Nature
PUBLISHED
Have you ever let out a deeply satisfied sigh after a long meal, or made happy noises when eating a delicious piece of cake? Well, a new study suggests that humans might not be the only ones, as rats have been found to make happy noises when eating chocolate.
Rats are known to produce ultrasonic vocalizations (USVs) in various different emotional states. These sounds are too high-pitched for humans to hear but can be recorded.
The researchers first recorded baseline USVs and then compared those to the sounds the rats made when they were given sweet milk chocolate to eat, as well as to a control group of rats that were not given any chocolate.
“When rats eat something delicious such as chocolate, they emit a distinct ultrasonic call that humans cannot hear,” says Dr Koshi Murata from the Division of Brain Structure and Function, Faculty of Medical Sciences, University of Fukui, Japan, in a statement.
The team used machine learning to identify and analyze the features of the USVs made by the rats. On the data it was clear to see a 40 kHz inverted-U USV that only appeared when the rats were munching on the chocolate.
To see if this was just a response to eating rather than the type of food, the team gave the rats naloxone, a drug that blocks opioid receptors, before giving them chocolate. This significantly reduced the number of those inverted-U USVs that the rats had made previously. This suggests that the opioid system, which is associated with processing pleasure, may influence how often these noises are produced.
“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,” said Dr Murata.
The researchers wondered whether the noises were just a generic response to food in general but by comparing the chocolate-eating rats' sounds with the control group fed normal chow, though the rats ate a similar amount of both foods, the chocolate-eating rats produced more 40 kHz inverted-U USVs.
The team also conducted a preferred food test in which 15 of the 16 rats chose chocolate over the chow.
The results showed that over 97 percent of those inverted Us occurred within 1 second of a feeding event. This reveals that the inverted-U USVs do not just mean the rat is eating, but suggests that the rat makes more of these vocalizations when eating food it thinks is tastier.
It could even suggest that the rat is happier, and further research could look at new ways to assess emotional states in different animals.
Unlike dogs, rats can consume small amounts of chocolate safely since their bodies can process the theobromine it contains. As for humans, well it would take a lot more to actually kill you – just how much are we talking before those health benefits turn into death by chocolate?
The study is published in eNeuro.
