Toothcomb origins: Support for the grooming hypothesis

Abstract

Debate over the original adaptive significance of the lemuriform toothcomb, whether it was principally a grooming organ or a scraper-feeding tool, currently hinges upon the functional morphology of the lower incisors and canines of lemurs and lorises, and the fossil adapids thought to be their ancestors or structural prototypes. We suggest that the morphology of the upper incisors and the oronasal complex of the latter, given the context of a more general theory of incisor evolution within the primates, exhibits preadaptive conditions foreshadowing the emergence of the toothcomb and also evidence of strepsirhine monophyly. We find in all underived lemuriforms and in most fossil adapids where the elements are known, a striking continuity in upper incisor form, including such derived features as an interincisal diastema, strong central incisor prong, low-crowned morphology and reduced premaxillary size. The pattern suggests a basic strepsirhine reduction in the functional significance of the anterior dentition in feeding and harvesting roles. These features may be related to a novel connection of the rhinarium with the vomeronasal organ via a sulcate pair of labial folds, which serves as a component of a specialized behavioral-physiological complex dealing with olfactory communication. Rather than being the anatomical nucleus of this system, the toothcomb may have been added secondarily in the lemuriform descendants of the preadapted adapids, possibly as a device to stimulate glandular secretion of pheromones by direct pressure, and to simultaneously distribute odorants through the fur.