A squirrel's front teeth never stop growing.

Squirrels belong to the order Rodentia, a group defined by a single pair of continuously growing incisors in the upper and lower jaws. These teeth are open-rooted, meaning they lack the closed ends found in human teeth, allowing for constant growth throughout the animal's life. On average, a squirrel's incisors grow between 5 and 6 inches (12 to 15 centimeters) per year.The front of these teeth is coated in a thick layer of hard orange enamel, while the back consists of softer dentin. As the squirrel chews on tough materials like hickory nuts or tree bark, the softer dentin wears away faster than the enamel. This uneven wear creates a self-sharpening chisel edge that is perfect for piercing hard shells.If a squirrel suffers from malocclusion, a condition where the teeth do not align properly, the teeth will fail to grind against each other. In these cases, the incisors can curve backward into the roof of the mouth or spiral out of the jaw. This often leads to starvation or fatal injuries as the teeth eventually penetrate the skull or prevent the animal from eating.Research published in journals like the Journal of Mammalogy highlights that this evolutionary trait is essential for survival in the wild. Because squirrels rely on a diet of abrasive seeds and nuts, their teeth would be worn down to the gums within months if they did not regenerate. This biological mechanism ensures they always have the sharp tools necessary to access high-energy food sources.