When rodents get a sniff of their predator's stench, one of their first reactions is to freeze. But many components of a predator's odour simply smell bad rather than striking fear into their prey's heart. Figuring out which scents simply stink and which scare the life out of an animal is critical for scientists that want to understand the physiology of fear. They have to be sure that animals are genuinely terrified rather than simply avoiding a repulsive smell. Thomas Endres and Markus Fendt from Universität Tübingen decided to test the effects of two stenches on rats to find out whether 2,4,5-trimethyl-3-thiazoline (TMT), derived from fox faeces, produces a genuine fear response (p. 2324).

Knowing that rats take evasive action when they sniff TMT, Endres and Fendt tested how young rats reacted to different concentrations of TMT and another unpleasant smell that rats avoid, butyric acid. Placing a scrap of filter paper carrying a tiny drop of either TMT or butyric acid in the corner of an arena, the team filmed the rats' reactions to the smells to find out how much time they spent avoiding the unpleasant smells. Endres and Fendt found that the rats avoided both smells, but the rodents really disliked visiting the corner that smelled of TMT. The animals tolerated a 0.387×10–6 mol drop of TMT, avoided a 3.87×10–6 mol drop and strongly disliked the large 38.7×10–6 mol drop while the rats only avoided the largest drop of butyric acid (54.7×10–6 mol) and were unaffected by small drops. So the rats avoided both smells but seemed much more sensitive to the TMT than the butyric acid.

But was the animals' avoidance behaviour a reflection of their dislike of the smells or a case of being frightened off? Endres and Fendt placed either a 3.87×10–6 mol drop of TMT or a 54.7×10–6 mol drop of butyric acid next to individual rats and filmed their reactions.

The rats placed in close proximity to the butyric acid seemed uncomfortable, but continued moving around the enclosure. They were not terrified; they just didn't like the smell. However, the rats that sniffed the TMT froze with fear, crouched down and barely breathed until the smell had gone; they were terrified.

So the rats avoided both butyric acid and TMT because of their unpleasant odours, but TMT terrified the animals. Endres and Fendt point out that avoidance and fear should not be confused, and conclude that TMT is a genuine smell of fear.

Endres, T. and Fendt, M. (
). Aversion-vs fear-inducing properties of 2,4,5-trimethyl-3-thiazoline, a component of fox odor, in comparison with those of butyric acid.
J. Exp. Biol.