Domestic pigs were introduced from Europe to America by Spanish explorers. Over time, some pigs escaped or were intentionally released into the wild. Subsequently, free-ranging populations established themselves on American soil. They are aggressive animals, posing serious ecological, economic, aesthetic, medical, and veterinary threats. Feral pigs have a strong sense of smell. They use their tough flexible nose to sniff out roots, tubers, and small animals living underground. They can run up to 30 miles per hour, and are good swimmers. Their lifespan is from 15 to 25 years. Biologists estimate the nationwide population of feral pigs to be around 4 million.