Cranes inhabit wetlands and grasslands on all non-polar continents. Their species numbers are diverse with Sandhill Cranes numbering 500,000 to Whooping Cranes numbering 350. Although they can be dangerously aggressive, a complex system of posturing and vocalization allow rivals to resolve fights before engaging. Other vocalizations are notable like the carefully orchestrated duet among mated pairs. Cranes fly up to 52 miles per hours at heights as great as 20,000 feet, however, preferably under 5,000 feet.