Rule number one of otter spotting: wear many more layers of clothing than you think you'll need.

This occurs to me while sitting on a bench at the Otter Pool viewing platform at the Wood of Cree RSPB Nature Reserve in Dumfriesshire. It's a sunny morning with a light wind that sends smudges of cloud skittering across the sky, creating an ever-changing pattern of light, but after an hour of quiet solitude by the River Cree the cold is seeping in through four layers.

I shouldn't be surprised. As otter spotter Miriam Darlington makes clear in her book Otter Country: In Search of the Wild Otter, otter spotting requires patience – and in Scotland, patience requires several layers of insulation.

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Darlington, from Devon, spent a year looking for otters from northern Scotland, to Somerset and west Wales. They are shy animals, usually coming out at dawn and dusk, and will dash off at the slightest whiff of a human, so most sightings are purely by chance.

Darlington became fascinated by otters as a child and her passion was rekindled by a sighting one night in her camper van near Little Loch Broom, in the north-west Highlands. She was awoken at 4am by a cracking sound and peered out of the window.

"An otter, three or four metres away, hunched low on a flat rock," she writes. "Just over a metre in length, he has the dimensions of a male or dog otter, with a broad, flat head, large back feet and a long tapering tail. It's the magnificent ruff of whiskers that surprises me, and the bulk of him, the fur sleek from fishing out in the loch."

After devouring a crab, he moved onto grass, rolling over and over to dry his pelt. It was that sighting that set her off on an odyssey which would make her an expert otter sleuth.

Otters are a conservation good news story. They seriously declined in the 1960s and 70s due to pollution, but by 2007, they had largely recovered and there were at estimated 8000 across Scotland. They are mustelids, part of the weasel family which also includes pine martens, stoats, polecats and badgers.

The Eurasian otter is the species found here and the best place to see one is on the west coast of Scotland and Shetland, where they are sometimes called sea otters. In fact, Darlington notes, the British otter is not a true sea otter as it could not last long in a purely salt water environment (it needs fresh water to rinse its fur and maintain its waterproofing). It eats mainly fish, but also frogs and insects, and sometimes birds and small mammals.

To narrow down the odds of seeing one, there are signs you can look out for. Otter spraints (faeces) are dark and tarry and often full of little bones, but have a scent that Darlington describes, unexpectedly, as "lavender or jasmine tea". They are typically located on large rocks and tree stumps to mark the otter's territorial boundaries. Sometimes next to them are "castlings", tangles of dried vegetation fashioned into markers to make their spraint more noticeable. On a beach, the animals might scrape together a sandcastle and leave their spraint on top. If there's a lot of spraint, the holt or den might be nearby. The root systems of ash trees are favoured places for them.

Other otter signs include rolling places – flattened, smooth areas of grass where otters roll around drying their fur – and muddy slides on riverbanks.

I have chosen Dumfries and Galloway because it's closer than Shetland and has one of the most abundant otter populations on mainland Scotland. The viewing platform at Wood of Cree overlooks a wide bulge in the river edged by swaying reeds, fed by Corcoran Burn. It is perfect otter habitat, but today there are no brown heads cutting through the water or spraint on the tree roots, just the rustling of the leaves and grasses in the wind.

"You must spend a good amount of time finding nothing; then and only then, perhaps an ambigious sign will turn up," writes Darlington, and the words reverberate round my head as I head back to the car to warm up my ears. En route, I notice an ash tree bending over the river and look for an otter slide, but to no avail.

Otters are like ghosts: everyone knows someone who swears they've seen one, but when you look for yourself, they don't show. Rule number two: enjoy the search.

I have no luck at Clatteringshaws Loch where I go next so I move on to Lochmaben and Castle Loch, where sightings have become more common recently, according to the website of the RSPB. I pick my way from the deserted castle ruins though the trees to the water's edge and in the lengthening rays of sunlight, look for signs; nothing.

Then my gaze snags on something and I take a closer look. Under a tree by the reed beds is a broad clump of grass about a foot high. Just inside it, with a few stray strands still standing on the edge, is a patch of smoothed and flattened grass. An otter's rolling place? It has to be. Rule number three: persevere.

Heading back to the car, I feel a sense of something like satisfaction. I may not have seen an otter, but I've seen likely evidence of one, and with this most elusive of creatures, that's a result.

Otter Country (Granta, £20) is out now

Wood of Cree: The otter platform is a very short walk from the second car park. Go through Minnigaff north of Newton Stewart and head three miles up the C50 minor road.

Castle Loch, Lochmaben - from the remains of Edward I's 14th century castle, it's a hop and a skip to the water where you can see numerous bird species, and possibly otters. Reached by the B7020. Website: