Factors affecting food comminution during chewing in ruminants: a review

A review is presented of the chewing effectiveness of herbivorous mammals dealing with the relationship between food comminution (i.e. reduction of particle size), morphological features of teeth, chewing behaviour (i.e. time spent chewing and chewing rate), and the chemical and physical properties of plant tissues. Chewing is the main food processing mechanism in herbivores, increasing the surface/volume ratio of the food, which is a key factor affecting the efficiency of digestion and, therefore, body condition, reproductive success and life history. Chewing effectiveness (CE) is defined as the reduction of a pre-determined amount and particle size of a given food after a known, but not necessarily determined, number of chews. The two main animal-centred factors influencing CE are tooth effectiveness and chewing behaviour. The most frequently used predictors of tooth effectiveness are molar occlusal surface area, molar occlusal contact area (defined as any surface of the upper and lower teeth in or near contact during occlusion) and the length of the enamel cutting edges of the occlusal surface. There is expected to be a direct positive relationship between the predictors of tooth effectiveness and chewing effectiveness. Chewing behaviour has particular importance to food particle reduction in ruminants, because they spend long periods chewing during both initial ingestion and ruminating. The majority of studies find significant unexplained variance when CE is predicted using tooth features or chewing behaviour parameters. There is also little agreement as to what is the key morphological factor determining tooth effectiveness, or what is the relationship between tooth effectiveness and chewing behaviour. The type, maturity stage and physical presentation of the food also contribute to the final particle size after food has been chewed, because of the involvement of the concentration of chemical components of the cell walls (acid detergent and neutral detergent fibres, lignin) and the architectural structure of the plant tissues in particle breakdown. The relationships between body mass and tooth effectiveness, chewing behaviour and CE are also discussed.