Some sharks go into tonic immobility when they are turned upside down. With tiger sharks 3–4 metres (10 to 15 feet) in length, tonic immobility may be achieved by placing hands lightly on the sides of the animal's snout approximate to the general area surrounding its eyes. Scientists believe that tonic immobility in sharks may be related to mating, because female sharks seem more responsive than males. During tonic immobility, the dorsal fin(s) straighten, and both breathing and muscle contractions become more steady and relaxed.