276 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1940 



or entirely lacking. The tiniest of these rodents are the silky- 

 pocket mice which weigh only from 7 to 10 grams and have fur so 

 fine and silky that it can scarcely be felt with hmnan fingers. They 

 dig burrows about three-quarters of an inch in diameter into the 

 soft sand, preferably selecting a location near a sagebrush or grease- 

 wood or some other kind of desert plant. They close the entrance 

 to the burrows when they go out and when they come in so that 

 enemies may have more difficulty in finding the entrance. It must 

 also be remembered that on warm dry sands, scent does not remain 

 long. Therefore, the enemies that would hunt them by scent have 

 no trail to follow a short time after pocket mice have passed by. 



The little hedgehogs {Erinaceus and related genera) that feed 

 mainly on insects and small snakes are also believers in passive re- 

 sistance. Their bodies are clothed with a dense coat of short, fairly 

 sharp spines that are more securely attached to the skin than are 

 the spines of the American porcupines {Erethizon and Coendou). 

 Around the lower edge of the body there is a circular muscle that 

 acts as a draw string. When danger threatens, the little fellow tucks 

 his head beneath himself, pulls in his legs and very short tail, then 

 tightens the draw string. This very effectively pulls him into a ball 

 about the size of a grape fruit, with his head, feet, legs, and tail 

 well enclosed near the center. At the same time the spines are 

 erected so that the animal appears as a spiny ball. In this form he 

 is quite inaccessible to almost any enemy. 



The tactics of porcupines in both the Old World and America are 

 alike. When danger threatens, they merely turn their backs to the in- 

 truder and let him hurt himself on their spines. However, the North 

 American porcupine {Erethizon) adds a bit to this trick by giving a 

 quick sidewise switch of his tail when the enemy is near. This often 

 drives the barbed quills into the flesh of the enemy, making painful 

 wounds ; and the movement is so quick that many people believe that 

 the porcupine throws its quills. Such however, is not the case. The 

 large African porcupine {Hystrix) will charge backward when an 

 enemy is close. The spines of porcupines are very lightly attached 

 to the skin. Usually they stick into and stay with the victim. The 

 quills of the American porcupines are barbed at the tips and so pene- 

 trate deeply into the enemy's flesh. The quills of Old World porcu- 

 pines are not barbed. Some are merely flattened bristles often deeply 

 grooved longitudinally. 



Many small rodents have remarkable abilities of leaping over dis- 

 tances equal to many times the animals' own length. Some kinds 

 can leap backward or sidewise as well as forward m an incredibly 

 short time from the actual alarm — in fact, so quickly that the human 

 senses do not detect the lapse of time from the alarm until the animal 



