Why do cats sneer?

Every so often a cat can be seen to pause and then adopt a curious sneering expression, as if disgusted with something. When first observed, this reaction was in fact called an 'expression of disgust' and described as the cat 'turning up its nose' at an unpleasant smell, such as urine deposited by a rival cat. This interpretation is now known to be an error. The truth is almost the complete opposite. When the cat makes this strange grimace, known technically as the flehmen response, it is in reality appreciating to the full a delicious fragrance. We know this because tests have proved that urine from female cats in strong sexual condition produces powerful grimacing in male cats, while urine from females not in sexual condition produces a much weaker reaction.

The response involves the following elements: the cat stops in its tracks, raises its head slightly, draws back its upper lip and opens its mouth a little. Inside the half-opened mouth it is sometimes possible to see the tongue flickering or licking the roof of the mouth.

The cat sniffs and gives the impression of an almost trancelike concentration for a few moments. During this time it slows its breathing rate and. may even hold its breath for several seconds, after sucking in air. All the time it stares in front of it as if in a kind of reverie. If this behaviour were to be likened to a hungry man inhaling the enticing smells emanating from a busy kitchen, it would not be too far from the truth, but there is an important difference.

For the cat is employing a sense organ that we sadly lack. The cat's sixth sense is to be found in a small structure situated in the roof of the mouth. It is a little tube opening into the mouth just behind the upper front teeth. Known as the vomero-nasal or Jacobsen's organ, it is about half an inch long and is highly sensitive to airborne chemicals. It can best be described as a taste-smell organ and is extremely important to cats when they are reading the odour-news deposited around their territories. During human evolution, when we became increasingly dominated by visual input to the brain, we lost the use of our Jacobsen's organs, of which only a tiny trace now remains, but for cats it is of great significance and explains the strange, snooty, gaping expression they adopt occasionally as they go about the social round.