By Kiera Howard 
The truth about Britain’s so-called snake invasion is stranger than the headlines suggest
What you need to know:
- Non-venomous and harmless to humans and pets
- Found only in Colwyn Bay, Regent’s Canal London, and Bridgend
- Been here since the 1970s, not spreading
- Legally protected species
The 60-second summary:
- Mediterranean snakes escaped from Welsh zoo in 1960s/70s
- Survive British weather by living in house attics and walls
- Three small populations totalling under 300 snakes
- Help control rat populations, no attacks on humans ever recorded
- Government debating whether to remove them despite no harm done
For 60 years, two-metre snakes have been living in British attics, and most of us had no idea.
They’re in the walls of Welsh houses. They hunt rats along London’s canals. They lay eggs in garden compost heaps. And according to scientists, we should probably thank them.
Around 240 Aesculapian snakes now call Colwyn Bay home. Another 40 or so patrol Regent’s Canal. A third group lives near Bridgend. Despite breathless headlines about invasions, these European giants have barely budged from where they first escaped decades ago.
From Italy to Wales
Robert Jackson founded the Welsh Mountain Zoo in 1963. Like many reptile dealers of the era, he imported exotic species from across Europe. His Italian Aesculapian snakes arrived in the mid-1960s.
Nobody’s quite sure how they got out. Some say a pregnant female nicknamed “Old Essie” escaped. Others blame an enclosure failure. By the 1970s, baby snakes were turning up in the zoo grounds. Staff initially mistook them for grass snakes because of their yellow markings.
Those babies grew up. They bred. They discovered something remarkable: British houses make excellent substitutes for Mediterranean hillsides.
Tom Major and Wolfgang Wüster spent two years radio-tracking 21 snakes around Colwyn Bay. Males, they found, practically live in people’s attics. Females prefer woodland but still use buildings when needed.
The snakes have turned central heating into survival strategy. Where Mediterranean cousins bask on sun-warmed rocks, Welsh Aesculapians coil up in warm lofts to digest meals.
The Slowest Invasion Ever
Call it an invasion if you like, but these snakes move at glacial pace. The Colwyn Bay population covers about 30 hectares. London’s snakes stick to two hectares along the canal. The Bridgend group, discovered in 2016, hasn’t expanded either.
Roads stop them. Winter kills stragglers. They need specific conditions: warm buildings, abundant rodents, and places to lay eggs. Without all three, they can’t survive.
Tom Major estimates Colwyn Bay has 70 adult snakes and 120 juveniles. London has perhaps 40 adults. Nobody knows how many live in Bridgend because surveys aren’t complete.
Radio tracking reveals lives of extraordinary sloth. Snakes rest 4-6 days, then move for 2-4 days. Males range over areas the size of 29 rugby pitches, females 23. They travel up to 500 metres daily when active.
One juvenile snake Major tracked shows just how tough British life is for Mediterranean reptiles. Born in September 2018, it weighed eight grams in 2019. By 2022, it had gained just seven grams. That’s three years to reach the weight of a pencil.
Is This Really an Aesculapian Snakes UK Invasion?
Sixty years on, calling this an invasion seems absurd. Grey squirrels conquered Britain in decades. Japanese knotweed spreads metres yearly. These snakes? They’ve colonised three tiny patches and stopped.
The government lists them under Schedule 9 of the Wildlife and Countryside Act 1981. It’s illegal to release them or help them spread. They’re also on the Management Priority Species list for Wales, monitored since 2004.
There’s talk of adding them to the “alien species of special concern” list, which would mean active removal. Dr Wüster thinks that’s nonsense: “They’re completely harmless and a cull would be completely unnecessary. They’re not doing anything and not going anywhere.”
He has a point. The GB Non-native Species Secretariat lists their environmental impact as “Unknown unrecorded” with “no indications that their presence is affecting native species.” Sixty years is plenty of time for problems to emerge.
How to Identify an Aesculapian Snake
You won’t confuse an adult Aesculapian with native snakes once you know what to look for:
Size matters: Adults reach 1.3-1.8 metres, sometimes over 2 metres. That’s twice the length of an adder and bigger than most grass snakes.
Colour and texture: Olive-green to almost black with a metallic sheen. Scales are smooth and unkeeled, giving them a polished appearance.
Location: The big giveaway. Found in buildings, especially attics and wall cavities. Native snakes avoid houses.
Young ones are trickier: Juveniles have yellow collars like grass snakes, plus dark spots and head markings. These fade with age.
When active: May to October in temperatures above 15°C. Morning baskers, sometimes active into warm evenings.
