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South American expedition (1913–1914)

In 1907 a friend of Roosevelt's, John Augustine Zahm, invited Roosevelt to help plan a research expedition to South America. To finance it, Roosevelt obtained support from the American Museum of Natural History in return for promising to bring back new animal specimens. Roosevelt's popular book, Through the Brazilian Wilderness describes his expedition into the Brazilian jungle in 1913 as a member of the Roosevelt-Rondon Scientific Expedition, co-named after its leader, Brazilian Cândido Rondon.

Once in South America, a more ambitious goal was added: to find the headwaters of the river Duvida and trace it north to the Madeira and thence to the Amazon River. Duvida was later renamed Roosevelt River. Roosevelt's crew consisted of his son Kermit, Colonel Rondon, naturalist George Kruck Cherrie, Brazilian Lieutenant João Lira, team physician José Antonio Cajazeira, and 16 skilled paddlers and porters. The initial expedition started tenuously on December 9, 1913, at the height of rainy season. The trip down Duvida started on February 27, 1914.

Roosevelt suffered a minor leg wound after he jumped in to try to prevent canoes from smashing against the rocks. The flesh wound he received, however, soon gave him tropical fever that resembled the malaria he had contracted while in Cuba. The infection weakened Roosevelt so greatly that six weeks in, he had to be attended to constantly by the expedition's physician and Kermit. By then, he could not walk because of the infection and an infirmity in the other leg, due to a traffic accident a decade earlier. Roosevelt had chest pains and a fever that soared to 103 °F (39 °C) and made him delirious. Regarding his condition as a threat to others' survival, Roosevelt insisted he be left behind to allow the poorly provisioned expedition to proceed, preparing to commit suicide with morphine. Only an appeal by his son persuaded him to continue.

Despite Roosevelt's continued decline and loss of 50 pounds (23 kg), Rondon reduced the pace for map-making, which required regular stops to fix their position by sun-based survey. Upon Roosevelt's return to New York, friends and family were startled by his physical appearance and fatigue. Roosevelt wrote, perhaps prophetically, to a friend that it had cut his life short by ten years. For the rest of his few remaining years, he would be plagued by flare-ups of malaria and leg inflammations so severe as to require surgery. Before Roosevelt had even completed his sea voyage home, critics raised doubts over his claims of exploring and navigating a completely uncharted river over 625 miles (1,006 km) long. When he had recovered sufficiently, he addressed a standing-room-only convention organized in Washington, by the National Geographic Society and satisfactorily defended his claims.