The lining of cats’ noses has a large surface area for trapping smells. It’s 5 times as big as ours. They also have large olfactory bulbs, which are the part of the brain where smells are initially analysed. Cats are very sensitive to scent and can discriminate more scents than they are likely to meet in a lifetime. As a result we have to minimise the number of strong scents we present to our cats as they are easily overwhelmed by them.
Mice leave scent marks to let other mice know they are about. Cats locate the mice using these scent marks, especially at night when vision is less reliable. When the cat finds the mark the marking mouse is long gone so the cat waits patiently for the next mouse to come along and sniff the mark - then he pounces on the poor mouse
Cats use scent to mark their own territories, too. An anxious cat will urinate around the house to warn other cats off. Less threatened cats rub their faces onto objects leaving a pheromone behind. This makes them feel more comfortable and lets other cats know they are there. Feliway is an analogue of this pheromone and we recommend it for cats who are anxious or taking a while to settle in to a new environment.
Cats also have a sense that we lack. While we are not quite sure what they are sensing, we think that odours from other cats are dissolved in saliva and moved up two tubes in the roof of the mouth to the vomeronasal organ. When you see a cat pulling up its top lip in a funny way while apparently sniffing an object she’s probably sensing another cat has been there.