Researchers in Japan report that rats emit a distinct 40 kHz inverted-U ultrasonic call while eating chocolate, according to a study in eNeuro. Using AI to analyze acoustic features, the team found the call increases with chocolate consumption and falls when the brain's opioid pleasure system is blocked. Fifteen of 16 rats chose chocolate over regular food, and most calls occurred within one second of feeding, suggesting a real-time indicator of animal pleasure.
Rats 'Cheer' in Ultrasound: A 40 kHz Call Signals Chocolate Pleasure
Rat vocalizations have been linked to different emotions and motivational states.
Many people relish chocolate — and so do rats. New research from Japan shows that rats emit a distinct ultrasonic vocalization when they eat chocolate, a sound humans cannot hear but detectable with specialized equipment.
What the Study Found
Researchers analyzed ultrasonic calls recorded while rats had access to chocolate, compared those calls with recordings when chocolate was absent, and included a chocolate-free control group. Using artificial intelligence to examine acoustic features, the team identified a characteristic call, officially labeled the 40 kHz inverted-U, that was strongly associated with chocolate consumption.
'When rats eat something delicious such as chocolate, they emit a distinct ultrasonic call that humans cannot hear,' said Koshi Murata, a study co-author at the University of Fukui. 'This call drops sharply when the brain's opioid pleasure system is blocked, suggesting the animals may be vocally expressing enjoyment.'
Key Experimental Details
The researchers compared vocalizations produced while rats ate chocolate with those produced while eating regular food. Although the animals consumed similar amounts of both foods, they emitted significantly more 40 kHz inverted-U calls while eating chocolate. In a separate choice test, 15 of 16 rats chose chocolate over the regular food. In two experiments, the vast majority of these calls occurred within one second of a feeding event.
Why It Matters
Because the 40 kHz inverted-U call diminishes when the brain's opioid 'pleasure' system is blocked, the vocalization could serve as a real-time, non-invasive indicator of positive affect in animals. This makes it a potentially valuable tool for studying brain mechanisms underlying reward, overeating, and conditions such as anhedonia — the reduced ability to feel pleasure, often seen in depression.
The study was published in the journal eNeuro and demonstrates how combining acoustic analysis with AI can reveal subtle behavioral markers of internal emotional states in animals.
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