 Regarding their senses , Charles Darwin observed , " The inhabitants believe that these animals are absolutely deaf ; certainly they do not overhear a person walking near behind them . I was always amused , when overtaking one of these great monsters as it was quietly pacing along , to see how suddenly , the instant I passed , it would draw in its head and legs , and uttering a deep hiss fall to the ground with a heavy sound , as if struck dead . " Although they are not deaf , tortoises depend far more on vision and smell as stimuli than hearing . 
