The grass snake Natrix helvetica belongs to the largest Family of snakes in the world, the Colubridae, containing over 1800 species, sometimes known as the ‘typical’ snakes.
The grass snake found in Britain had long been considered a subspecies within a species that is widely distributed across Europe. However in 2017, based upon molecular evidence, it was recognised as a species in its own right. Therefore the name changed from Natrix natrix helvetica to Natrix helvetica to reflect this discovery. At the time this change occurred, it was reported in the UK press that Britain now had an additional species of snake. This is not the case, as all the grass snakes in Britain previously belonged to the subspecies that has now been elevated to species level and the species from which they have been split does not exist here. Therefore, the only change here is in the name. On the continent, however, both forms exist, therefore the realisation that what was once thought to be one species of grass snake is actually two, does affect its distribution beyond the UK, which was previously considered to be more extensive than it actually is. It has been proposed that the ‘new’ species of grass snake should be referred to as the ‘barred grass snake’ to differentiate it from the ‘grass snake’ from which it has now been separated. However, the distinction is rather superfluous here in Britain where only one species of grass snake exists.
Grass snakes are long, slender, fast moving animals, quite different in build from the short, stocky adder, and are quick to flee from any perceived threat. It takes considerable stealth to observe this species.
Colouration does not differ between the sexes. The overall ground colour varies from shades of pale brown to olive green with a pattern of dark vertical bars along the flanks and two rows of small dark spots along the back. The belly is pale, overlaid with bold black markings, often giving a checkerboard effect. The scales above the mouth, while pale, are boldly edged in black giving a pattern of oblique black bars reaching up the face to the level of the eye. The pupil of the eye is round unlike the elliptical pupil of the adder.
Perhaps their most recognisable characteristic is a yellow or orange ‘collar’ around the nape of the neck. In the older literature grass snakes are often referred to as ringed snakes, on account of this marking. Although the impression may be of a collar or ring, on closer inspection it becomes apparent that it does not encircle the neck but is in fact simply two yellow crescents behind the head, each bordered posteriorly in black. It makes the grass snake instantly recognisable and is usually quite conspicuous. In rare cases this collar may be missing, most frequently among elderly females, in whom it sometimes fades.
Grass snakes can grow large, the females significantly larger than the males, and a veteran female may comfortably exceed a metre in length, while males rarely exceed 80 cm. The build of the two sexes is noticeably different, the females having a wide girth and broad, heavily-built heads and faces, compared to the slender girth of the males with narrow, rather delicately built heads.
In common with all the native reptiles, the winter months are spent in hibernation underground, either in small mammal burrows or natural crevices, often under tree roots. The hibernacula may be shared, either with other grass snakes or sometimes with adders, but they frequently hibernate alone.
Being a little less cold-tolerant than adders, grass snakes are rarely seen quite as early in the year and sightings during February, when the first adders may be emerging, are unusual for grass snakes. The main emergence begins in March and is followed by a ‘lying out’ period close to the hibernacula, during which the animals take advantage of the available basking opportunities to attain their breeding condition.
Mating occurs in the vicinity of the hibernacula, sometimes as early as March, but more regularly during April or May and mating behaviour differs considerably from that of the adder. Grass snakes do not display the male combat seen in adders which results in a single male attempting to mate with the female after all competitors have been driven away. Instead, mating among grass snakes gives the impression of a bit of a scramble, sometimes with several males attending a receptive female during which a ‘mating ball’ develops with the female at the centre. Within the ‘mating ball’ all the males become highly excited, twitching frantically and shoving each other off the female in an attempt to manoeuvre themselves into a position to successfully copulate. Unsurprisingly, the larger and stronger males have a considerable advantage in the melee. Once mating has occurred the snakes will disperse from their hibernation sites.
The hibernacula are by necessity located on high ground where the risk of flooding during hibernation is reduced. However, grass snakes are a semi aquatic species and, although they can be found in dry habitats long distances from any aquatic features, they are more frequently associated with ponds, rivers, canals, marshes and other wetlands, where they will spend most of the summer. They are a species that can tolerate all but the most intensive farmland landscapes, particularly the more neglected corners such as the damp ditches and dense hedgerows, and they can often be found visiting suburban gardens or allotments, especially those with a pond, even turning up occasionally in urban parks or along the course of urban riverbanks.
Grass snakes are a highly mobile species. They occupy large home ranges covering ten hectares or more and will move considerable distances in search of foraging opportunities or between focal sites within their home range. They are not, however, territorial and the home range may be shared with many other individuals, often overlapping with neighbouring home ranges.
