Species of the day: Grass Snake

By James Duncan

Learning and Engagement Officer

The Grass Snake (Natrix helvetica) is the largest of the three British snake species and indeed our biggest terrestrial reptile. It's the females that grow the longest, often exceeding a metre in length, though the average for males is around 65cm, the females 80cm. They're certainly capable of attaining significant proportions and the very largest may measure 1.8m. Those with Ophidiophobia may be petrified at the thought, but the Grass Snake is utterly harmless to humans and is in fact a wary, timid, fast-moving species with no desire to spend any time in our presence. The most regular sighting is usually the tail of a rapidly fleeing snake as it slinks off into cover. A casual glance at a Grass Snake may cause misidentification with the venomous Adder (Vipera berus), though the Grass Snake's olive green-grey colouration, distinctive creamy-yellow and black colour, round pupil (as opposed to a vertical slit), slender body and lack of 'zig-zag' patterning are all diagnostic. British Grass Snakes were once considered to be a subspecies of the widely distributed European Grass Snake (Natrix natrix), but it was only after DNA testing in 2017 that our native population was declared to belong to a distinct species in its own right. It's now the 'Barred Grass Snake', so-called owing to a propensity to display more obvious black barring along its sides. It's incredible that the genetic identify of this supremely widespread European species has remained a mystery for quite so long.  

Once emerged from a humid hibernation spot in early spring, the snakes will be particularly sluggish and spend a lot of time 'basking' to raise their body temperature and 'fire-up' their metabolism. This is often the very best time to see them as once mating commences they tend to become secretive. However, if many males are present competition may become fierce and 'mating balls' can occur. This will involve a number of males surrounding a female, attracted by the pheromones she produces. Much wiggling, thrashing, writhing and shoving will commence and inevitably it's the largest males that are the most successful in copulation. Once courtship is complete, hunting may begin and Grass Snakes will utilise hedgerows, ditches and banks as 'commuting corridors' to take them to both feeding and egg-laying sites. These will typically be found close to fresh water ponds, streams and marshland, for the Grass Snake is very much semi-aquatic. It is in fact a highly accomplished swimmer, using lateral undulations of the body to create forward momentum whilst holding the head above the surface like a scaled mini-periscope. Their abilities in water mean they're also able to dive and may stay submerged for up to half an hour while hunting.

Grass Snake © Derek Middleton

Swimming is an essential skill owing to the Grass Snake's favoured amphibian prey. Though frogs, toads and newts are not widely available all year round in temperate climates they still provide the bulk of sustenance. A big meal may satisfy a Grass Snake for a significant time and they only need eat once every twenty days on average. They have no evolutionary adaptions to assist in subduing prey and unlike many other snakes, neither possess venom or use constriction to suffocate prey. They instead rely on a sneakier approach, using the element of surprise to strike out at their prey. This results in it being swallowed whole, preferably head-first for ease. The Grass Snake's recurved teeth help the jaw to work sideways along the prey, gradually engulfing it until it succumbs through either suffocation or the digestion process itself. The activity of Grass Snakes is usually confined to daylight hours and the ambient conditions are critical. Rain, wind and overcast skies are entirely unsuitable as activity relies on the snake achieving a body temperature close to 30°c, something that's difficult to achieve without thermoregulation in strong sunlight. 

Whilst outside of amphibian 'season', Grass Snakes will feed on fish, small mammals, nestlings and eggs, though they themselves are subject to extensive predation. Avian predators are numerous and a real danger, birds such as crows, herons and raptors. They're also a popular food source amongst mammals, those such as badgers, hedgehogs, foxes, mustelids (weasels, stoats, etc.) and of course the domestic cat. Pregnant (gravid) and therefore sluggish females are particularly vulnerable owing to the significant amounts of time they spend basking. Should a Grass Snake avoid predation, it may survive for up to fifteen years or so. They do have a few defence tactics up their reptilian sleeve and may hiss, thrash and strike. However, it's all bluff as typically the mouth will be closed and this serves purely as a diversionary tactic. They'll also feign death using a strategy known as thanatosis, turning the body flaccid and holding the mouth open with the tongue out, occasionally secreting blood through autohaemorrhage. If handled or provoked further, the defence will involve the release of a rather pungent and foul-smelling discharge from their anal glands.