Does being a dad change a man's brain? The answer isn't clear in humans, but a study with tiny monkeys called marmosets suggests fatherhood may alter gray matter.

Researchers say the structure of the brain is different in marmoset dads vs. non-dads. They also found that the brains of fathers were more receptive to a hormone linked to learning.

The Princeton University study is "very exciting" because it's apparently the first to link paternal parenting to physical and chemical changes in the brain of any primate, said Jon E. Levine, a professor of neurobiology and physiology at Northwestern University who's familiar with this research.

Marmosets live high in trees in the jungles of South and Central America. They're small, with adults sometimes reaching just 6 inches long.

"Marmoset fathers, unlike many other male mammals, are very involved in offspring care," noted study lead author Yevgenia Kozorovitskiy, a graduate student in neuroscience at Princeton.

In fact, research suggests that marmoset fathers carry their young 70 percent of the time during the first month of life. "In addition, the complexity of their brains makes them a good model for examining the processes that might occur in humans," Kozorovitskiy said.

In the new study, the researchers examined the brains of marmoset fathers and non-fathers. Their findings appear in the September issue of the journal Nature Neuroscience.

When compared to non-fathers, the prefrontal cortexes of marmoset fathers had a higher density of "spines" that formed on dendrites, the microscopic branches of neurons. Dendrites allow neurons to communicate with each other.

In humans, the prefrontal cortex is considered a center for emotion and higher thinking, including learning about the consequences of actions.

The researchers also found that the brains of marmoset fathers had more receptors for a hormone known as vasopressin, a neuropeptide. That means their brains could process more of this chemical than the brains of non-dads.

Vasopressin is strongly connected to parental behavior, the researchers added. In humans, the hormone -- produced in the pituitary gland -- is crucial for learning and memory.

In essence, then, "the experience of being a father dramatically alters brain regions important for cognition," Kozorovitskiy said.

According to Levine, it's not yet clear how these changes came about in the marmoset brains. "Do these changes mediate some aspect of paternal behavior, or are they secondary to physical or hormonal changes that may occur as a consequence of the behavior?" he asked. "Cause and effect still need to be explored."

And, of course, researchers would like to know if there are similar effects in human fathers. For now, though, "these are primates that exhibit paternal behavior, which is about as close to human relevance that you are going to get with an experimental animal," Levine said.