It’s surprising how loudly your heart appears to thump in your chest when your survival depends on silence. Treading stealthily through sun-scorched scrub in the Sera Conservancy in northern Kenya, I’m conscious of every quickening beat, and flinch at each clumsy crunch, as parched branches and leaves underfoot betray me. A few paces ahead my Samburu guide, Sammy Lemiruni, is shaking an incongruous, ash-filled knotted sock to reveal the direction of the wind, and soundlessly signalling directions: walk in single file; stay quiet; crouch behind a tree and then, with considerably more resolve: Do. Not. Move.