202 MICHIGAN ACADEMY OF SCIENCE. 



Grinding the Teeth. 



The habit of grindiug or grating- the teeth has already been mentioned as bsing 

 performed usually before eating and at times while eating. At such times the 

 animal extended the head slightly and partially closed the eyes. The action of the 

 jaws was accompanied by a peculiar squeaking or grating sound caused by the 

 friction of the teeth and which could be heard distinctly across the room. It is a 

 well-known fact that the incisor teeth of rodents continually grow at the roots and 

 that this growth is normally compensated by the wear of the teeth, keeping them of 

 approximately the same length. Familiar cases of the disastrous results of a fail- 

 ure to wear away rapidly enough are those brought about by the breaking of the 

 opposing teeth, and other causes, allowing the remaining incisors to grow very 

 long, curling around outside the mouth or growing within and prying it open so that 

 it cannot be closed, or even sometimes piercing through the roof of the mouth into 

 the nasal cavity. The manner of wearing also results in keeping the teeth sharp, 

 the softer back portion wearing away faster than the hard enamelled front, which 

 is left as a chisel-like edge. 



In squirrels and those rodents which gnaw hard substances like nuts and wood 

 a great deal, the ordinary wear is probably nearly sufficient to compensate for the 

 growth.^ The muskrat, however, lives almost entirely upon soft foods, the eating 

 of which alone does not seem to be sufficient to keep the teeth in the right condi- 

 tion. 



When at rest the upper and lower incisor teeth of the muskrat' have about the 

 relative positions to one another shown in Fig 1. In grinding the teeth the first 

 action is usually to extend the lower jaw so that the upper incisors rest upon the 

 lower, as shown in Fig. 2. The jaw is then moved rapidly backward and forward 

 in the horizontal plane, the result being to grind the softer portion of the lower 

 incisors with the hard otiter portion of the upper. After a moment the positions 

 are reversed, the lower teeth are brought to bear upon the softer portion of the 

 upper, as shown in Fig. 3, and now a motion in a more or less perpendicular plane 

 results in sharpening the latter. There is also a third method, a side-to-side mo- 

 tion of the lower jaw, the exact result of which could not be determined, but it 

 probably helps to sharpen either set of teeth, depending upon their position at the 

 time. 



When first captured the incisors; viewed from the front, presented nearly square 

 ends, as is indicated in Fig. 5. It was soon noticed, however, that the outer edges 

 of the lower teeth were wearing away faster than the middle portion, resulting in 

 a wedge or V-shape (Fig. 6), while a smaller corresponding notch was developed 

 in the upper teeth. For the cause of this we have no explanation to offer unless it 

 may in some way have been due to the change in food while in confinement. 



Activities in General. 



As noted above the muskrat was for the most part rather quiet in the daytime, 

 but if one entered the room at night and suddenly ttirned on the lights he was 

 usually discovered otit eating or moving about the cage. The sudden turning on of 

 the lights seemed usually to frighten him and he would dive quickly into the water 

 in the tank below, a thing he seldom did in the daytime unless he happened to be 

 near the hole leading down to it when some unusual noise or movement frightened 

 him. At either time he could nearly always be coaxed up again by offering him 

 something to eat. Sometimes, however, when unusually frightened, as when a 

 crowd of strangers was about, he would dive under the water and remain for some 

 time. He would also dive and get kernels of corn from the bottom; these he 

 usually ate floating at the surface of the water, and he also sometimes ate in the 

 same way other food which he carried down into the tank himself. More often, 

 however, instead of floating entirely free he would eat resting on the inclined 

 screen which served for a ladder, with just his head and shoulders out of water. 



When in his cage the muskrat seldom retreated from the hand or any other 

 object put in towards him, but held his ground and struck at the intruder with his 

 fore feet. This seemed to be the most common method of defense. Drawing back 

 on his hind legs with his fore feet brought in close to the body he would then 

 suddenly lunge forward and at the same time extend the fore legs and bring the 

 paws downward with a raking motion, the whole movement being very quick. He 



1 Wp have noticed, however, that the fox squirrel sometimes grates Its teeth in much the 

 same manner as here described for the muskrat, but not with any such reRularity nor with 

 such persistence. 



