Functional histology of quill erection in the porcupine, Erethizon dorsatum

Abstract

The histology of the porcupine's skin and subcutis is described. The functional relationships of the thoracic skin during elevation of the quills involve a smooth arrector muscle attached to quill sets, one set behind the other, with the muscle at a slant usually from a single root tip of one set to the distal part of the follicles in the caudal set. Stretched elastin returns the quills to their resting position. There is a low-resistance pathway for the tilting quill roots through loose connective tissue and fat until a collagenous strand attached to the quill set stops further tilt. The lumbosacral region (rosette) has a two-layered arrector, but it attaches to only its own quill set and inserts into the dermis. Both regions have a varied and complicated system of transverse and oblique muscles associated with the roots in a set. (The occurrence of obliques is variable in the thoracic region.) A hypothesis is presented concerning the mechanism of quill detachment from the porcupine after the quill is rammed into a predator. This involves the shearing of the conical root tip along its adherent follicular cells and a stopping device (spool) to prevent the root from self-stabbing.