reproduction rate. Diet and foraging Pigs are omnivores, which means that they consume both plants and animals. In the wild, they are foraging animals, primarily eating leaves, roots, fruits, and flowers, in addition to some insects and fish. As livestock, pigs are fed mostly corn and soybean meal with a mixture of vitamins and minerals added. Traditionally, they were raised on dairy farms and called "mortgage lifters", due to their ability to use the excess milk and whey from cheese and butter making combined with pasture. Older pigs will consume three to five gallons of water per day. When kept as pets, the optimal healthy diet consists mainly of a balanced diet of raw vegetables, although some may give their pigs conventional mini pig pellet feed. Relationship with humans Most pigs today are domesticated pigs raised for meat (known as pork). Miniature breeds are commonly kept as pets. Because of their foraging abilities and excellent sense of smell, people in many European countries use them to find truffles. Both wild and feral pigs are commonly hunted. Apart from meat, pig skin is turned into leather, and their hairs are used to make brushes. The relatively short, stiff, coarse pig hairs are called bristles, and were once so commonly used in paintbrushes that in 1946 the Australian Government launched Operation Pig Bristle. In May 1946, in response to a shortage of pig bristles for paintbrushes to paint houses in the post-World War II construction boom, the Royal Australian Air Force (RAAF) flew in 28 short tons of pig bristles from China, their only commercially available source at the time. Use in human healthcare Human skin is very similar to pig skin, therefore many preclinical studies employ pig skin. In addition to providing use in biomedical research and for drug testing, genetic advances in human healthcare have provided a pathway for domestic pigs to become xenotransplantation candidates for humans. Species The genus Sus is currently thought to contain nine living species. Several extinct species (†) are known from fossils. Extant species Sus ahoenobarbus Huet, 1888 – Palawan bearded pig Sus barbatus Müller, 1838 - Bornean bearded pig Sus cebifrons Heude, 1888 – Visayan warty pig Sus celebensis Müller & Schlegel, 1843 – Celebes warty pig or Sulawesi warty pig Sus domesticus Erxleben, 1777 – Domestic pig (sometimes considered subspecies of S. scrofa) Sus oliveri Groves, 1997 – Oliver's warty pig or Mindoro