Rat-natomy: Teeth and Chewing Ability

Amazing pearly... yellows?

   Rats have an amazing mouth!  Our furry friends have a total of 16 teeth in their little mouths, 12 tiny molars for grinding (that we’ll probably never see ourselves) and four long, sharp, yellow incisors in the front (the ones you can’t miss). The upper incisors are shorter and yellower than the lower ones. That’s it!  Yet rats have been known to gnaw through almost anything in their path, including metal and concrete.

    This amazing ability comes from a number of specializations built into that amazing mouth of theirs;

    First, those long teeth are “open rooted”, meaning they never stop growing. The average rat grows a whole 5 inches worth of incisor each year (2.5 mm per week).  If constantly trimmed, they grow faster, up to 1.0 mm per day.  On top of that,  individual incisors can grow at different rates: if one incisor is shortened more than its neighbors, it will grow back faster than the others.  This ability allows the rat to chew fervently on anything with out needing to worry he’ll run out of tooth to do it with.

   The only drawback to this growth habit is that if the upper and lower incisors do not meet properly (called a malocclusion), they cannot be worn away normally and become overgrown. If allowed to grow without restraint, the rat's incisors would grow in a spiral with an angle of 86º (Seen by Herzberg and Schour in their 1941 study).

   In addition to that growth rate, rat teeth are amazingly hard.  They’re actually harder than copper, platinum and even iron.  On the Mohs hardness scale the rat’s lower incisors rate 5.5, which is about the same as a steel nail (diamond is 10).  This incredibly hard material is only on the front of the incisors so that the incisors wear at an angle, with the softer material at the back wearing before the enamel in the front. This guarnatees a sharp beveled cutting edge, much like a knife. Rats sharpen their teeth by bruxing (also called thegosis), or grinding the different sides of their incisors together.

   The rat takes full advantage of its hard teeth with matching jaws of steel; the design of the rats jaw is similar to that of the alligator, giving the rat an almost unimaginable amount of power in its jaws – more than either the pitbull or lion!  Consider the proof discovered by scientists measuring various animals’ pressure per square inch: 

   Another amazing aspect, is the rat’s ability to close off their mouths while using their teeth to gnaw. The rat has small flaps of cheek tissue on either side of the inside of their mouths that close behind the incisors, protruding into the gap betwen the incisors and the molars (the diastema).  The flaps work like a seal, closing off the rest of the mouth and throat to protect is from non-food items.  This allows rats to use their teeth to dig or gnaw their way into a building without swallowing debris.

   A lesser known ability the rat carries in its mouth is the capability to adjust its incisors as needed; unlike the teeth of a human, that are firmly set in place, rats’ teeth are set in a jointed jaw.  The human mandible is one stiff piece, but the rat’s manible has a joint in the middle (mandibular symphysis) formed of fibrocartilage and ligaments.  This joint allows each side of the lower jaw to rotate slightly, seperating the lower incisors.  The rat can actually seperate and angle its lower incisors as much as 40 degrees as it gnaws and bites.  This ability gives the rat the ability to fine tune every movement to achieve it’s goal, whether holding and carrying and item or chewing through it.

   Oh, and that yellow color?  It’s not caused by a lack of brushing – they’re actually pigmented that way.  Baby rats have white teeth, but by 20 days, that color’s already growing in.  The yellow tint is actually caused by an iron compound in the tooth enamel.