Porcupine feeding patterns: selectivity by a generalist herbivore?

Abstract

The North American porcupine (Erethizon dorsatum) is a feeding generalist, but can feed primarily on phloem of the ponderosa pine (Pinus ponderosa) during winter months in areas of the Rocky Mountains where ponderosa pine occurs in nearly pure stands. This pine is highly variable both in resin monoterpene composition and at the allozyme level. Porcupines can girdle branches or whole trees when eating phloem, and in the process often damage or kill the trees. Trees that had been fed upon (target trees) differed biochemically and genetically from adjacent trees that had not been fed upon (nontarget trees). Target trees had lower levels of the monoterpene limonene in the xylem oleoresin than matched nontarget trees, and there was a significant negative association between levels of limonene and the amount of phloem removed from individual trees. Target trees differed only slightly from nontarget trees in allele frequencies at two of nine polymorphic loci tested. When the feeding patterns of porcupines were compared with those of a previously studied mammal that feeds primarily on the phloem of ponderosa pine, Abert's squirrel (Sciurus aberti), the level of feeding selectivity was found to be far less pronounced in the porcupine.