Animal Life in the Yosemite
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YELLOW-HAIRED PORCUPINE. Erethizon epixanthum epixanthum (Brandt)

Field characters.—Size larger than that of Sierra Marmot; tail less than half head and body; pelage very long; upper surface and sides of body and tail with many barb-pointed yellow quills which can be raised at will. Head and body 21-1/2 to 27 inches (545-682 mm.), tail 7 to 9 inches (175-225 mm.), hind foot 3-3/4 to 4-3/4 inches (95-120 mm.), ear about 1 inch (27 mm.); weight 15 to 20 pounds (estimated). [Measurements from California specimens taken outside Yosemite region.] Body coloration blackish, a few long hairs and the quills yellow except for black tips. Workings: Areas on coniferous trees denuded of bark and showing paired marks of incisor teeth, each mark about 1/4 inch (6 mm.) broad (pl. 37a). Droppings: Found on ground beneath 'barked' trees, about 1 inch long, 3/8 inch in diameter, rounded and slightly curved, composed of undigested wood pulp.

Occurrence.—Moderately common resident in boreal region on Sierra Nevada. Workings noted from Porcupine Flat eastward to Tuolumne Meadows. Individuals occur (rarely) on floor of Yosemite Valley and are reported at even lower stations. Lives in coniferous trees, chiefly lodgepole pines. Solitary.

The Yellow-haired Porcupine is perhaps one of the best known of our native mammals, by reputation at least, though not all the stories which are current concerning it are accurate. Being of sluggish disposition and active by day as well as night it gives visitors in the high Sierras many opportunities to observe it at close range. And should these fail, the work of the animal is evident in many places.

In general, the porcupine is to be found in the high Sierras above the level of Yosemite Valley; its range is practically the same as that of the lodgepole pine. Occasionally, however, individuals are observed at much lower levels. We were told, for example, of one trapped on the floor of Yosemite Valley in September, between 1916 and 1918. We have been told of individuals seen on Bullion Mountain. And at Snelling one resident told of a porcupine which he had shot in the river bottom a mile from town, and of two or three others, possibly castaways brought down the river in drift, which had been observed in the same locality.

Fig. 26. Quill from Yellow-haired Porcupine showing details of tip and base. At the tip are numerous small barbs which when the quill penetrates skin or flesh keep it from being pulled out; at the base is the slender and weak connection which makes for ready separation of the quill from the skin of the Porcupine.

The porcupine's chief claim to attention lies in its covering of sharp-pointed hollow quills which are especially developed on its back and tail. These quills are specialized or modified growths which supplant some of the underfur normally present on a mammal's body. (See fig. 26.) Individually, a quill consists of a hollow tube, closed at both ends, about 1/16 to 1/8 inch in diameter and 1 to 3 inches in length. The tip is supplied with a great number of backward-projecting small barbs which upon touching the flesh of another animal instantly engage and hold fast. The bases of the quills are constricted and are weakly held in the porcupine's skin so that they become detached readily when the barbs are imbedded in some victim.

When a porcupine expects attack from another animal it draws its head down, and erects the quills of its back; if the enemy approaches too closely, it also sweeps the tail quickly to one side or the other. This is sufficient warning for most animals, but certain species, and particularly young and inexperienced individuals of those species, do venture to attack porcupines. Often the attacker receives a load of quills for his pains. We have seen a coyote with jaws and feet literally filled with the barbed quills of a porcupine. Occasionally wildcats are found with porcupine quills imbedded in their flesh, particularly about the head and forelegs. Local information is rather scant as to the exact measure of success attained by those species which prey or attempt to prey upon the porcupine.

The Yellow-haired Porcupine is a bark feeder, that is, it subsists on the soft growing tissues (cambium) found at the junction of the inner bark and outermost wood in a growing tree. Its preferred food tree in the Yosemite region is the lodgepole pine, probably because least effort is required to obtain the cambium in that thin-barked tree. This sort of food is available at all seasons of the year and the porcupine therefore has no need to lay up a food supply nor to hibernate, or migrate, as do many of the other herbivorous mammals. The abundance of easily obtained food probably accounts in large measure for the porcupine's sedentary nature. At several points in the Yosemite region we saw trees whose condition indicated that a porcupine had wintered in a very restricted area, and had probably spent many days in each one of the few trees which had been peeled. To get at its food the porcupine shells or scales off the outer bark and then actively gnaws off the tender growing tissue.

A most favorable locality in which to study the work of this animal is the place on the Tioga Road called Porcupine Flat. The territory comprising Porcupine Flat was once occupied by one or more glaciers, and the terminal moraine of one small glacier forms a lake there. This glaciation cleared the surface down to bedrock. Since the glaciers have disappeared only a small amount of soil has accumulated on the surface of the rock. All the trees are therefore shallow rooted and may be easily struck down by storms. The trees newly prostrated provide easily accessible food for the porcupines.

Porcupine Flat is well named, to judge from the amount of the porcupine work which we saw there. The forest on parts of the nearly level floor is purely of lodgepole pine ranging from well spaced trees as much as 3 feet in diameter down to small saplings in dense groves of an acre or so in extent. There was, in June, 1915, much fallen timber, including scores of trees which had been downed the preceding winter or possibly within three months, for much of the foliage was still green. These trees showed that the porcupines had been very busy; they bore marks of incisor teeth on trunks, on large branches, and even on twigs down to half an inch in diameter. (See pl. 37a). These gnawed places were all 'bleeding' much pitch at the time of our visit. Evidently, for years, the porcupines had regularly taken advantage of these local circumstances, and so avoided the 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 particular set of gnawings) cut a strip about 1/4 inch (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 interesting 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 otherwise, prey upon the slow-moving porcupine. The identity of these effective enemies has already been intimated (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.


Animal Life in the Yosemite
©1924, University of California Press
Museum of Vertebrate Zoology

grinnell/mammals52.htm — 19-Jan-2006