Mice that don't fear cats

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Fear may be linked to the sense of smell, and can be switched off simply by shutting down certain receptors in the brain, Japanese scientists have found. In an experiment with mice, the researchers identified and removed certain receptors on the olfactory bulb of their brains -- and the result was a batch of fearless rodents. To prove their point, the scientists showed pictures of a brown mouse within an inch of a cat, sniffing up its ear, kissing it and playing with its predator's collar. Experts have long thought that fear in animals may be prompted by their keen sense of smell. But this is the first time scientists have discovered that smell detection and how that translates to fear take place in different parts of the olfactory bulb. Two lines of mice were created. One lacking the receptors to translate odors and the other lacking receptors for smell detection. They were then exposed to the urine of predators such as snow leopards and foxes. The first group would keep smelling and they turn around and they show very strong curiosity but they never can tell any danger, whereas the second group are very poor in detecting smell, but as soon as they detect the fox urine, they would freeze and they will pretend to be dead. Mice have about 1,000 smell receptor genes, while humans have only 400 functioning ones and about 800 non-active ones.