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.