1350 AD as a probable arrival date for the main founding canoes from which most Māori trace their descent. The descendants of these settlers became known as the Māori, forming a distinct culture of their own. The latter settlement of the tiny Chatham Islands in the east of New Zealand about 1500 CE produced the Moriori; linguistic evidence indicates that the Moriori were mainland Māori who ventured eastward. There is no evidence of a pre-Māori civilisation in mainland New Zealand. The original settlers quickly exploited the abundant large game in New Zealand, such as moa, which were large flightless ratites pushed to extinction by about 1500. As moa and other large game became scarce or extinct, Māori culture underwent major change, with regional differences. In areas where it was possible to grow taro and kūmara, horticulture became more important. This was not possible in the south of the South Island, but wild plants such as fernroot were often available and cabbage trees were harvested and cultivated for food. Warfare also increased in importance, reflecting increased competition for land and other resources. In this period, fortified pā became more common, although there is debate about the actual frequency of warfare. As elsewhere in the Pacific, cannibalism was part of warfare. Leadership was based on a system of chieftainship, which was often but not always hereditary, although chiefs (male or female) needed to demonstrate leadership abilities to avoid being superseded by more dynamic individuals. The most important units of pre-European Māori society were the whānau or extended family, and the hapū or group of whānau. After these came the iwi or tribe, consisting of groups of hapū. Related hapū would often trade goods and co-operate on major projects, but conflict between hapū was also relatively common. Traditional Māori society preserved history orally through narratives, songs, and chants; skilled experts could recite the tribal genealogies (whakapapa) back for hundreds of years. Arts included whaikōrero (oratory), song composition in multiple genres, dance forms including haka, as well as weaving, highly developed wood carving, and tā moko (tattoo). New Zealand has no native land mammals (apart from some rare bats) so birds, fish and sea mammals were important sources of protein. Māori cultivated food plants which they had brought with them from Polynesia, including sweet potatoes (called kūmara), taro, gourds, and yams. They also cultivated the cabbage tree, a plant endemic to New Zealand, and