Sharks go into trances with the loosening of their muscle and respiratory processes. When they're upside down, their dorsal fins become straighter and more streamlined. The atypical stance is believed to have an effect on reciprocity with their surroundings, both through their motor and sensory reactions. Some believe tonic immobility might be a defense mechanism for sharks. If they stay completely still, they can be inconspicuous to others. Many doubt that theory because of sharks' impressive predation powers, however. Tonic immobility is also believed to be a breeding behavior in sharks, as rubbing them sometimes causes immobility.