Publications

New World monkeys display a wide range of masticatory apparatus morphologies related to their diverse diets and feeding strategies. While primatologists have completed many studies of the platyrrhine masticatory apparatus, particularly morphometric analyses, we collectively acknowledge key shortcomings in our understanding of the function and evolution of the platyrrhine feeding apparatus. Our goal in this contribution is to review several recent, and in most cases ongoing, efforts to address some of the deficits in our knowledge of how the platyrrhine skull is loaded during feeding. We specifically consider three broad research areas: (1) in vivo physiological studies documenting mandibular bone strains during feeding, (2) metric analyses assessing musculoskeletal functional morphology and performance, as well as (3) the initiation of a physiological ecology of feeding that measures in vivo masticatory mechanics in a natural environment. We draw several conclusions from these brief reviews. First, we need better documentation of in vivo strain patterns in the platyrrhine skull during feeding given their empirical role in developing adaptive hypotheses explaining masticatory apparatus form. Second, the greater accuracy of new technologies, such as CT scanning, will allow us to better describe the functional consequences of jaw form. Third, performance studies are generally lacking for platyrrhine jaws, muscles, and teeth and offer exciting avenues for linking form to feeding behavior and diet. Finally, attempts to bridge distinct research agendas, such as collecting in vivo physiological data during feeding in natural environments, present some of the greatest opportunities for novel insights into platyrrhine feeding biology.

The three species of vampire bats (Phyllostomidae: Desmodontinae), Desmodus rotundus, Diaemus youngi, and Diphylla ecaudata, are the only mammals that obtain all nutrition from vertebrate blood (sanguinivory). Because of the unique challenges of this dietary niche, vampire bats possess a suite of behavioral, physiological, and morphological specializations. Morphological specializations include a dentition characterized by small, bladelike, non-occlusive cheek teeth, large canines, and extremely large, procumbent, sickle-shaped upper central incisors. The tips of these incisors rest in cuplike pits in the mandible behind the lower incisors (mandibular pits). Here, we use microCT scanning and high-resolution radiography to describe the morphology of the mandible and anterior dentition in vampire bats, focusing on the relationship between symphyseal fusion, mandibular pit size, incisor size, and procumbency. In Desmodus and Diaemus, highly procumbent upper incisors are associated with relatively small mandibular pits, an unfused mandibular symphysis with substantial bony interdigitations linking the dentaries, and a diastema between the lower central incisors that helps to facilitate the lapping of blood from a wound. In Diphylla, less procumbent upper incisors are associated with relatively large mandibular pits, a completely fused mandibular symphysis, and a continuous lower toothrow lacking a central diastema. We hypothesize that symphyseal morphology and the presence or absence of the diastema are associated with the angle of upper incisor procumbency and mandibular pit development, and that spatial constraints influence the morphology of the symphysis. Finally, this morphological variation suggests that Diphylla utilizes a different feeding strategy as compared to Desmodus and Diaemus, possibly resulting from the functional demands of specialization on avian, rather than mammalian, blood.

The mammalian masticatory apparatus is distinguished from the intra-oral processing systems of other amniotes by a number of morphological and functional features, including transverse movements of the teeth during the power stroke, precise occlusion, suspension of the teeth in the socket by a periodontal ligament, diphyodonty (reduction to two generations of teeth), a hard palate, and the presence of a single bone (the dentary) in the lower jaw which articulates with the skull at the temporomandibular jaw joint. The evolution of these features is commonly argued to have improved the efficiency of food processing in the oral cavity. The present aricle highlights the existence in mammals of the fusimotor system and afferent fibers from the periodontal ligament through which the CNS modulates the responses by the muscle spindles. Published data suggest that the fusimotor system and the periodontal afferents are important components in feed-forward (or anticipatory) control of chewing behavior. We hypothesize that this feed-forward control is used to maintain relatively constant cycle lengths in mammals in the face of intra-sequence and inter-sequence variation in material properties of the food, and that this enables them to maintain a higher average chewing frequency than that of lizards. These predictions were evaluated using data on mean cycle length and its variance from the literature and from our own files. On average, mammals have less variable cycle lengths than do lizards and shorter cycle lengths than do lizards of similar size. We hypothesize that by decreasing variance in cycle length, presumably close to the natural frequency of their feeding systems, mammals minimize energy expenditure during chewing, allowing them to chew for longer, thereby maintaining the high rates of food intake required for their high metabolic rates.