How does an animals physiology dictate its diet?

An animal's physiology can indicate it's diet based on what the animal's body can handle processing. For example, animal's with strong bones and teeth can break down food that is tough to eat, this helps obtain energy alot quicker. For instance, giraffes have long and slender necks that are perfect for reaching up to trees. The giraffe's diet would consist of vegetation located in high levels.