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There’s nothing better than sinking your teeth into a sweet, creamy chocolate bar. Rats think so, too—and they may announce their chocolate-infused joy to the world at ultrasonic frequencies.
Researchers have previously linked ultrasonic rat vocalizations with different emotional and motivational states in the animals, such as in reaction to rewarding stimuli. For example, the rodents vocalize in this way when eating something sweet.
Now, a team of researchers in Japan have discovered that rats emit a particular ultrasonic vocalization while eating chocolate. Their findings are detailed in a study recently published in the journal eNeuro.
To come to this conclusion, the researchers compared ultrasonic vocalizations that rats emitted when they had access to chocolate, with those they made when they didn’t have access to the candy. The team also compared them with a chocolate-free control group of rats. Researchers then used AI to analyze the vocalizations’ acoustic features, identifying a distinct ultrasonic vocalization—officially 40 kHz inverted-U—strongly associated with chocolate snacking.
“When rats eat something delicious such as chocolate, they emit a distinct ultrasonic call that humans cannot hear,” Koshi Murata, a study co-author and researcher at the University of Fukui, said in a statement. “This call drops sharply when the brain’s opioid ‘pleasure’ system is blocked. This suggests that rats may be vocally expressing the enjoyment of eating, offering a new window into whether animals experience pleasure as we do.”
To see if this distinct vocalization was related to the tastiness of food or just eating in general, Murata and colleagues compared chocolate-eating vocalizations with the sounds the rats produced while eating regular food. And while the rats ate similar amounts of both, they emitted significantly more 40 kHz inverted-U USVs while eating chocolate.
In a separate food-choice test, the rats also demonstrated an obvious preference for chocolate: 15 out of 16 rats picked chocolate over the regular food (and can we blame them?). In two experiments, researchers also found that the vast majority of 40 kHz inverted-U USVs were emitted within one second of a feeding event.
“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,” said Murata. “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.”
Understanding more about how the brain processes rewarding experiences could shed light on behaviors like overeating, as well as conditions like anhedonia, or the inability to feel pleasure and a common symptom of depression.
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