SCOTLAND has a new beastie.

The first firm evidence that the grass snake is now living in the wild north of the Border has been uncovered by a naturalist, upsetting decades of conventional wisdom.

It was believed the grass snake, a non-poisonous creature native to England, has never colonised Scotland. A few kept as pets had escaped, but there was no proof that any lived here in the wild, or had spread north naturally.

But now Chris Cathrine, a member of the Clyde Amphibian and Reptile Group and director of the Caledonian Conservation company, has discovered grass snakes in Dumfries and Galloway. There have been three verified sightings of the animals since 2009 and three more possible sightings in the area, according to a study he has completed.

"This is very exciting because it's the first incontrovertible proof that grass snakes are present in the wild in Scotland," he said. Along with the adder, the common lizard and the slow worm, it brings the number of native Caledonian reptiles to four.

Grass snakes could have spread north from England, encouraged by the warmer temperatures triggered by climate change in recent years. Or they could have lived here undetected for a while.

Cathrine's investigation started in 2010 after he saw a grass snake in a pond in Upper Nithsdale. "I was balanced on a raft of vegetation searching for great crested newt eggs when I flushed out the grass snake," he recalled.

"I nearly fell into the water in surprise. I don't know who got the worse shock – me or the snake. This piqued my curiosity – as far as I had read and been told, grass snakes did not occur in the wild in Scotland."

His follow-up research revealed that there had been six sightings in Dumfries and Galloway, and good evidence of the reptile near Loch Lomond as well as at two sites in Aberdeenshire.

Cathrine has now presented his findings to Scottish and UK herpetologists – experts in amphibians and reptiles.

"The next step is to gather more records through surveys and from members of the public," he said. "Grass snakes are at the very edge of their UK range in Scotland, making their conservation particularly important in our country."

Some fellow experts hailed Cathrine's discovery as thrilling news. "We've been speculating about this for years, and he has done a good forensic job on it," said Dr Chris Gleed-Owen, a reptile specialist from Dorset.

"I look forward to seeing grass snakes officially recognised as Scotland's fourth native reptile species. It was only a matter of time before someone found native grass snakes in Scotland."

Dr John Wilkinson, from the Amphibian and Reptile Conservation (ARC) trust, argued that grass snakes had probably been prevented from colonising Scotland in the past by the cold. He speculated that the sightings "might at least partly be due to the warmer summers we are now getting."

Not everyone, however, was as enthusiastic. John McKinnell, a reptile expert with the Government's wildlife agency, Scottish Natural Heritage, said: "Though people have seen grass snakes in Scotland recently, there's no way of knowing if they're here naturally or as a result of accidental or deliberate releases. It's also possible snakes could have arrived accidentally in, for example, lorry loads of hay or straw."

The grass snake is not the only animal new to Scotland. Rising temperatures caused by climate change have caused three species of butterfly to come north of the Border – the small skipper, the Essex skipper and the holly blue.

The hummingbird moth has also moved north, as has the nuthatch, a small chestnut and grey woodland bird. In recent decades, collared doves have also arrived, as part of a general expansion across Europe.

Other species have been deliberately reintroduced to try to make up for past persecution. They include sea eagles, greylag geese, a fish known as vendace and, famously, beavers, below.

And then there are the animals that the government wildlife agency, Scottish Natural Heritage (SNH), regards as alien invaders. They include the sea squirt, the North American signal crayfish and mink.

They have joined other invaders who've been here for longer like grey squirrels, Japanese knotweed and giant hogweed. According to SNH, there have also been "occasional sightings" of parakeets in Scotland.

Snakes have a serious image problem. As serpents in the Garden of Eden, Slytherin and Voldemort in Harry Potter and stars of the Hollywood hokum horror film, Snakes On A Plane, they are not loved.

In the real world, though, things could be different. The discovery that Scotland has grass snakes, one of the most benign of reptiles, could help people overcome their repugnance.

"Grass snakes are beautiful and charismatic animals," says the naturalist who made the discovery, Chris Cathrine. "They are the UK's largest native snake species and, unlike the adder, they are not venomous."

Grass snakes (Natrix natrix) can reach almost two metres long, and are the only UK snake to lay eggs. Dark green with yellowish collars behind their heads, they eat frogs, toads and fish, swallowing them whole, and tend to live near water.

They are very shy, hard to spot in the wild and hibernate in the winter. If cornered, they can play dead to try and fool predators. Failing that, they emit a foul smell and try head butting.