For now, they simply enjoy working their incisors.
Rodents' incisors grow throughout their lives, and if they are not worn down on something hard, they can become so long that the rodent cannot eat. Therefore, constant gnawing for mice, rats, hamsters, beavers, and others is literally a matter of life and death. But what drives them to gnaw? For a long time, biologists believed that it was an automatic reaction, similar to blinking – the body simply does something on its own, based on reflexes.
Scientist Bo Duan and his colleagues at the University of Michigan noticed that some of their laboratory mice had excessively long incisors, even though they had everything necessary at hand – or rather, in their mouths – to wear them down. Other mice, living alongside the "long-toothed" ones, had normal-sized teeth. The researchers hypothesized that something had gone wrong in the neurobiological apparatus that controls gnawing in some animals.
In the laboratory, there were genetically modified rodents, in which it was possible to deactivate a specific group of nerve cells or a particular area of the nervous system using a toxin. By turning off a group of nerve cells, researchers could observe what processes they were involved in and what behaviors depended on them. This method revealed that sensory neurons responding to touch on the incisors are connected in rodents to two neural circuits. One monitors the bite and the positioning of the teeth relative to each other.
The second goes into the reinforcement system – a group of nerve centers that provide pleasure from the anticipation of a completed task through the neurotransmitter dopamine, stimulating motivation. Simply put, rodents enjoy working their incisors. If this neural pathway is disabled in mice, they lose interest in gnawing, and their incisors grow longer than they should. It is likely that there are (though rarely) natural failures in this mechanism, resulting in the emergence of rodents that do not like to gnaw – like those that caught the authors' attention. The results of the study were published in the journal Neuron.
Rodents may not be the only ones with such a neurobiological mechanism. One can also think of dogs that incessantly chew on some toy, and humans who chew gum or bite their fingers. However, regarding humans, one must be more cautious – although our compulsive actions are reinforced by the brain's dopamine system, human motivation for such behavior is very complex and is unlikely to be reduced to signals from the teeth.
