In skimming journals to find articles to discuss on this blog, I frequently see studies that, based on title alone, seem a bit unbelievable. Recently I came across the most unbelievable-sounding one so far: Laughing Rats are Optimistic. Rats can be optimistic? How would you even measure that? Wait – rats can laugh?
To begin with the last question, yes, actually, they can. Rats make certain Ultrasonic Vocalizations (USVs) – sounds that are too high-pitched for humans to hear – that seem to be analogous to human laughter. For example, rats emit these USVs in response to pleasant stimuli, and when playing or being tickled. Additionally, stimulating the reward circuit of the brain increases the rates of USVs in rats.
If these USVs are truly analogous to human laughter, then they similarly serve as an indicator of a positive affective state in rats. Put more plainly, these USVs are a sign that a rat is happy. And this could be good information to know because, in humans, emotional state can greatly affect cognitive processes like decision-making. What if the same is true for animals?
Rygula et al. (2012) wanted to know if rats in a positive affective state are also more optimistic. We know how to tell us if a rat is in a positive affective state (USVs), and also how to put a rat in a positive affective state (tickling). But how can we tell if a rat is optimistic?
The researchers began by teaching the rats a very simple task. If a particular (“positive”) tone was played, the rat could push one lever (the “positive” lever) to receive a reward. If a different (“negative”) tone was played, the rat had to push another lever (the “negative” lever) to avoid a punishment (a small shock to the foot). The rats easily learned these associations, and were able to discriminate between the two tones.
But how would the rat respond if the researchers played a tone in between the positive and negative tones? An optimistic rat would expect the tone to indicate a reward, so it would be more likely to push the positive lever. A pessimistic rat, on the other hand, would expect the tone to indicate a punishment, so it would be more likely to push the negative lever.
After training the rats on the initial task, Rygula et al. tested the rats on the intermediate tone in two different sessions. In one session, the rats were tickled before being tested; in the other session, they were merely held before being tested.
Overall, the rats didn’t push the positive lever significantly more often after being tickled than after being held. However, when they took a closer look at the data, the researchers found individual differences in how much the rats laughed in response to the tickling; some rats laughed a lot, while others barely laughed at all. The researchers thus divided the rats into two groups (those that laughed when tickled and those that did not) and analyzed the data again. They found that the rats that laughed when tickled did press the positive lever significantly more often after being tickled than after being held. There was no significant difference found for the rats that didn’t laugh when tickled.
These results indicate that laughing rats are indeed more optimistic, and more generally, that affect can influence cognitive processes like decision-making in rats. This study also serves as an important reminder to be aware of individual differences in subjects; although it had been previously shown that tickling could elicit USVs, it clearly didn’t do so for all rats (or at least not to the same degree). By just combining the results of all the rats, the significant relationship between USVs and optimism was obscured.
Here’s a short video about detecting rat laughter (and how to tickle a rat). In addition to rats, non-human primates and dogs also laugh. In fact, it’s been found that playing recordings of dog laughter for shelter dogs actually decreases their stress behaviors!
Rygula, Rafal, Helena Pluta, and Piotr Popik. “Laughing rats are optimistic.” PloS one 7.12 (2012): e51959.