The diet consists primarily of amphibians and fishes although small mammals and nestling birds are taken less frequently. Where toads exist, they may constitute the bulk of the diet and grass snakes are completely immune to the effects of toad toxins. Prey is actively hunted both on land and in the water, where grass snakes are accomplished swimmers, and once caught, is simply overpowered and swallowed alive.
Grass snakes are the only oviparous species among our native snakes, the eggs being laid during June and July. Favoured nesting sites include piles of rotting vegetation such as compost heaps or undisturbed dung piles around farmyards and stables, although sometimes eggs may be laid under fallen logs or in warm, moist soil. Often, where suitable nesting sites are in short supply, many females will congregate to share a convenient compost heap and can be seen basking on or around the heap for several days before the eggs are laid deep inside.
Clutches consist of anything from 10 to 40 eggs although the average is around 20 to 25. Each egg measures approximately 25mm long and is covered in a tough parchment-like shell. When laid, the eggs are covered in mucus which, as it dries, cements the clutch together into a clump.
There is no parental care. Once clutches are laid the females will leave the nesting site and incubation is reliant on heat generated by the fermentation of the surrounding medium, or on ambient temperatures. The incubation period is temperature dependent, but hatching usually occurs in late August or September after approximately six to eight weeks. The hatchlings rupture the egg shell using a sharp egg tooth located on the tip of the snout, which is shed soon after hatching.
The hatchlings are exquisite miniature replicas of the adults, each being approximately 17cm long and as thin as a pipe cleaner. They shed their first skins within a few days of hatching and rapidly disperse to find suitable hibernation sites.
Grass snakes have a range of predators, including raptors and wading birds, such as herons, together with mammals such as stoats, weasels, cats and badgers which will also excavate and eat the eggs. Speed and secrecy are their best defence, however when captured they will excrete a pungent musk from their anal glands, together with copious amounts of faeces which if voided into the mouth of a predator might be expected to be quite an efficient deterrent! If this fails, many individuals will make an impressive display of playing dead, laying on their back, the body completely flaccid, with the tongue hanging limply out of the mouth. If subsequently left alone they will, once danger has passed, miraculously recover and glide away.
Grass snakes are widespread in Somerset and the unitary authorities, although particularly concentrated wherever there is fresh water. Nowhere are they more abundant in the county than on the wetlands, where there is plenty of ideal habitat and good feeding opportunities. They are common throughout the levels and moors, including those in North Somerset, as well as on the lowland valleys such as the Gordano, Chew and Avon Valleys and the Vale of Taunton. But they achieve their greatest densities on the peat moors, attracted by the large lakes in the old peat diggings and the abundance of amphibians and fish. On one occasion I counted fourteen individuals basking on a twenty metre stretch of bank in the Avalon Marshes, where the large numbers of introduced green frogs Pelophylax perezi probably contribute significantly to their abundance.
While the lowlands are their preferred habitat they are not uncommon at higher elevations. For example, they are widespread on the low hills of mid-Somerset, as well as the scarplands in the east and the south, and across the coalfields. At higher altitudes on the major hill ranges they are present where suitable habitat exists, but their distribution becomes increasingly patchy as they tend to avoid the dry upland moors such as those covering Exmoor, or the extensive tracts of heathland found on the Brendons, Blackdowns, Quantocks and Mendips which are better suited to the adder.
The grass snake is both secretive and mobile making it a difficult species to survey, and while populations would appear to be stable in most parts of the county an accurate assessment of its status has not been made.
Beyond Somerset the species is widespread across England and Wales although it is more common in the southern counties, becoming increasingly scarce further north and only just making it across the border into southern Scotland where they are rare. As with all species of snakes, they are absent from Ireland.
On the European mainland their recognised distribution has been markedly reduced by the recent taxonomic changes. The species Natrix natrix, to which our native grass snake was previously thought to belong, has a wide distribution covering most of Western Europe from the Atlantic coasts of the Iberian Peninsula, north to southern Scandinavia and south to the Mediterranean, reaching east across Russia to central Asia. However, with Natrix helvetica now separated from Natrix natrix it becomes clear that the true distribution of ‘our’ grass snake concurs with that formerly considered to be the distribution of the subspecies Natrix natrix helvetica. Therefore as well as Britain this species is found throughout France, Belgium, the Netherlands and Switzerland east as far as the Rhine region; and south throughout the Italian peninsula including the islands of Sicily, Sardinia and Corsica. The grass snakes either side of this Central European distribution no longer belong to the same species as ours!