Contrary to earlier suggestions that Mr Lees had died of a heart attack, a Home Office pathologist said the 45-year-old campaigns director for Friends of the Earth probably died from severe heatstroke as he waded through the dense Madagascan jungle.
He collapsed and died in the Petriky Forest on New Year's Eve. Seven days later his body was found by a villager, who led police and friends of Mr Lees to the spot.
His fiancee, research chemist Dr Christine Orengo, 34, had waited at the edge of the forest for news.
Mr Lees' friend, Dr Jonathan Kaplan, told the inquest at Great Yarmouth, Norfolk, how he had broken the news to Dr Orengo.
They found Mr Lees' body lying at the edge of a clearing about a mile into the forest. He was clutching his camera bag. There was nothing to suggest Mr Lees had been attacked, robbed, or molested by a wild animal, Dr Kaplan said.
A post-mortem examination by Home Office pathologist Dr David Harrison failed to establish the cause of death. He said the body was too badly decomposed. But Dr Harrision said he examined the heart and found no evidence of disease.
"There are a number of possibilities but if I had to choose, heatstroke would seem the most likely," he said.
Mr Lees had an almost missionary zeal towards his work. He had been in Madagascar for two weeks making a film about the effect on the forest of a nearby mining project.
He had been due to return to London on 1 January but made a last journey into the forest on New Year's Eve. He told his driver he was going into the forest for an hour and would return. When he failed to reappear a rescue operation was launched.
"Conditions were extremely hot and Mr Lees had had an exhausting two weeks," said deputy coroner, Nicholas Holroyd.
Recording an open verdict he said: "There is no evidence that foul play may have played a part in his death and I cannot speculate about the cause of death."Reuse content