Chimpanzees and humans are both omnivorous (eat plants and meat). Humans are more carnivorous than chimpanzees, and have intestines more refined towards the digestion of meat. Chimpanzees will occasionally hunt and kill other mammals, often monkeys, but otherwise restrict themselves to fruit and sometimes insects. Humans are much more dependent on meat – humans can only obtain vitamin B12 naturally through eating animal products. Based on our digestive system and the lifestyles of extant tribes, it is thought that humans have evolved to eat meat at least once every few days. Humans also tend to eat in meals rather than continuously eating throughout the day, another carnivorous trait. This may be due to meats only being available after a successful hunt, and so are eaten in large quantities but infrequently. Chimpanzees will graze on fruits constantly whereas most humans will eat no more than three times in a day.