7 things you never knew about the grass snake

Discover fantastic facts about Britain’s largest native terrestrial reptile. 

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Close-up of a grass snake © MDParr / Getty

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1. Cold-blooded killer

Grass snakes feed mostly on toads, frogs and newts, but also take small mammals, fish and, occasionally, birds. They cannot digest their meal if the temperature drops below 5°C.

2. Size and scales

Grass snake - Natrix natrix. swimming across canal, Wiltshire, UK

A grass snake swimming across a canal in Wiltshire © Education Images/ Getty

Regularly exceeding 1m in length, the grass snake is our biggest native terrestrial reptile. Only the introduced Aesculapian snake, which now breeds in London and North Wales, grows larger.

3. Empty threats

Lacking venom, grass snakes defend themselves by exuding a foul-smelling secretion from their anal glands. They also hiss and feign strikes at attackers, and as a last resort play dead.

4. Collared frog-catcher

The grass snake’s characteristic yellow and black collar, which is most obvious in juveniles, may deter predators through mimicry of the warning colours of wasps and other toxic insects.

5. Blue period

Prior to moulting, a grass snake’s eyes turn a cloudy blue. It’s the result of a layer of oil forming between the old and new eye scales.

6. Identity kit

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A grass snake scenting the air © Geoff Scott-Simpson / Getty 

Grass snakes’ markings are rather variable, especially the black patches on the belly and under the eyes, allowing individual animals to be recognised.

7. Big is beautiful

Mating occurs soon after emergence from hibernation in spring. Several males attempt to copulate with a receptive female simultaneously in a writhing “mating ball.” The largest females are the most popular.

By playing dead grass snakes dissuade predators from tucking in. © Frank Paul Fietz/Imagebroker/FLPA
By playing dead grass snakes dissuade predators from tucking in. © Frank Paul Fietz/Imagebroker/FLPA

Plus, why do grass snakes tie themselves up in knots? Zoologist Jules Howard explains…

Such is the flexibility of grass (and other) snakes that they can sometimes tie themselves in knots as they settle into tight grasses or slither into narrow cracks. How intentional this behaviour is remains largely unknown. It certainly doesn’t seem to cause grass snakes any obvious problems. They usually simply pull the knot wide with their super-muscular skeleton and slide through the loop as if the knot weren’t there. 

Tying itself in a knot can also be an accidental by-product of the grass snake’s famous ‘play dead’ response. When threatened by predators, these reptiles often bunch their coils and stay stock still, lolling their tongues and rolling their eyes into their skulls with theatrical aplomb. The behaviour is reasonably common in adults, but rarer in young snakes. One study found that 66 per cent of wild-caught grass snakes exhibited the death-feigning response when threatened, yet no (lab-reared) hatchlings appeared to rely on this adaptive behaviour.

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Read more amazing facts about wildlife in BBC Wildlife Magazine