Mechanical Breakdown of Foods

"The purpose of mechanical manipulation of food is to improve the access of digestive enzymes. Biting teeth can puncture an otherwise impermeable exoskeleton (arthropods) or protective armor (bony armor) of prey and allow digestive enzymes to invade tissue. Some fishes and aquatic salamanders often spit out captured prey only to snatch it with the jaws again. As they repeat this process, their tiny teeth tear down the tough outer skin of the prey." - Kardong (2002), Vertebrates: Comparative Anatomy, Function, and Evolution.

Two common mechanisms for mechanical breakdown of food are:

Mastication

• Chewing (mastication) is a hallmark of mammalian evolution, but does occur in some fish and lizards as well. Mastication allows digestive enzymes access to greater surface area of the food by breaking a whole organisms or large "chunk" into many small pieces.
• The number and type of teeth observed in an animals mouth, along with the musculature of the jaw, indicate how important mastication is to digestion in the organism and what types of foods it typically deals with.
• Each group of specialized teeth seen in the mammalian toothrow are adapted to efficiently processing a particular type of food. This specialization of individual teeth, called heterodonty (meaning "different teeeth") is a characteristic of mammals and is not well-developed in other animal groups.
• Many non-mammalian animals simply swallow large pieces of food and depend primarily on chemical digestion.

Mastication in mammals.From Kardong (2002), Vertebrates: Comparative Anatomy, Function, and Evolution.

(click on image for larger version)

Gizzards

• The gizzard, seen today in many seed-eating bird species as well as crocodiles and alligators, can be thought of as a "toothed-stomach". The gizzard is a specialized region of the digestive tract whose primary function is mechanical breakdown of food.
• Animals with gizzards typically swallow hard stones or sandy grit. These materials are held in the gizzard and act as the "teeth" of the structure. The thick, muscular walls of the gizzard grind the stones against the mass of food and break it into smaller pieces, exposing more of the surface area of the food to digestive enzymes that will are present in adjacent regions of the digestive tract.
• Interestingly, smooth, highly-polished stones found amongst the remains of some fossilized dinosaurs suggest that some of these extinct animals were herbivorous and depended on a gizzard for mechanical digestion of food.
• "The incredibly strong avian gizzard has been reported to completely crush 24 walnuts in under four hours and turn surgical scalpel blades into grit in less than 16 hours!" - Streseman (1927-1934) as cited in Proctor and Lynch (1993), Manual of Ornithology: avian structure and function.

The structure of the avian (bird) stomach. The gizzard is tough and muscular and may be lined by the "cutica gastrica", a layer of sandpaper-like tissue that aids the mechanical digestion of food. You are not responsible for any additional terminology shown here. Figure from: Proctor and Lynch (1993), Manual of Ornithology: avian structure and function.	Stomach of a Blue-hooded Euphonia, showing the reduction of the gizzard (the small knob on the right wall of the proventriculus) in fruit-eating birds. Figure from: Proctor and Lynch (1993), Manual of Ornithology: avian structure and function.