The digestive system is one of the most reactive physiological systems to changes in environmental conditions as a result of a trade-off between functional benefits and maintenance costs. Empirical studies indicate that highfiber diets promote an increase in the rate of food intake, and, thus, an increase in the size of the digestive organs. Here, we investigated variation in the size of digestive organs in 4 populations of Abrothrix olivacea and A. longipilis, both inhabiting 2 contrasting habitats. Based on published data on diet composition, we predicted that individuals from the more-productive forest habitat should show larger digestive organs than individuals from the less-productive steppe habitat. As expected, we found that individuals from the forest had significantly larger small and large intestines (relative to body mass) than individuals from the steppe. In addition, we found that individuals from the forest had a larger body size than individuals from the steppe, suggesting that habitat productivity has an important effect on body size.