Fig. 2. (A) Tested adult newts inhabited stream pools along a 4 km stretch of Tuna Canyon Creek in Malibu, California, USA. In each 5-min trial, an investigator hid behind rock boulders that lined the stream, and continuously released solution (treatment or control) though transparent polyethylene tubing. The tip of the tubing was placed, nominally, 30 cm upstream of a free-ranging adult newt. (B–E) Successive images (at 3 s intervals) showing an adult (gray arrow) detecting an attractant (arginine, at 10^–5 mol l^–1) odor plume, turning, and swimming upstream to the source (white open circle). The principal along-channel axis of stream flow is denoted by a white arrow.