And she's off, her keen nose to the ground. With Karl barely keeping up. Cautious pelts through the heather, down into the valley, up the other side and then disappears over the brow of the hill.
Soon she reappears with her quarry, RSPCA supervisor Sheila Rowe, who was attempting to hide in the next valley.
This is just a demonstration. In a proper training session the day before, another of Karl's hounds, Grayling, managed to track somebody who's scent had been cold for eight hours, across moorland ripe with the distracting scents of rabbits, deer and New Forest ponies.
Cautious and Grayling are otterhounds, an old and rare breed. The earliest references of hounds being used to hunt otters go back to the thirteenth century. But it's believed that the otterhound as we know it was established as a breed in the 19th century.
When otters became protected the hounds were used to hunt mink, for the show ring and occasionally as pets. Today there are reckoned to be only between 200 and 300 left in this country.
Now Karl Hopton has found a new job for the magnificent otterhound - hunting people. He is training Cautious, Grayling and two other hounds for a new Track and Search Dog Service for Dorset and the New Forest.
The scheme has financial backing from a number of local councils and the approval of Dorset police, and is expected to be launched in January. Karl and a group of fellow otterhound owners will take turns to be on call, ready to respond in the event of an emergency.
"About 18 people a year go missing here," says Karl. "OK, not a great problem if you compare it with somewhere like the Brecon Beacons. But it's still a problem. The New Forest attracts a lot of visitors, and there are children who wander out of tents. Then there's the Dorset Coastal Path, which is absolutely deadly.
Until now they've been using the police dog section to find people, but the otterhounds have the ability to track much longer and much colder scents than a police dog.
Karl, 31, was a dog trainer with the Army when he first saw an otterhound being used by the Garda in Ireland to sniff out explosives. When he left the Army he became an animal welfare officer with East Dorset District Council. Then one day he went to license a local kennels that bred otterhounds.
I said I remember those - can I take one out, see whether they'll track men? They said by all means. So we kept going out every day, taking a different one. None of them had ever done it before, but they all had this ability.
"They're the hardiest breed I know - they just keep going and going. You could go through thickest brambles and gorse with an otterhound and it wouldn't bat an eyelid. A bloodhound wouldn't even attempt it - it would go around and try to pick up the scent on the other side."
He's been training four otterhounds since April this year, and is now starting on a bloodhound-otterhound cross - a "blotterhound" called Bowman.
Out on moorland on the edge of the New Forest, Cautious and Grayling go lolloping through treacherous-looking bogs, and plunge into a ditch of brackish water, basking in it as if that's where they belong. In a sense, it is.
"They're built totally for the job they used to do, explains Karl. "They've got a very thick double coat, one of the coats is very wiry, rather like a waxy Barbour jacket. If you tip a bucket of water over them it just runs off. You have to really soak them to get them wet.
"All dogs have a web foot, but the otterhound's web is wider, and they've got long ears that are heavily insulated. They are made for the cold Scottish rivers - they're tremendous swimmers.
Training them, however, is not easy. "Hounds are a dog-trainers' nightmare. They're very independent - they'll do exactly what they want to do - suddenly pick up the scent of a rabbit and they're off. You have to be really patient with them, you have to try and channel that hunting instinct.
"We've put a logo on all our letterheads - 'Living in a world of scent' - because I believe these hounds are living in a world of their own. They're not in our world most of the time - they're wandering along, and they're so busy discovering the scent that they wander up to you and crash straight into your feet.
Another aspect of the otterhound he loves is its placid nature. They're completely non-aggressive both to other dogs and humans. Although some people do keep them as pets, Karl sees them more as working hounds.
"Mine live in a kennel outside and I wouldn't have them in the house. The reason is that when they're dirty, they're really dirty. Try and imagine them coming out of the river, that coat full of water. And they have an odour about them. No - they're working animals."
So impressed is he with the otterhound's nose, its power and its hardiness, that he would like to see one based with every search and rescue organisation in the UK.
"That would be the ultimate for me. It would also give the otterhound a complete role in life. At the moment it's only the show ring that's keeping them going, and that's a damn shame because they've got so much to offer."
Jean Pretious, a retired breeder, has some 25 otterhounds at her kennels near Holsworthy in Devon and has helped keep the breed going. Unlike Karl, she believes they do make good pets.
"The strength of the breed is that they're totally adaptable and have lovely natures. They love children.
"The only thing against them as pets is their ability to track. You have to have a secure garden because if they do get a scent of something, they're off. Then again I've had three go off for 24 hours and they all came back.
"I've heard some huntsmen say they're thick, but I think that's because they only see them doing one job. I find they're extremely intelligent, but you do have to stay one step ahead of them.
"I'm delighted with what Karl Hopton's doing, because this gives the otterhound another outlet. It's a real shot in the arm for the breed."Reuse content