The Natural History Museum confirms these are now one of four snake species in Britain, the only non-native among them.
Living with Mediterranean Squatters
Colwyn Bay residents have mixed relationships with their serpent neighbours. Lydia Mary Fernandez-Arias finds them in her Llanrwst Road garden regularly. She doesn’t mind. Bangor University researchers used to visit her garden twice daily during tracking studies.
Tom Buckley takes a more active approach. He’s seen three dead snakes on roads near the zoo recently. “The more people know of their whereabouts, the better,” he says. “From now on, I will always be on the look-out for them on the roads. If I see one, I’ll try to move it.”
Road deaths hit breeding females hardest. During Major’s study, two of eight tracked females died on roads while carrying eggs. A third died too, her transmitter found inside a male snake. Cannibalism happens.
Dr Major received worried messages from residents during his research. People finding six-foot snakes in their attics tend to panic. He reassured them: “shy and harmless to humans.”
The Climate Change Angle
These snakes aren’t supposed to be here, except they are. Or were. Fossil evidence proves Aesculapian snakes lived in Britain 300,000 years ago. Ice ages drove them out. Climate change accidentally brought them back.
Now southern Europe is becoming too hot for them. Spanish and French populations are declining. The species that shouldn’t survive in Britain might need Britain to survive.
Some conservationists see them as returning natives. Others insist non-native means non-native, history be damned. The government hasn’t decided which view wins.
London Zoo is currently supporting monitoring efforts along Regent’s Canal. They ask visitors to report sightings for their database. Similar work continues in Wales through various conservation groups.
What to Do If You Meet One
Finding a two-metre snake in your house is memorable. Here’s what to know:
They won’t attack. They’re non-venomous constrictors that eat rodents, not people. Sixty years, zero attacks on humans or pets. Even handled, they might scratch but nothing worse.
If one’s in your house, it followed mouse trails or wants warmth. Open doors and windows. It’ll leave when disturbed. They’re legally protected, so don’t kill or capture them.
Check compost heaps carefully June through September. Females lay eggs in warm, decomposing material. The eggs look like leather pouches. Leave them alone and report to Amphibian and Reptile Conservation Trust or Froglife.
Summer is peak activity. Watch roads near known populations. These snakes move slowly and die quickly under wheels.
Not Your Average British Wildlife Story
Tom Major’s research published in PLOS ONE reveals creatures perfectly adapted to suburban life. They’ve turned human infrastructure into habitat. Our houses keep them warm. Our gardens feed them. Our compost heaps incubate their eggs.
Other non-native species wreak havoc. Signal crayfish destroy rivers. Grey squirrels eliminate reds. Himalayan balsam chokes waterways. Aesculapian snakes? They eat rats and hide in attics.
The Colwyn Bay population remains the world’s most northerly colony of this species. They’ve survived Welsh weather for 60 years through sheer adaptability and central heating.
Maybe that’s the real story here. Not an invasion, but an accident that worked out. Mediterranean snakes found a way to live in Britain. British humans found they quite like having free rat control. Everyone wins except the rats.
Climate change will scramble wildlife distributions worldwide. Species will move or die. The Aesculapian snakes got a 60-year head start, courtesy of a zoo escape. They’re not going anywhere fast. But they’re not going extinct either.
Your Questions About British Aesculapian Snakes
How big do Aesculapian snakes get in the UK?
Most adults measure 1.1-1.6 metres, though some reach two metres. The cold limits their growth compared to Mediterranean cousins. One tracked Welsh male measured 1.4 metres.
When are they most active?
May through October, mainly during daylight in temperatures above 15°C. They bask mornings and may stay active into warm evenings. Winter is spent hibernating in buildings or underground.
How can I tell them apart from grass snakes?
Size gives it away. Aesculapians are much larger. They lack the grass snake’s yellow and black collar (though juveniles have yellow markings). Most obviously, they climb walls and enter buildings. Grass snakes don’t.
Are they dangerous to humans or pets?
No. Non-venomous, no attacks recorded in 60 years. They eat rodents, birds, and eggs. Even if handled, worst case is minor scratches. They actively avoid confrontation.
What do they eat in the UK?
Mainly rats and mice. Also birds’ eggs and nestlings when available. The London population survives almost entirely on brown rats. Young snakes take smaller prey like lizards.
Will climate change help them spread?
Unlikely. They depend on heated buildings and can’t cross major roads easily. Any expansion would be extremely slow, taking decades not years. Current populations show no signs of spreading.
For verified sightings and identification help, contact Amphibian and Reptile Conservation Trust or Froglife. Last reviewed: May 31, 2025