154 ANIMAL LIFE IN THE YOSEMITE 



necessity of climbing trees, for other downed trees in various stages of 

 decay indicated that some trees had probably fallen in the storms of each 

 year. Only one standing tree was seen upon which recent work had been 

 done. 



An intensive study of the work of the porcupines on one particular tree 

 developed the following facts and inferences. The peelings were confined 

 to the upper, younger half of the tree ; the bark there is thinner, and there 

 are fewer rough outer layers; hence the porcupine was able more easily 

 to get at the nutritious parts. The gnawings were further restricted to 

 the sides of such branches as could be reached from some convenient resting 

 place — the ground, or another branch, or a log. The tooth marks were all 

 vertical; if a branch was resting in a horizontal position, the pairs of 

 grooves marking the paths cut by the incisor teeth would cross the grain, 

 and then the bark and outer wood for a distance of from 1 to 4 inches 

 along the grain would be stripped off. Each incisor tooth (in this par- 

 ticular set of gnawings) cut a strip about 14 i^ich (6 mm.) wide, and there 

 were of course always two such strips side by side. One branch 9 inches 

 in diameter had been peeled all around. 



Ordinarily when working on a standing tree the porcupine makes use 

 of branches or stubs as supports for its body while it gnaws off the bark. 

 If the branches of a tree are so placed that the animal can work all around 

 the tree at one level the tree will be girdled and so killed, unless one of 

 the lower branches is able to take on the function of a new top. An inter- 

 esting and rather unusual departure with respect to a feeding place was 

 observed on a split tree at Tuolumne Meadows. The porcupine had climbed 

 up by holding to the sides of the crevice and had gnawed off the bark on 

 each side of the split. 



The factor which keeps the number of porcupines within bounds is not 

 obvious, but it does not seem to be that of food supply, so potent with most 

 other animals. The lodgepole pine forest could to all appearances support 

 a much larger population of these animals. Possibly the check is occasioned 

 by those of the larger carnivores which remain in the mountains through 

 the winter and, in dire necessity or othei*wise, prey upon the slow-moving 

 porcupine. The identity of these effective enemies has already been inti- 

 mated (p. 153). To judge from the frequency with which we found the 

 remains of porcupines, a good many individuals must come to grief in 

 some way or another. A factor apparently figuring here, however, is the 

 relative imperishableness of the quills ; they withstand the usual processes 

 of decay for a long time, much longer than the bones do. In some cases 

 only a mat of quills was to be found, as if every other part of the animal 

 had decayed or possibly been made away with by mice. After all, then, 

 despite the frequent remains found, porcupines, as compared with most 

 other rodents, probably enjoy an 'expectation' of long life. 



