Cattle have been domesticated since the neolithic era, but can survive for months or even years with little or no supervision. Their ancestors, the Aurochs, were quite fierce. Modern cattle, especially those raised on open range, are generally more docile, but when threatened can display aggression. Cattle, particularly those raised for beef, are often allowed to roam quite freely and have established long term independence in Australia, New Zealand and several Pacific Islands. There are small populations of semi-feral animals living the southwestern United States and northern Mexico. Such cattle are called Mavericks, Scrubbers or Cleanskins. Most free roaming cattle, however untamed, are valuable and are usually rounded up in closely settled regions.