The research was conducted by Assistant Professor Koshi Murata, Dr. Yuki Ikedo, and Dr. Yugo Fukazawa at the Division of Brain Structure and Function in the Faculty of Medical Sciences at the University of Fukui. According to the team, the motivation was methodological: they wanted a more direct, non-invasive way to assess how much an animal enjoys food, beyond simply measuring food intake. While recording rats during chocolate feeding, they noticed that the animals produced a characteristic vocalization while eating, which led them to investigate whether these calls carried information about how much the rats enjoyed the food. That observational starting point shaped the entire experimental design, which combined acoustic recording, machine learning-based classification, pharmacological manipulation, and food-choice testing.

Ultrasonic vocalizations, or USVs, are sounds produced by rats at frequencies too high for humans to hear. Previous studies have associated these vocalizations with various emotional and motivational states, including responses to rewarding stimuli, and rats have been shown to emit them both when consuming sweetened foods and when anticipating receiving them. Understanding how the brain processes rewarding experiences could help researchers better understand behaviors such as overeating, as well as conditions such as anhedonia — the inability to experience pleasure — which is a common symptom of depression. Animal models are one way researchers study reward-related processes, and a vocal signal tied to palatable food consumption would give them a new measurable variable to work with.

In their experiments, the researchers first recorded USVs from rats to identify patterns associated with chocolate consumption. They compared the sounds produced when the rats had access to chocolate with those recorded during baseline periods, and with those from a control group of rats that were not fed chocolate. To analyze the acoustic features of these vocalizations, the team used machine learning, applying computational classification to sort the calls into distinct categories. This analysis revealed a distinct subtype — the 40 kHz inverted-U USV — that was strongly associated with chocolate consumption. The designation refers to the call’s frequency and to the inverted-U shape by which the researchers classified it; the study identifies it as a category separable from other vocalizations in the rats’ repertoire.

The temporal coupling between these calls and feeding was striking. In the initial experiments, 97.2% of 40 kHz inverted-U USVs occurred within one second of a feeding event. This close association was reproduced in an independent test group, in which 96.6% of these calls occurred within one second of feeding. Such tight timing indicates that the vocalizations are not random background signals but are locked to the moment-to-moment act of eating. The researchers report that this pattern held across both the discovery experiments and the independent validation group, strengthening the case that the call subtype is specifically tied to consumption episodes rather than to general activity in the recording environment.

A central question was whether the vocalizations reflected the palatability of the food specifically, or whether they were simply a general response to eating. To find out, the researchers compared the vocalizations rats produced while eating chocolate with those produced while eating regular chow. Although the rats consumed similar amounts of both foods, they produced significantly more 40 kHz inverted-U USVs while eating chocolate. The rats also showed a clear preference for chocolate in a separate food-choice test, with 15 of the 16 animals choosing chocolate over regular chow. Together, the researchers conclude, these results suggest that the calls do not simply reflect the act of eating: the rats produced them more frequently when consuming the more palatable food, and the vocalizations may reflect the rats’ internal state while eating highly palatable food.

The researchers then probed the neurochemical basis of the signal. They administered naloxone, a drug that blocks opioid receptors, and found that it reduced the 40 kHz inverted-U vocalizations during chocolate consumption. This suggests that the endogenous opioid system, which is involved in processing reward-related and hedonic processes, may influence the production of these vocalizations. In the researchers’ words, when rats eat something delicious such as chocolate, they emit a distinct ultrasonic call that humans cannot hear, and this call drops sharply when the brain’s opioid ‘pleasure’ system is blocked. The pharmacological result links the vocal signal to the same neurochemical system the study identifies as relevant to hedonic processing, though the experiment demonstrates an influence on call production rather than a direct measurement of the animals’ subjective experience.

The researchers suggest that these vocalizations could provide a measurable signal associated with palatable food consumption and help investigate the brain mechanisms underlying food-related reward and positive emotional states, with potential relevance to conditions such as overeating and obesity. Because the signal is vocal, it can in principle be recorded without handling the animal or altering the feeding situation, which addresses the limitation the team identified in intake-based measures. The finding could open up new ways to investigate positive emotional states in animals and to further study the brain mechanisms involved in reward and pleasure. The researchers are careful to frame the calls as a candidate signal associated with an internal state, a hypothesis their data support but which remains a subject for further investigation.

Dr. Murata emphasized the broader promise of the approach. “Our discovery suggests that animal vocalizations could serve as a real-time, non-invasive readout of internal emotional states — specifically, how much an animal is enjoying its food,” he says. “Such a tool would allow researchers to measure ‘pleasure’ more directly than current methods permit, and could accelerate research into the brain mechanisms that generate positive emotions.” He also noted that the finding suggests rats may be vocally expressing the enjoyment of eating, offering a new window into whether animals experience pleasure as we do. The study, titled “A Subtype of Ultrasonic Vocalizations during Palatable Food Consumption in Rats Identified by Machine Learning-Assisted Classification,” was supported by the Lotte Research Foundation, the Urakami Foundation for Food and Food Culture Promotion, the Takeda Science Foundation, and the Japan Society for the Promotion of Science. The authors declare no competing financial interests.

Subject of Research: A machine learning-identified subtype of ultrasonic vocalizations in rats during palatable food consumption and its link to the endogenous opioid system

Article Title: What can a rat’s ultrasonic call tell us about food-related pleasure?

Article References: What can a rat’s ultrasonic call tell us about food-related pleasure?. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: ultrasonic vocalizations, rats, food reward, palatability, endogenous opioid system, naloxone, machine learning, hedonic state, chocolate, eNeuro, University of Fukui, reward