Rats succumb to peer pressure too

Blogging on Peer-Reviewed Research
This week's New Scientist includes a short piece from me about conformist rats. i-41c67941727f301e6e2d7d063ff24499-brownrat.jpg
Until now, only humans and chimps were known to succumb to peer pressure, to the extent that we often ignore our own experiences based on the preferences of others. But a new study in brown rats shows that these rodents are similarly prone to following the Joneses. They can even be persuaded to choose a piece of food that they know makes them sick if they smell it on the breath of a 'demonstrator' rat.

Bennett Galef and Elaine Whiskin at McMaster University in Hamilton, Ontaria, Canada, trained rats to avoid cinnamon-flavoured food pellets by injecting them with a nausea-inducing chemical (lithium chloride) after their meals. When given a choice between pellets flavoured with cocoa or cinnamon, they only went for the cinnamon ones about 5% of the time.

But after spending time in the company of other rats that had just eaten cinnamon themselves, they regained their liking for it and chose cinnamon pellets about a third of the time. In contrast, rats that were left alone still had a strong aversion.

The rats didn't get to see the demonstrators actually eating any pellets so their change in behaviour was most likely motivated by the leftover scent of cinnamon on the tutors' breath or fur. Residual odours explain how the rats conform but not why, and that's the big remaining question. Why should social learning override individual experience so completely? Why would a rat, or for that matter a chimp or a human, cast aside what it knows from its own experience and adopt a potentially inferior strategy just because someone else is doing it?

Reference: GALEF, B., WHISKIN, E. (2008). Conformity in Norway rats?. Animal Behaviour DOI: 10.1016/j.anbehav.2007.11.012

Image: Rat by Reg McKenna

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Well, one obvious situation is if many others are doing something they may know something that that rat doesn't know (ok this seems pretty unlikely with rats. I could see this making sense with monkeys or apes but with rats it seems unlikely).

So you're saying that if all my friends jump off the Empire State building I have to jump off the Empire State building after all? I could have justified all those things I wanted when my friends had them. Where was science when I was 8?!

As usual, I will look forward to your article in New Scientist which I will receive in Canada about 2 weeks later then you.

Your neighbors make great poison detectors. If you see them drop dead from eating something, then you know you should avoid it yourself. Relating to this example, even though the cinnamon food had gained a bad reputation for some of these critters, I bet they actually liked the flavor of it, and noticing that someone else ate it now brought back their own pleasant associations of the cinnamon. Perhaps if others ate it, they knew that it had previously just been a bad batch of cinnamon and that it was alright to eat it again. Besides, whats a little vomiting and hangover between friends? Negative reactions aren't always much of a deterrent for humans either.

The aversion the rats learned to cinnamon would have been primarily an unconscious conditioned response (taste/smell aversion learning is mediated by the amygdala, which is able to influence behavior without necessarily impinging on consciousness). In contrast, the social influence from other rats would have to be mediated by the cortex (and would possibly be conscious). So, in a sense, the rat doesn't "know" that cinnamon is bad for it--it has a "gut reaction" against it. What they DO know is that other rats seem to find cinnamon tasty. Overcoming the aversion to cinnamon is thus a case of valuing a reasoned response--"the other rats don't seem to mind cinnamon, so maybe I should give it a try"--over an emotional/visceral one--"I don't know why, but I just can't eat cinnamon." Thus, the really interesting question here is the second-to-last sentence of the post, paraphrased as "why should social learning override strong conditioned responses so easily?"

I'm wondering if maybe peer pressure has something of an adaptive benefit in this kind of situation. I don't know how it is with rats, but with humans, one bad enough experience is usually all it takes to make you averse to a certain kind of food - if shrimps made you puke once, chances are that you'll avoid shrimps from now on. But just because you puked after eating those shrimps doesn't have to mean it was the shrimps that made you vomit. It could have been a stomach bug, or something you ate before or alongside the shrimp, and the shrimp were just the flavour that got associated with the puking event. For a species that relies on a wide variation of only seasonally available food sources, not eating one of them because of a "faulty trigger" would be a disadvantage, and that's where peer pressure could come in - if you see/smell that others are eating shrimps without having to vomit afterwards, it's a good sign that they're safe to eat after all. Peer pressure could be a kind of override system erasing the "faulty trigger" and enabling you to going back to eating those nutrition-rich shrimp.

By Darwin's Minion (not verified) on 12 May 2008 #permalink