Nesting Instinct

Credit: KAT CHADWICK

There may be a biological basis for separate doll and dump-truck aisles in the toy store.

In a study of baby mice, researchers at Vanderbilt and the University of Southern California found that males and females respond differently to the hormone arginine-vasopressin (AVP), which plays an important role in social behavior.

The study, published online last December in the journal Hormones and Behavior, found that baby girl mice tend to move toward familiar odors if they previously had smelled that odor near their mothers. Baby boy mice didn’t seem to be influenced by the odors.

Baby girl mice missing a gene that codes for a receptor known as vasopressin 1a (V1aR), which binds AVP, were especially drawn to familiar odors. Loss of the gene did not change male behavior. Conversely, baby girl mice that had the gene seemed to lose interest in the odors—just like the males—if they were given an extra dose of AVP.

This implies that a rise in AVP levels early in life, which can result from genetic differences or stress or exposure to hormones in the diet, “might lower the probability of orienting toward things associated with mom or home,” says Elizabeth Hammock, first author of the study and instructor in pediatrics at Vanderbilt.

Hammock is also a Vanderbilt Kennedy Center investigator. Her research program focuses on understanding the genes, neural circuits and developmental trajectories that lead to typical and disordered social behavior and emotion regulation. She uses rodent model systems to dissect genes and circuits during postnatal development, with an interest in the mechanisms by which stress and social experience may alter typical trajectories.

Hammock says she is motivated to pursue these questions in order to broaden our knowledge about development of the social brain. Such knowledge should facilitate the creation of more effective intervention strategies used to promote better outcomes in individuals with atypical development such as autism and schizophrenia.

Her findings suggest that AVP, which is more abundant in male brains, may reinforce the tendency of male mice to leave home to establish a new territory elsewhere, a phenomenon known as “natal dispersal.”

The hormone may also play a role in social learning. AVP and its receptor, V1aR, previously have been linked to neurodevelopmental disorders such as schizophrenia and autism.

“Throughout early life, repeated exposure to individuals, experiences and things are required to build a brain with knowledge about the world,” Hammock says. “If AVP reduces the probability of orienting to things associated with the center of one’s social world, then that reduces the exposure required to enhance social expertise.” This might help explain why, on average, women have more social expertise than men and are better able to “read” emotions in others, she says.

During early development, male and female brains may be “wired” to experience the world differently. This could help explain why—in humans and some species of monkeys—girls gravitate toward dolls that replicate the family and home environment, while boys can be captivated by dump trucks, for example.

Hammock’s co-authors were former Vanderbilt research assistant Caitlin Law and Pat Levitt of the Zilkha Neurogenetic Institute at USC’s Keck School of Medicine. The research was supported by National Institutes of Health grants.