15. Nov, 2016

To tickle or not to tickle

Tickling is the act of  touching  a part of a body in a way that causes involuntary twitching movements or laughter (‘giggling’)*.  It  usually leads to a pleasant playful sensation when carried out in a consensual  context. Non-consensual -or aggressive- tickling can be uncomfortable, and  even painful for the recipient. Tickling  is intrinsically dualistic: it simultaneously triggers an approach (more!) and avoid (stop!) reaction to the source of tickling. Primates like  chimpanzees, gorillas, bonobos and orangutans when tickled also show laughter-like vocalizations, that sound like  alternating inhalations and exhalations.  Similar reactions can be observed  in  domesticated dogs when  tickled or stroked by their keepers. Even rats,  supposed to be lower on the evolutionary  scale, show reactions to tickling. When a rat is tickled on its  favorite spot, being the back or  belly, it may show two types of reactions. It first seems  resist the human intrusion  by wildly  moving its legs. Then it surrenders and seems to enjoy the treatment. When tickling stops, they will first move away  but  quickly return to the hand  like it's ‘asking for more’. During tickling, rats produce very high (ultrasonic) squeaking sounds. The  same kind of sounds are produced during spontaneous playful contacts between young rats, suggesting that they are a genuine expression of a pleasant, playful state, and not of distress or pain. Recently, German investigators found a brain regions that generates  these reactions*.  This ‘tickle  area’ lies in  the somatosensory cortex, an area in the upper-posterior brain where physical touch is processed.  Furthermore, microstimulation of this brain region evoked the same ‘ticklish’  behavior.  Interestingly they found that you can’t tickle rats in   anxiety-inducing situations. This also  suppressed the cells' firing, and the animal could no longer be tickled. So, just like humans  a rat  has to be in a good mood to enjoy tickling,

Tickling in fish: tonic immobility. Tickling, or rather: touching  and manipulating the body surface  not only affects mammals but also fishes. But here the effects are  different from tickling in mammals: they lead to  inactivity or even complete immobility. A remarkable phenomenon  observed traditionally  by fishermen and poachers (who don’t want to be caught with a fishing line)   is called  ‘trout tickling’. Trout tickling  is the art of rubbing the underbelly of a trout with fingers.  If done properly, the trout will go into a trance after a minute or so. The method is described  as follows:  ‘...the fish are watched working their way up the shallows and rapids. When they come to the shelter of a ledge or a rock it is their nature to slide under it and rest. ….he  then kneels on one knee and passes his hand, turned with fingers up, deftly under the rock until it comes in contact with the fish's tail. Then he begins tickling with his forefinger, gradually running his hand along the fish's belly further and further toward the head until it is under the gills. Then comes a quick grasp, a struggle, and the prize is wrenched out of his natural element, stunned with a blow on the head, and landed in the pocket of the poacher’**

Amazing anecdotes indeed, that are reminiscent of  tonic immobility exhibited  in sharks.*** When a shark like a lemon shark  is turned on it back,  it  will remain in a state of  paralysis for an average of fifteen minutes before it recovers. During tonic immobility, the dorsal fin(s) straighten, and both breathing and muscle contractions become more steady and relaxed. Other sharks like tiger sharks  will go into tonic immobility when they are inverted, or stroked  on the snout in the area  around  its eyes. They may even sink to the sea floor in their tonic state. Tonic immobility might not be a pleasant but rather a stressful experience for sharks.  The specific origin or function of this  reaction  remains a  source of speculation. Some believe its related to mating, when a bite of a male shark  immobilizes the female and thus facilitates mating.  It might also be a manifestation of  an old evolutionary reflex called ‘thanatosis’ or apparent death: a defense mechanism that is found in  a wide range of animals  to  deceive a predator in a situation  of  acute danger.  My personal -I think more simple- explanation is that manipulations like inverting or stroking a shark  disturb its sensory-spatial orientation system. For example, snout stroking could affect the Lorenzini  cells  that sharks  use to locate their  prey in the immediate environment,  while  they are  sensing the surrounding  electrical field. Stroking this hypersensitive area might not only  lead to a  ‘black out’  of its sensory environment, but also of  its  motor system, comparable to fainting in humans. 'Sensing' and 'moving'   are closely related functions, seen  from the perspective of the brain.

But it cannot be excluded that stroking the nose of a shark also elicits a pleasant feeling,  similar to tickling or stroking of a dog. Some video clips of  tiger sharks at Tiger Beach, a famous location to meet tiger sharks in the Bahamas,  suggest that certain tiger sharks once stroked on the nose return to the diver like its ''asking for more’’. This fits in the pattern that sharks -like dogs- tend to become familiar with and even recognize the regular bait providers.The only difference being that the tiger sharks favorite snack is a piece of tuna and not a cookie.





 Panksepp, J.; Burgdorf, J. (2003). "Laughing" rats and the evolutionary antecedents of human joy?"(PDF). Physiology & Behavior. 79 (3): 533–547. doi:10.1016/s0031-9384(03)00159-8.