Functional Design of Cranial Muscles Comparative and Physiological Studies in Pigs

Abstract

Synopsis. The adaptive significance and evolutionary history of the internal architecture of the masseter muscle in suid artiodactyls are investigated. Hypotheses are developed on the basis of anatomical and physiological studies in the pig, Sus scrofa, and used to make predictions about the expected anatomy in the giant forest hog, Hylochoerus meinertzhageni and the warthog, Phacochoerus aethiopicus. The hypotheses are then tested by anatomical examination of these animals. In the pig, pinnation of the masseter serves to increase masticatory force and to partition the muscle into three semi-independent parts: vertical anterior fibers, which are used especially to close from wide gape; a central pinnate por? tion, which is used for all activities; and horizontal fibers covering the jaw joint, which are used especially for anterior translation of the condyle. In the giant forest hog and warthog the structure of the masseter is more uniform and the physiological cross section is larger than in the pig. These changes are interpreted as correlates of decreased emphasis on gape and increased emphasis on masticatory force in the African suids. In order to clarify the evolutionary sequence, masseteric architecture in the suids is compared with that of tayassuids and the hippopotamus.