December 3, 2017 Kilombero Valley, Tanzania—Andy Marshall, a biologist, yanks on the steering wheel of a battered Nissan station wagon and swings it off a track in the Kilombero Valley of southern Tanzania. Rain from the night before has left hubcap-deep puddles across the road. Mr. Marshall downshifts, swerves onto a recently harvested field of sugar cane, and parks on the furrows. The Nissan shudders for an instant before going quiet.

The biologist – a researcher on the staffs of the University of the Sunshine Coast in Queensland, Australia and the University of York in England – and three Tanzanian villagers slog a short distance through dirt clods and stubble toward a tall