As implausible as it sounds, I believe that under certain circumstances of angle, distance and arrow speed, the animals can see the arrow in flight coming towards them, especially if they are alert to your presence, even if they have not actually made you out. I've also seen video footage (in slow motion) of an elk that clearly drops as if to make a fast get-away with his eyes firmly fixed on the arrow coming towards him all the way into its chest. His head clearly follows the arrow as it closes in.