Some wild mice are diligent parents, building elaborate grassy nests to keep their young safe and warm. Others are less attentive and construct shoddier homes for their offspring. Researchers recently identified a gene that regulates nest building—one of only a few genes known to affect parental behavior in mammals, including humans.

In a study published in Nature, geneticist Andrés Bendesky of Harvard University and his colleagues worked with two kinds of mice that are genetically similar but differ in their pairing behavior: a subspecies of oldfield mice (Peromyscus polionotus subgriseus) form monogamous pairs, whereas deer mice (Peromyscus maniculatus) mate with multiple partners. They also parent differently. Oldfield mice build more elaborate nests than their promiscuous counterparts, and they spend more time grooming and huddling with their pups, as well as retrieving stray ones. In the study, these parenting behaviors did not depend on the care that the mice received as babies—when pups of one species were fostered by parents of the other species, they still grew up to mimic their biological parents' nurturing habits.

In search of a genetic basis for these parental behaviors, Bendesky and his colleagues crossbred the two types of mice twice to yield nearly 800 grandpups. Among the second-generation offspring, one gene linked to nest building stood out—it regulates the hormone vasopressin, which, like oxytocin, has a strong effect on social and bonding behaviors in mammals and birds. The researchers followed up with two experiments: in one, they injected vasopressin into the brains of mouse parents from both of the wild species, and in the other, they genetically manipulated vasopressin neurons in the brains of house mice (Mus musculus) to excite them. Both sets of mice built lower-quality nests, suggesting that vasopressin and the gene responsible for producing it play a critical role in nest building.

The results surprised Oliver Bosch, a biologist at the University of Regensburg in Germany, who was not involved with the new study. In his own research with rats and voles, vasopressin promoted some maternal behaviors. It is possible that the hormone serves different functions before the birth of pups than it does afterward, depending on whether parents are preparing for their babies' arrival or tending to them. “Maybe it's suppressed in the beginning, and then later it kicks in and promotes maternal behavior,” he says. It should come as no shock, after all, when the affairs of mice go astray.