This morning I was walking my elderly spaniel Rowan on open access land. She was on a lead, as the law requires, when a hare crossed the track about 40 yards ahead of us. She didn’t spot it but when a minute or so later we came to the point where it had crossed the path, she suddenly became animated. Her nostrils must have picked up the heady scent of the passing animal and if she hadn’t been on a lead she would certainly have investigated the line. (Read our list of the best slip leads.)

It was a clear indication that though her eyesight is not so good as it once was and her hearing is now poor — but she is not quite deaf — my dog’s sense of smell appears undiminished. This is something I have noticed in all my dogs as they have got older, because smell seems to be the last of the senses to decline. (Read more about deafness in dogs.)

A dog’s sense of smell

As dogs rely strongly on scent for so many things, this is a good thing that undoubtedly helps them survive as life gets more challenging. There’s no way of knowing how good Rowan’s sense of smell is compared with 16 years ago when she was a puppy.

We all know that a dog’s sense of smell is far more sensitive than ours. A dog’s nose contains around 220 million smell-sensitive cells, compared with our measly five million. So even if Rowan has lost a few hundred thousand, she’s still got a lot left and can smell things that I’m totally unaware of. (Read more on how scent works.)

Dogs may not perceive colours as we do, but this is more than compensated for by the fact that their sense of smell is the equivalent of glorious technicolour. (Read how dogs see colour.)

The experience of Rowan and the hare prompted me to undertake research into whether dogs do lose their sense of smell. The consensus seems to be that all senses decline as a dog ages, but the sense of smell is usually the last to go. Some old dogs do lose their sense of smell and with its loss comes a decline in interest in food. The smell of food is something that induces a dog to eat, so if it can’t smell what you put in its dinner bowl, it’s less inclined to consume it.

Interesting food

Veterinary advice in such circumstances is to provide the dog with more interesting food. Dry food fed straight out of a sack probably won’t smell inviting, but a bit of gammon fresh from the oven is much more likely to appeal and nudge even an old dog’s olfactory senses.

The fact that Rowan still loves her food is another indication that she’s not doing too badly for an elderly spaniel. I always reckon that when a dog no longer wants to eat, the end of life is fast approaching.

Though we have long known that dogs have far more sensitive noses than our own, there’s still an awful lot we don’t know about how their noses work. We have all heard gundog owners speak proudly of what a great nose their dog has, but does the effectiveness of the nose really vary from dog to dog, or is it a case of some dogs being better able to process what their nose tells them?

I’m also intrigued about that oft-repeated fact that dogs have 220 million scent cells. Is this the case for all dogs, whatever their breed, or do gundogs have more sensitive noses than, say, poodles or collies?

I suspect that brachycephalic breeds such as pugs and bulldogs have an impaired sense of smell because their muzzles are so short.

These breeds generally have great difficulty breathing, so it’s difficult to believe that their sense of smell isn’t impaired too, but I may be wrong.

Funnelling scent

Hounds undoubtedly have a great sense of smell, but is it any better than that of gundogs? According to some sources, bloodhounds and bassets were bred with long ears because they helped funnel the scent to the dog’s nose when it was hunting. I have always wondered whether there was any truth in this, but it makes a pleasing story.

Gundogs are usually the first choice when it comes to sniffer dogs. I recall a meeting with a labrador sniffer dog in Bhopal, India, where its policeman handler gave me special permission to photograph him — he was clearly very proud of his dog.

Some years ago I was told by the Metropolitan Police’s dog unit that they liked to use cockers as sniffer dogs. This wasn’t because they had better noses than labradors, but their small size allowed them to get to places where labradors couldn’t go. It is easy to pick up a cocker to help it check the overhead lockers of an airplane, for example, something that you can’t do with a labrador.

With gundogs we rely on the dog’s sense of smell in many ways. Pointers and setters working on the moors are expected to be able to distinguish between the scent of grouse and other ground-dwelling birds, such as pipits and skylarks, while we want our picking-up spaniel or retriever to be able to select runners rather than unshot birds, which most manage with impressive certainty.

Intriguingly, man has yet to invent an instrument with the sensitivity of the canine nose.