harmonious cooperation, the marmoset family unit includes offspring that stay at home past sexual maturity to help care for their younger siblings and fathers that share the duty of carrying newborns so mothers can take a break. As it turns out, all this sharing and caring may be facilitated by a fluid sense of identity among the tiny monkeys, which carry around not only one another’s infants but bits of one another as well. The phenomenon could offer new insight into human immune reactions to foreign cells.