This habit of rodents might explain why you bite your nails

More Than Just a Matter of Teeth

Why does a rodent spend its time chewing on almost anything it can get its teeth on? At first glance, the answer seems obvious. A mouse or squirrel gnaws on wood and other hard objects for a purely mechanical reason: to wear down its teeth and keep them at a functional length.

However, new research conducted by the University of Michigan challenges this assumption. The act of gnawing may be much more than a simple physical need. The brain itself benefits from it, actively rewarding this behavior. Scientists have discovered that this action triggers the release of dopamine, a neurotransmitter associated with pleasure and motivation.

This discovery suggests that gnawing isn’t just about dental maintenance. It’s a behavior that the brain actively encourages. And this mechanism might even shed light on some of our own habits, such as biting our nails or grinding our teeth.

The Vital Need to Gnaw

The rodent family is vast, including animals such as mice, rats, squirrels, beavers, and capybaras. What they all have in common is constant chewing throughout their lives. Their incisors—the front teeth—grow continuously. Without regular wear, they would become too long, leading to serious complications. The word “rodent” actually derives from a Latin term meaning “to gnaw.”

Oversized teeth can indeed disrupt jaw alignment and make eating extremely difficult, if not impossible. For years, the scientific community therefore viewed this behavior as a simple reflex—a mechanical response to a physiological problem. The prevailing view was that rodents gnawed solely to maintain their teeth.

However, researchers at the University of Michigan were intrigued by an unusual observation in laboratory mice. Some individuals had longer teeth than expected. This simple observation sowed doubt: what if gnawing involved mechanisms more complex than a simple reflex?

A Surprising Discovery in the Brain

To unravel this mystery, Bo Duan of the College of Literature, Science, and the Arts at the University of Michigan collaborated with Joshua Emrick of the university’s School of Dentistry. Their investigation revealed a previously unsuspected neural circuit. "According to the old view, everyone sort of believed that teeth grinding was a very passive behavior, dictated by mechanical factors," explains Bo Duan.

Their work proves the opposite. “What we’re learning is that this is indeed a motivated behavior. There is a specific neural circuit that connects sensory information from the teeth to dopaminergic neurons in the midbrain,” the researcher continues. This neural pathway activates a reward system in the brain.

In other words, the release of dopamine encourages rodents to continue gnawing, transforming a necessity into a rewarding action. “This tells us that even the most basic maintenance behaviors are actively reinforced by the brain,” concludes Bo Duan.

How the Brain Orchestrates Gnawing

The team studied mice to understand precisely how signals travel from the mouth to the brain. They identified specific touch-sensitive neurons located around the teeth. These neurons detect the pressure exerted during chewing and send signals that then split into two distinct pathways.

The first neural pathway manages the purely physical aspect of the action. It controls jaw movements and helps position the teeth correctly. This is the mechanical circuit. The second pathway, however, is quite different. It travels to the midbrain, a region of the brain, to activate dopamine neurons there. It is this circuit that creates motivation by rewarding the behavior.

The importance of this motivation is significant. “If you block the motivation pathway, the sensorimotor pathway remains intact and helps maintain the teeth,” explains Bo Duan. “But without motivation, it’s simply not very effective. The motivational component is therefore very important.” This discovery highlights the brain’s active and essential role in encouraging a vital behavior.

From Rodents to Humans: Striking Parallels

While this study focused on mice, its findings may well have implications for humans. Although our teeth stop growing after a certain age, similar brain circuits could still influence our behaviors. “Even though human teeth stop growing, the brain mechanisms that drive repetitive oral behaviors may still be active,” suggests Bo Duan.

This connection could help explain persistent habits such as nail-biting or chewing on objects. Understanding this circuit sheds new light on how sensory signals from the mouth influence motivation and, potentially, oral health.

In fact, certain dental conditions are already linked to brain chemistry. Bruxism, which causes people to grind or clench their teeth involuntarily, is one example. Malocclusion—a misalignment of the upper and lower teeth—may also be involved.

New avenues for oral and mental health

For Joshua Emrick, who studies oral sensations at the University of Michigan School of Dentistry, this discovery could transform our understanding of dental problems. “I think the most lasting impact of this is that it helps us understand why animals engage in repetitive oral behaviors and how that relates to human pathology,” he says.

The researcher highlights a current gap in knowledge: “If you have a systemic dysfunction at a higher level, it can ultimately be very destructive to our oral tissues, and, honestly, we don’t have targeted treatments for the underlying problem. We need a fundamental understanding of how and where these behaviors are controlled in the brain.” Links have already been observed between certain brain disorders and dental health. People with autism or depression sometimes have higher rates of dental misalignment. Parkinson’s disease affects dopamine levels, and some treatments can lead to chronic teeth grinding.

Published in the scientific journal Neuron, this research opens new avenues of inquiry. “We now have evidence of a biological circuit link that could contribute to this,” adds Emrick. The team hopes this breakthrough will help develop better treatments. They are now investigating whether similar circuits control other behaviors. “We believe this could represent a more general principle,” concludes Duan. “Understanding how these circuits are organized could eventually help us target them when behavior becomes maladaptive.”

Source: earth.com

This habit of rodents might explain why you bite your nails