As of 2016, there were 29,500 rhinos left in the world, 70 percent of which were living in South Africa. Photographer Brent Stirton, who is himself South African, has won international acclaim for his coverage of one of the biggest threats to the remaining rhino population: the illegal trade of their valuable horns. In 2015 rhinos were killed by poachers at a rate of nearly four a day. In recognition of World Rhino Day, we bring you a selection of Stirton's most compelling images, photographed on assignment for National Geographic magazine.