The Grass Snake is typically dark green or brown in colour with a characteristic yellow collar behind the head, which explains the alternative name ringed snake. The colour may also range from grey to black, with darker colours being more prevalent in colder regions, presumably owing to the thermal benefits of being dark in colour. The underside is lighter in colour. In Great Britain the Grass Snake is the largest reptile reaching up to 190 centimetres total length, though such large specimens are rare. Females are considerably larger than males, typically reaching a size of 80-90 cm when fully grown. Males are approximately 20 cm shorter and significantly smaller in girth.
They prey almost entirely on amphibians, especially the common toad and the common frog, although they may also occasionally eat mammals and fish. Captive snakes have been observed taking earthworms offered by hand, but dead prey items were never taken . The bite is non-toxic and the snake is a hunt/ambush predator, consuming the prey live without using physical constriction. Grass Snakes are strong swimmers and may be found close to fresh water, although there is evidence that individual snakes often do not make use of water bodies throughout the entire season . The preferred habitat appears to be open woodland and 'edge' habitat such as field margins and woodland borders as these may offer adequate refuge while still affording ample opportunity for basking activity and thermoregulation. Pond edges are also favoured and the relatively high chance of observing this secretive species in such areas may account for their perceived association with ponds and water.
Grass Snakes, as with most reptiles, are at the mercy of the thermal environment and need to overwinter in areas which are not subject to freezing. Thus they typically spend the winter underground where the temperature is relatively stable. As spring approaches, the males emerge first and spend much of the day basking in an effort to raise body temperature and thereby metabolism. This may be a tactic to maximise sperm production as the males mate with the females as soon as they emerge up to 2 weeks later in April or earlier if environmental temperatures are favourable. The leathery skinned eggs are laid in batches of 8–40 in June to July and hatch after about 10 weeks. As eggs require a temperature of at least 21° C and high humidity to survive and hatch, rotting vegetation, including compost heaps, are preferred locations. The young are about 18cm long when they hatch and are immediately independent.
After breeding in spring, snakes tend to hunt and may range widely during this time, moving several hundred metres in a day . Prey items tend to be relatively large compared to the size of the snake and this impairs the movement ability of the snake. Snakes which have recently eaten rarely move any significant distance and will stay in one location, basking to optimize their body temperature until the prey item has been digested. Individual snakes may only need 2-3 significant prey items throughout an entire season.
Ecdysis also impacts upon the movement of grass snakes at least once during the active season. As the outer skin wears and the snake grows, the skin loosens from the body, including from the eyes, which may turn a milky white colour at this time. This presumably impacts upon the eyesight of the snake and they do not move or hunt during this time. The outer skin is eventually sloughed in once piece and normal movement activity is resumed.
Not being venomous, the snake's only defence is to produce a foul-smelling fluid (containing asafoetida) from the anal glands and/or feigning death by becoming completely limp. Rarely, they may also perform fake attacks, striking without actually opening their mouths. They bite in defense rarely.
This snake is distributed throughout lowland areas of England and Wales but is almost absent from Scotland and not found in Ireland, which has no resident snakes. It is widely distributed in continental Europe, ranging from mid Scandinavia to southern Italy. It is also found in north-western Africa. British Grass Snakes belong to the subspecies N. n. helvetica, but experts differ on the number of subspecies.
The species has various predator species, including corvids, owls and perhaps other birds of prey, foxes and the domestic cat. In England, the species is scheduled under the Wildlife and Countryside Act 1981 and cannot be harmed or even handled without appropriate permissions.
Two of the subspecies are considered critically endangered: N. n. cetti (Sardinian Grass Snake) and N. n. schweizeri. In 2007, the Grass Snake was included on the updated UK Biodiversity Action Plan as a species in need of conservation and greater protection.