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.

When University of Nebraska biologist Corinna Ross set out to determine whether hair samples could be a suitable source of genetic material for marmoset paternity testing, she found that many samples contained a patchwork of cells—some bearing an animal’s own gene assortment and others with a DNA profile that was only about 50 percent similar, like that of a sibling. Indeed, the cells belonged to the animals’ fraternal twins. Further testing revealed that this microchimerism—the harboring of another’s cells or DNA—extended to all the 17 organs sampled, as well as to eggs and sperm. “We panicked a little at the beginning,” Ross says of her original paternity project. “It suddenly became a much more complex issue.”