Ingestion in Mammals

Ingestion is a series of biologically complex activities (capture, incision, transport, mastication, swallowing and, in infant mammals, suckling) performed by the oral apparatus. The oral apparatus includes the dentition, the masticatory muscles, numerous bones of the cranium, the squamosal–dentary joints connecting the lower jaw to the skull, the tongue, and many other structures in the head. Mammals are distinguished from other vertebrates in many aspects of ingestion. For example, in most other vertebrates, mastication does not occur. Also, food transport and swallowing are less complex in other vertebrates and generally involve the coordination of fewer muscles and other soft tissue structures. Furthermore, suckling is a behaviour that is unique tomammals. It is not surprising, therefore, that mammals have many bony, muscular, neural and other specializations for ingestion that distinguish them from other vertebrates. Among mammals, interspecific variation in these anatomical specializations frequently reflects the dominant component of a species’ diet, so that we speak of herbivores, carnivores, insectivores, frugivores and omnivores. Structures that are adapted in mammals for use during ingestion include the dentition, the chewing muscles, the lips and the lip musculature, the cheek musculature, the cranial bones, the palate, the tongue, and the pharynx. The dentition of mammals is heterodontic, whichmeans that the teeth do not all look alike as they do in fish, reptiles and most other vertebrates. Instead, mammals have incisors, canines, premolars and molars in both the upper and the lower jaw. These tooth types are structurally different and each has a specialized function. The incisors are at the front of the mouth and are usually spatulate in shape. They are used to grab food items, to break small pieces of food away from larger pieces, to kill live prey and to gnaw. In some mammals, for example rodents, the central incisors are robust, ever-growing teeth specialized for gnawing and chiselling (Figure 1a). The canines sit behind the incisors. They are usually long, sharp and pointed, and are used to pierce food objects and for selfdefence. In some mammals, for example lions and other carnivores, the upper canines are quite large and specialized for live prey capture and killing (Figure 1b). In some mammals, particularly in herbivores, the canines are absent, leaving a space, called a diastema, between the incisors at the front of the mouth and the premolars and molars at the back of the mouth (Figure 1c). The premolars and molars are commonly referred to as the postcanine dentition or the cheek teeth. These teeth are sometimes used during food capture, but they are specialized to initiate the digestive process by breaking down the food so that it is the proper size and consistency for swallowing and further digestion by the gut. Premolars andmolars have bumps (called cusps), ridges (called crests and lophs) and basins that function to crush, grind, tear, cut and/or pulp food items. The cusps on the lower postcanine teeth fit snugly into the basins on the upper postcanine teeth, and vice versa. This is called occlusion. The shape of the postcanine dentition varies dramatically among mammals because it strongly reflects diet. Carnivores and insectivores have postcanine teeth with tall, sharp, vertically oriented cusps and crests that are good for tearing and cuttingmeat.Herbivores have postcanine teeth dominated by elongate, sharp, horizontally oriented lophs and crests that are good for cutting and grinding grasses and leaves. Frugivores have postcanine teeth with low cusps and large basins that are good for crushing and pulping fruits. The cranial bones support and serve as attachment sites for the teeth, the masticatory muscles and many oropharyngeal structures. The teeth are attached to three jaw bones: the premaxilla, the maxilla and the mandible. The premaxilla and maxilla together form the upper jaw. In manymammals, the mandible is two separate bones called dentaries which are joined anteriorly in the midline at a mobile symphysis. In somemammals, the dentaries fuse at the mandibular symphysis to form a single bone in the lower jaw. Each dentary articulates with the squamosal surface of the temporal bone on the undersurface of the skull, forming a squamosal–dentary joint (SDJ). Mammals are also distinguished by the number and structural complexity of their masticatory muscles. Whereas reptiles typically have two major jaw-elevating muscles, mammals have three major muscles: the medial pterygoid muscle, the masseter muscle and the temporalis Article