THEY have been vilified as a pest, persecuted for centuries and hunted for their fur.
But now one of Scotland's native animals is attracting a new body of support for its role as a potential saviour of the red squirrel.
The pine marten, a small mammal once prolific throughout the country's woodlands, only existed in remote pockets in the far north of Scotland a century ago.
As it has spread south again, however, it has come into contact with American grey squirrels which are taking over the territory of the native reds, now a threatened species.
Forestry researchers have now found that where the two species have moved on to the same ground, grey squirrel numbers have gone into reverse.
They believe that carnivorous pine martens prey on greys, which, unlike reds, mainly forage for food on the forest floor. But they are unable to catch the faster, lighter native species, which normally inhabit the treetops.
They are now calling for habitats for pine martens to be created to help stop the invasion of the greys in its tracks. Red squirrels are now one of Scotland's most endangered species, largely due to the bigger, more aggressive greys moving into their former territory.
The pine marten effect has been observed in woodlands in Perthshire, near Pitlochry and Aberfeldy, and published in a paper for the journal of the Royal Scottish Forestry Society.
Rob Coope, the Forestry Commission's Tayside region biodiversity manager, said: "Pine martens are opportunistic animals who will exploit any new food source that moves into their territory.
"Grey squirrels are a big threat to red squirrels, but what we have noticed is that in areas where pine martens are active, grey numbers go down and they almost disappear.
"Their march up the country has been pretty relentless, but in these areas it had been halted.
"We tried to think of reasons for this happening, and the theory is that expanding pine marten populations come up against expanding grey squirrel populations. Colleagues in Ireland feel they have seen the same thing happening."
Coope suggests that the answer to why pine martens hunt greys rather than reds is that the American invader mainly hunts for its food at ground level.
"We know that pine martens feed mainly on the ground and we know that greys spend more time on the ground than reds," he said.
"Reds tend to be in the treetops eating seeds from pine cones, whereas greys will run around on the forest floor looking for acorns and beech nuts. That's when they come into contact with pine martens.
"So it may be that in areas that are poor habitats for grey squirrels, pine marten predation tips the balance in favour of the red."
Pine martens, which grow to the size of an adult cat with a bushy tail, are part of the weasel family of animals, native to Northern Europe. Its fur is usually light to dark brown and grows longer and silkier during the winter months. It has a cream to yellow-coloured 'bib' marking on its throat.
Although they are preyed upon occasionally by golden eagles and even more rarely by foxes, humans are their largest threat. They were pursued so relentlessly by hunters and gamekeepers that by 1915 the only population in Scotland was confined to a small area of the north-west Highlands. Since then, the expansion of conifer forests led to a partial recovery in numbers, but populations were still so scarce in the 1980s they were declared a protected species under the Wildlife and Countryside Act.
Now the animal, which has a diet of small mammals, carrion, birds, insects and fruits, is present throughout the Highlands region, and is expanding its range south into Perthshire, Fife and down Loch Lomondside in the west. There are now believed to be around 3,500 breeding adults.
Scottish Natural Heritage, the Government's countryside agency, said it was aware of anecdotal evidence that pine martens were preying on grey squirrels.
"It is a very interesting theory on which more work needs to be carried out," a spokesman said.
Various methods have been tried to stop greys invading the territory of the native reds – including poisoning, shooting, trapping and contraception – but success has been elusive.
Coope believes more research is now required to establish whether providing a more extensive habitat for pine martens – they prefer established woodlands and breed in holes in trees or rock faces – will help save red squirrels.
"We really should encourage pine marten habitats to help the reds cling on," he said. "With good mixed forests we will have the right habitat for pine martens and red squirrels, and it may also stop the spread of the greys."
Mel Tonkin, co-ordinator of the official Scottish Squirrel Survey, said: "There is evidence from areas that grey squirrels occupy that when pine martens move in, their numbers decline and reds make a bit of a comeback. It's an area worthy of more research."
Into the red
Once widespread across the UK, the red squirrel is now largely restricted to the north of England, Scotland and Northern Ireland. Scotland hosts about 75% of the estimated UK population.
The species, immortalised for a generation of children as Tufty, the road safety squirrel, has already been lost from an extensive area in the Central Belt and is found primarily in the Borders, Dumfries and Galloway, Argyll, Perthshire, Grampian and the Highlands.
The red builds a nest known as a drey in a branch-fork of a conifer by laying down twigs to make a domed structure about 25cm to 30cm in diameter, then lining it with moss, leaves, grass and bark. It mostly eats the seeds of trees, neatly stripping conifer cones to get at the seeds within. Excess food is put into caches, either buried or in nooks or holes in trees, and is eaten when food is scarce.
The grey squirrel population out-competes the red squirrel for various reasons. The grey can easily digest acorns, while the red cannot. The grey carries a disease, squirrel parapoxvirus, that does not appear to affect their health but will often kill the reds.