The flight of the narwhal Animals tend to respond to threats with the well-known behaviors of fight, flee, or freeze, each of which requires a different suite of physiological responses. Marine mammals face particular challenges because they may flee into an environment where oxygen is not available and pressure must be accommodated. Williams et al. placed a submersible electrocardiograph, depth, and acceleration recorder on narwhals after they were freed from entanglement. The animals displayed contrary cardiovascular responses simultaneously, which placed extreme stress on the cardiovascular system and the tissues that it protects. Science , this issue p. 1328