Police dogs (K-9's) evolved from military uses primarily as trackers and territorial guards to perform a number of tasks that continue to expand. Initially used as guards or to find missing persons and pursue fleeing suspects, K-9's are now partners with police personnel in search and rescue and the detection of various hidden items critical to crime prevention, crime detection, and other public safety concerns. With their heightened sense of smell, intelligence, and ability to enter areas not accessible to police officers, K-9's have sniffed out smuggled drugs, foodstuffs, animals, and plants at U.S. borders; and have detected bombs, stolen prescription medications, hidden tunnels, arson accelerants, and even bodies under 35 feet of water. The German Shepherd is often the breed selected by officers and security personnel for their work. This is because of their capacity to learn, their loyalty, and their ability to transition quickly from being sociable and friendly to being protective and aggressive. They are also capable of distinguishing between threatening and benign situations.