In carnivores the gut length is reduced because meat is easy to digest. The premolar and molar teeth are designed as scissor like blades (carnassials) which act like a guillotine to slice the meat into chunks which are swallowed. The incisor teeth are slightly pointed to help grip and rip the flesh, while the canine teeth are long and dagger-like to grip into the prey to hold and kill it. The jaw joint is highly developed to give little lateral or backward motion which reduces the likelihood of jaw dislocation when catching prey. The joint is designed to give maximum gape or opening and the lack of cheeks helps this to happen. The jaw muscles are very large and there is often a crest of bone along the top of the skull to enable the bigger muscle to attach securely.

 In herbivores the main problem is that the food contains large amounts of cellulose and the digestive enzyme cellulase needed to reduce this to sugar is not produced by mammals. The food must be processed by symbiotic micro-organisms which can produce cellulase and for this to happen the food must be broken down into the smallest possible particles for rapid fermentation. The incisors are flattened and chisel shaped. the canines are frequently absent leaving a gap between the incisors and the premolars which is referred to as the diastema. The jaw joint is undeveloped and allows complete dislocation of the jaw which makes a rotary grinding action possible. The teeth are flattened with sharp ridges for grinding the food material into very tiny pieces which are easier to digest. The jaw muscle is relatively small since biting power is not important. Cheeks are present to prevent food loss whilst grinding the food. The gut is longer than the carnivores and often shows specialisation to develop areas in which complex cultures of symbiotic micro-organisms grow. In the ruminants (cows etc) this occurs in the stomach, while in rabbits it occurs in the large intestine in an area called the caecum. The stomach of the ruminant has three major sections, food is chewed quickly and passed down into the reticulum, it is then regurgitated and chewed to give a fine particle size and passed down into the rumen where fermentation takes place. The symbiotic bacteria and protozoa digest the cellulose in the food by intracellular digestion. The food is then passed into the true stomach where acid is added and digestion of the fermented material and the symbiotic organisms begins. The first two chambers of the stomach are likely to become acid due to the secretion of organic acid wastes by the micro-organisms during fermentation and the ruminant produces huge amounts of alkaline saliva which it swallows to keep these chambers slightly alkaline. In rabbits the caecum and appendix acts as the fermentation device but this is unfortunate since the absorptive area has already been passed through. Consequently the rabbit must swallow its faeces and pass them through for a second time to absorb the products of the fermentation.