Johannesburg - First there was Cecil, a Zimbabwean lion whose 2015 killing by an American hunter ignited international outrage. Now Cecil's son Xanda has been killed in the same area, bringing fresh scrutiny on the "trophy" hunting of a species whose numbers in the African wild have plummeted.Some conservation groups denounced 6-year-old Xanda's reportedly legal killing, saying commercial hunting bans and robust wildlife tourism in countries such as Kenya and Botswana are among the best ways to protect threatened species. The hunting industry, meanwhile, counters that it has a conservation role if it is well-regulated, channeling revenue back into wildlife areas that otherwise could end up neglected or turned into livestock farms.Many researchers agree that Africa's lions face greater threats, including human encroachment on habitats and the poaching of animals for food, which deprives lions of prey.A more recent concern is the legal export of South African lion skeletons to a traditional medicine market in Asia, which some critics believe could lead to increased poaching of wild lions to meet demand."The species is in free-fall," said Will Travers, president and co-founder of Born Free, an international conservation group. He cited estimates that there are only 20 000 wild lions left in Africa.Xanda was killed on or around July 7 just outside Zimbabwe's Hwange National Park and the shooter is a client of Zimbabwean professional hunter Richard Cooke, a group called World Heritage Species said on Facebook. Group members include Brent Stapelkamp, a researcher who monitors lions in Hwange park, where Cecil and Xanda lived."Although the hunt was reportedly legal (all the proper papers were in order, Xanda was over the age of 5, and he was outside the boundaries of the park), Cooke has refused to divulge the identity of his client. Cooke also led the hunt in 2015 that killed Cecil's other son," the group said.Efforts to reach Cooke by email and phone were not immediately successful. Zimbabwean wildlife authorities have not commented publicly.Like Cecil, Xanda was wearing a GPS collar that allowed researchers from WildCRU, a group affiliated with the University of Oxford in Britain, to monitor his movement, according to World Heritage Species. The group said researchers "were aware that Xanda had been spending more time outside the park" and that the dead lion's GPS collar, which was fitted in October, was returned to the researchers.Cecil was killed in a hunt in which he was illegally lured out of the wildlife park with bait and initially wounded by an arrow, according to authorities.The death unleashed an extraordinary outpouring of anger at Walter Palmer, the American dentist who shot the lion, and other foreigners who have traveled to Africa to kill wildlife.Xanda had a pride with cubs, said the World Heritage Species group. It said "their safety and survival is now in jeopardy if a new male comes along and attempts to take over."