Porcupines are herbivores, and as such, possess micro-organisms which digest the cellulose in the food that porcupines consume. These microflora, primarily bacteria, are located in a pouch called the caecum, located at the beginning of the large intestine. One of the most essential nutrients for porcupines is nitrogen, and their winter diet of bark does not provide enough to sustain them. Thus, most porcupines depend on reserves they build up in the summer and fall, and in so doing, lose weight during the winter.
One might wonder why porcupines don’t simply increase the amount of bark that they eat in order to get the required amount of nitrogen. The reason is that the process of bacterial digestion of cellulose is relatively slow – it takes two days for food to pass through the porcupine’s digestive system. If it were to pass through any quicker than this the bacteria would not have enough time to properly digest the bark. (Source: The North American Porcupine, by Uldis Roze)
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