Have no fear

Male rats and female rats respond to stress in different ways...
23 December 2015

Interview with 

Becca Shansky, Northeastern University, Boston


Male rats and female rats respond to stress in different ways.


Over a century ago, Pavlov showed that, when he fed his dogs and rang a dinner bell at the same time, the dogs soon learned the association between the two and, after a while, the dinner bell alone was enough to make the animals salivate. Similar experiments can be done with rats, where a tone forewarns of a small ensuing electric shock. Rather than salivate though, the rats freeze, showing that they are fearful of the shock they anticipate receiving. But when she was using this approach, some unusual behaviour by some of her subjects made Becca Shansky realise that, for decades, we might have been missing something critical, as she explains to Chris Smith...

Rebecca - We were originally interested in animals that naturally seem to have very low levels of fear and see if there were differences in the brains of these animals. What we found was that there were females that seem to be expressing low levels of fear. But then my graduate student was looking at the videos - so we take videos during the entire experiment and she said, "You know, some of these females, they're not freezing but when the tone comes on, they start running around like crazy. It's hard to say that these animals don't look afraid." So we said, "Is there a way that we can try and quantify this? Is this a real thing or is this just some random weird animal that we should just take out of analysis or something like that?"

Chris - And so at this stage, you're saying, "Well perhaps, we're calling fear incorrectly. Perhaps there are other manifestations of fear that aren't just animals freezing. Perhaps it's animals also running very fast."

Rebecca - Exactly. So maybe instead of freezing, for some animals, their instinct is to try and escape. It's not very common and I think part of the reason that hasn't been really described before is that 95 per cent of these kinds of experiment have been done in male animals and not female animals. So, it's possible that it just hasn't been observed.

Chris - And when you did the reanalysis, does it hold water? Do they show this running behaviour rather than freezing?

Rebecca - Yes. So that's what it looks like and about 40 per cent of the females that we analysed, instead of freezing, they start to run around. The analysis that we conducted allows us to detect individual instances of this escape behaviour which we, in the paper called Darting. What we saw is that in some animals, what you normally see if you're measuring freezing is that freezing increases every time the tone gets presented and then occurs with a shock after it. And so, you look at sort of an upward trend in freezing as a representation that the animal's learning is getting better and they really know that that tone means they're going to get shocked. What we see in some of these females is the exact same trend except we see an increase in this darting behaviour instead of freezing.

Chris - Just the females or do both males and females do this?

Rebecca - We found a very, very small percentage of males that qualified as darters. Only around 10 per cent of the males did. So, it's still up for debate whether or not this is something that we can truly say the males are doing at the same level that the females are.

Chris - Why do you think they do it at all?

Rebecca - Well, I think that's going to be the 64,000 dollar question as we move forward with the research. What was really interesting about these animals that are darting as they learn what that tone means, is that there were some interesting behavioural effects when we did a couple more tests later, a couple of days later. It seems that animals that exhibited this escape-like behaviour had low levels of fear in any measure two days later in another kind of test that we did. And so, there seems to be possibly something protective or adaptive about trying to escape during a stressful event. So, that's one of the more provocative findings: there may be in a sort of long term consequences that are positive of having this kind of behavioural response as opposed to a freezing response.

Chris - What are the implications of what you found and what has been missed for many years previously? 

Rebecca - What it shows is that it's possible that female rats have a more diverse repertoire of fear expression. And so, that's something that's going to have to be taken into consideration because the biggest problem that this sort of highlights is that if you're only looking at freezing, animals that are fearful are being presented as not fearful. If your freezing levels are low because you're engaging this darting behaviour then that's a problem for interpreting your data.


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