Rat Biology 101
By: Sarah Paterson-Farrand

Many of us overlook some of the basic biology of rats, but they really are fascinating creatures, and their uniqueness, even just in the face, goes beyond the fuzzy faces and quizzical expressions.

Did you know that rats have a Harderian gland, a pigmented, tear-producing gland, which is located right behind the eye and encircling the optic nerve? It secretes fluids that are rich in porphyrin, a pigmented, naturally occurring compound, especially at times of stress, creating the ‘blood-looking’ discharge sometimes seen around the eyes and nose.

Did you ever look at those teeth and wonder why they are so dark in color? Rat's teeth have a hard, yellowish-orange enamel coating, which protects the softer, white dentine, which makes up the rest of the tooth. The dark color is due to an additional iron compound in the teeth, which beyond appearance, also keeps them very strong. Rat's teeth actually have no nerve tissue or root, either. The normal or average length of healthy rat teeth is 1/4" for the upper and 1/2" for the lower. They do grow continuously at a rate of about 2.5mm/week. And although you might have heard that your rats need wood chews or something similar to keep those front teeth at a healthy length, rats actually grind them against each other to keep the length and bevel just right, a phenomenon known as bruxing in the rat fancy community. Of course, that doesn't mean that offering wood chews is a waste of time. Most rats enjoy them thoroughly, and they are a great source of enrichment. In fact, SMRF currently sells bundles of willow chews in varied sizes for rats and other small animals in our webstore. Teeth can become misaligned, creating a condition called malloclusion, and in that case, teeth may need to be trimmed to try to realign them to allow normal grinding patterns. But that is the only time where a rat owner might have to step in to intervene. The rats will take care of the rest. And since we're on the subject, what about that bruxing? The sound you hear is actually the repetitive grinding of the front teeth (incisors) against one another. It sharpens and maintains length, but rats also tend to brux both in times of deep relaxation and in times of stress. It is often accompanied by the appropriately named eye boggle. The eyeball itself literally vibrates in and out of the eye socket as the masseter muscle below it is moving. When a rat bruxes, the muscle, which passes beneath the eye, causes the jaw to move up and down. These muscle contractions vibrate the eye at the same time, and it is most often associated with pleasure or contentment. So since this is a jaw muscle, why don't you see the eyes ’boggle’ when a rat is gnawing? There are actually three jaw muscles used for gnawing or chewing, and although it includes the masseter muscle, two other muscles (temporal and pterygoid) are also used. A completely different tooth and jaw placement occurs than for bruxing. When a rat is gnawing, the bottom jaw actually is jutted forward so the incisors line up, which is why the teeth have such a sharp beveled edge. For chewing, the jaw relaxes back so the molars line up. Rats have 12 molars, 6 upper and 6 lower. They have 4 incisors, 2 upper and 2 lower. The 2 lower incisors are not fixed, rather they are in a cartilage like tissue that allows them to separate and rotate as needed, up to a 40 degree angle even.

Although these bits of information don't involve the face, they are equally interesting. A normal body temperature for a rat is 99.5º - 100º F. Their temperature is maintained by something called non-shivering thermogenesis. In human infants and some mammals, brown adipose tissue or brown fat is present in the body. It is in this tissue that heat is produced through a chemical pathway. When temperature needs to be decreased, that chemical reaction is slowed. The ears and tail also become more vascularized to release heat as the blood circulates. Like horses, rats have no gall bladder. Rats lungs are unique, as well. The left lung has only 1 lobe, but the right lung has 4 lobes. Respiration rate is 70-115 breaths/min compared to that of the average adult human, whose rate is around 12-20 breaths/min. A rat's normal heart rate is 250-450 beats/min, while that of a human is 72-80 beats/min. Rats have 5 pairs of mammary glands (with does having 12 nipples), and they are diffuse, distributing tissue over most of the body's surface. This is why, unfortunately, mammary tumors can show up under the skin's surface nearly anywhere on the body, and can even occur in males. In females, estrus cycles occur every 4-5 days and last around 12 hours. Reproduction has an average gestation period of 22 days, and while litter size averages between 6 and 12, many more or less are certainly possible. Once a rat gives birth, she is fertile immediately. This means that a single female rat has the potential to create a LOT of offspring.

These are a few of the many fascinating facts about our rats. We hope to add to this in the future!

References available upon request.

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