Rats are indeed ticklish, a discovery that provides unique insights into animal emotion and the evolutionary roots of laughter. Ticklishness is a complex, non-reflexive behavior linked to social play and positive emotions. This finding demonstrates that fundamental positive affective states are conserved across mammals. Researchers now use this natural behavior to objectively study happiness and play in laboratory animals.
How Scientists Confirmed Ticklishness
Neuroscientist Jaak Panksepp and his colleagues confirmed ticklishness in rats in the early 2000s. They used human-hand stimulation to mimic the rough-and-tumble play observed between juvenile rats. The technique involved two components: a “dorsal contact” on the rat’s neck and a “pin” or light pressure on the rat’s stomach area, simulating wrestling.
Rats did not merely react to the touch; their response was distinctly playful and voluntary. When tickled, the animals often exhibited “frollicking” or “joy jumps” (Freudensprünge). They actively chased the researcher’s hand, soliciting more tickling, which indicated a rewarding experience.
The Rat’s Neurological and Vocal Response
The rat’s vocal response provides compelling evidence that tickling is a positive experience. When tickled, rats emit high-frequency 50-kilohertz (kHz) ultrasonic vocalizations (USVs), which are inaudible to the human ear. These 50-kHz chirps are considered a reliable marker for positive emotional states, analogous to human laughter. The frequency of these USVs increases the more a rat enjoys the tickling.
Further research pinpointed the neurological mechanism behind this joy. During tickling, researchers observed distinct activation patterns in the somatosensory cortex, the brain region responsible for processing touch. Nerve cells in the deep layers of the trunk cortex showed intense activity. When scientists artificially stimulated this specific area, the rats produced the 50-kHz vocalizations, linking this brain region directly to the ticklish response.
Significance for Animal Emotion Studies
Laughter-like behavior in rats has advanced the study of animal emotions and comparative neurobiology. This finding suggests that the basic neural circuitry for experiencing positive affect is ancient and shared by mammals that diverged over 90 million years ago. It challenges the notion that complex emotions like joy are exclusive to humans or primates.
The measurement of 50-kHz USVs now provides a quantifiable, objective metric for positive emotion, useful for assessing animal welfare. Researchers use tickling and the associated vocalizations to understand how environments or treatments affect an animal’s happiness and well-being. Social play and positive experiences are established as fundamental components of mammalian development and health.