(Press-News.org)
Pleasant tactile stimulation drives social bonding in many animal species, especially mammals. Tactile stimulation forms the basis of the infant-caregiver relationship and often leads to affinity-like behavior, indicating the formation of social bonds. Juvenile-adolescent rats often engage in rough-and-tumble play, during which they emit characteristic ultrasonic vocalizations. These vocalizations, typically in the 50 kHz frequency range, are thought to be associated with pleasurable emotional states. Juvenile adolescent rats also emit 50 kHz ultrasonic vocalizations in response to tickling with human hands, which mimics natural rough-and-tumble play. Moreover, these rats develop a strong affinity for human hands as a result of repeated tickling.
In a study that was recently made available online on June 4, 2025, and was published in Volume 35 Issue 12 of the journal Current Biology on June 23, 2025, researchers have found that repeated tickling of human hands results in increased expression of oxytocin receptors in a region of the rat’s brain called the ventrolateral part of the ventromedial hypothalamus (VMHvl). In the study, the inhibition of oxytocin signaling in this brain region reduced affinity-like behavior in response to human touch among rats.
“We revealed part of the neural circuitry underlying why naturally wary animals become attached to humans. We discovered that during the process of rats becoming tame to human hands, the action of oxytocin in a specific brain region (VMHvl) mediates the effects of pleasant tactile stimulation and controls the formation of social bonds across species,” explains Dr. Himeka Hayashi of Okayama University, Japan, the lead author of the study. In addition to Dr. Hayashi, the study received significant contributions from Professor Hirotaka Sakamoto of Okayama University, among other collaborators.
In the study, rats received 10 days of tickling training, during which period researchers measured their 50 kHz vocalizations. While the rats rarely emitted 50 kHz vocalizations initially, they consistently produced these vocalizations in response to tickling starting on day five. These vocalizations became more frequent through day 10, indicating that repeated tickling produced pleasant sensations in rats. At the end of the training period, rats were administered a conditioned place preference test. The rats expressed affinity for repeated tickling by spending more time in the tickling room after the test as compared to before.
“We have always been curious about how humans and animals can form bonds despite having no shared language or lifestyle,” says Dr. Hayashi. “We wondered whether the connection we felt with animals was real or just our imagination. Through this study, we discovered that rats genuinely enjoy interacting with us—which brought us great happiness.”
To explore the neural connections of oxytocin receptor neurons in the VMHvI that are implicated in affinity-like behavior, the researchers conducted nerve fiber tracing studies. Tracing studies revealed that a subset of these neuronal fibers originates from the supraoptic nucleus region of the brain, suggesting the role of oxytocin signaling from magnocellular neurons in controlling affinity-like behavior in rats. The identification of specific neural mechanisms underlying the development of affinity-like behavior offers crucial insights into social bonding between humans and rats.
“The discovery that pleasant tactile stimulation facilitates cross-species social bonding through oxytocin pathways could lead to a better understanding of the neural mechanisms underlying both social interaction difficulties and the therapeutic effects of physical contact with companion animals such as cats and dogs. This knowledge may contribute to the development of novel therapeutic and support approaches for individuals who struggle with social interactions,” Dr. Hayashi concludes.
By uncovering the neural basis of cross-species affinity, it paves the way for developing therapeutic strategies that leverage physical interaction to enhance social connection and emotional well-being.
About Okayama University, Japan
As one of the leading universities in Japan, Okayama University aims to create and establish a new paradigm for the sustainable development of the world. Okayama University offers a wide range of academic fields, which become the basis of the integrated graduate schools. This not only allows us to conduct the most advanced and up-to-date research, but also provides an enriching educational experience.
Website: https://www.okayama-u.ac.jp/index_e.html
About Project Assistant Professor Himeka Hayashi from Okayama University, Japan
Dr. Himeka Hayashi is a Project Assistant Professor at the Faculty of Environmental, Life, Natural Science and Technology, Okayama University, Japan. Her research explores behavioral neuroendocrinology, and she has authored five scientific publications in these areas.
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