Barred grass snake

Latin name
Natrix helvetica
Snakes & Lizards

Also known as simply as the grass snake and colloquially as “grassy”. Formerly a subspecies of Natrix natrix (the Eastern European grass snake), the barred grass snake was split out as a distinct species in August 2017 based on genetic sequencing of mitochondrial and nuclear microsatellite DNA. Currently one of five recognized Natrix species, with five subspecies proposed; N. helvetica helvetica found in UK.

Natrix Latin for “water snake”, from natare “to swim”, while Helvetica is the Latin name for Switzerland.

A male (right) and larger female (left) barred grass snake (Natrix helvetica) in a mating coil on the New Forest. - Credit: Marc Baldwin

Size: UK's longest snake species, males commonly reaching a metre (3.3 ft.) in length and females occasionally growing to 180 cm (6 ft.). Females mature at around 60 cm (2 ft.), while males typically reach maturity at 40-50 cm (~ 1.5 ft.). Adults weigh 90-240 grams (3-8.5 oz.). Some evidence average female size declining in UK.

Colour/Appearance: Variable, but typically a uniform olive-green colour on the upper surface along the length of the head and body, with a black and off-white to cream coloured underside and lower jaw. As the common name alludes to, these snakes have dark/black bars at regular intervals along their flanks, and rather indistinct dark/black spots along the upper back. Both sexes exhibit a creamy-white collar flanked by semi-circular dark/black patches (nuchal spot). The pupil is circular (unlike the vertically slit one of the adder) with a gold/yellow iris, and the eye is often encircled by black scales. Black (melanistic) animals occasionally reported in UK, and rare reports of white (leucistic and albino) individuals from Europe, although white phenotypes are likely selected against owing to poor thermoregulatory potential and being more obvious to predators.

Distribution: Grass snakes probably the most abundant, and one of most widely distributed, snake species in the Palaearctic region. The dividing line for N. natrix and N. helvetica is roughly the River Rhine in Germany; helvetica in the west and natrix in the east. Thus, barred grass snakes are found throughout England and Wales, including the southern islands, and south into Europe through France, western Germany and throughout Italy. There are occasional reports from southern Scotland, including confirmed reports in Dumfries and Galloway during 2009/10, but it's uncertain whether a population exists here.

Habitat: Found in a wide variety of habitat types, up to about 2,400 metres (7,900 ft.) elevation, including unimproved grasslands and meadows, deciduous woodland, parkland, roadside verges, golf courses, larger gardens, allotments, heathland, and hedgerows. Seem to be an “edge species”, preferring field and woodland margins, riverbanks, and pond sides. Indeed, generally regarded as a “damp-loving” species, it is frequently associated with freshwater, from rivers and canals to small garden ponds.

Longevity: Very little data but appears relatively long-lived species, with lifespans estimated at 19 years in wild by one study, although some suggest many probably survive no more than five or six years. There are reports of captive animals reaching 20 years old and at least one wild individual surviving 28 years.

Sexing: Females tend to be larger than males (i.e., ~25% longer, and broader), with a “heavier” appearance. It's often said that any snake longer than ~70 cm (2 ft.) will be a grassy and any longer than 90 cm (3 ft.) will be a female grassy. The number of post-ocular scales is generally regarded as ca. 75%-85% accurate in sexing, males having three and females two, and the tail can be indicative with experience (tapering more in females than males). The nuchal patch is often paler in females, and can even be absent in old animals.

Activity: Primarily diurnal species owing to its need for environmental warmth, although further south activity may continue into and past dusk if temperatures remain high. Swims strongly and commonly sighted hunting in water during late spring and summer, apparently able to remain submerged for up to 30 mins. Like all reptiles, grass snakes are “cold-bodied” and must employ behavioural thermoregulation to manage their body temperature; basking in direct sunlight to warm-up and moving to cover or water to cool down. Tends only to be active when air temperature climbs above 10C (50F) and will undergo a hibernation-like state call brumation during the coldest months, often underground where the temperature is more stable, or in deep bracken litter. Average ground speed appears to be ca. 4 mph (6.5 kmph). Proficient climber and will climb off ground to bask.

Territory/Home Range: Non-territorial. Radio-tracking data suggest home ranges of three to 120 ha (7.5-300 acres), depending on habitat diversity/quality. Swiss study in 1980s estimated combined home range (i.e., that utilized most of the time) averaged 12 ha (30 acres) in high quality rural region; females moved over larger areas than males (10 vs. 14 ha). May move up to 300 m (985 ft.) per day and some individuals have been tracked four kilometres (2.5 mi.) in a single year, particularly between hibernacula and feeding/breeding sites. More sedentary during June and July.

Diet: In Britain, bulk of the diet is frogs and toads, although newts and fish are also taken along with small mammals and nestling birds. In Europe there are reports of predation on other snakes, lizards, and slugs. In captivity, earthworms and larvae have been accepted. Prey is swallowed whole while alive, as this species is neither a constrictor nor venomous. Observed scavenging on newt carcasses in Spain, but generally carrion is rarely included in diet. Hatchling snakes will feed on tadpoles. Study in Dorset found adults ate large meals (toads) every 20 days or so May-September, with females fasting for ~45 days during gestation and egg-laying. Mean food consumed per day estimated at 2.3% and 1.6% of male and female bodyweight, respectively.

Reproduction: Unlike adders, male grass snakes do not “dance” when competing for females, instead they (22 being the largest following I've seen in the literature) pursue a female and jostle with one another to form a “mating ball” around her, each rubbing her with their chin while trying to align their cloaca with hers to insert one (of paired) hemipenis. Copulation occurs primarily during April and May and lasts up to three hours. Multiple paternity is common, one Swiss study estimating 91% of clutches were fertilized by two to five males. Gestation takes two to five weeks, with up to 105 white eggs being laid (ave. 30-40) each measuring 20-40 mm (0.8-1.6 in.) between early June and mid-July. Larger females lay larger clutches and may aggregate at good incubation locations such that several thousand eggs are deposited at the same site. Incubation is dependent on environmental warmth (ideally 27-28C / 81-82F) and large piles of rotting vegetation and dung heaps are typically sought, although may be laid under rocks, in mammal burrows and even among seaweed. Incubation lasts 6-10 weeks in south of range (inc. southern England) after which 14-22 cm (5.5-8.6 in.) long hatchlings emerge in August and September. Sex is determined at conception by sex chromosomes, at a ratio of 1:1, and is not influenced by incubation temperature. No parental care is provided. Males mature in about three years, and females at five years old. Shows gradual senescence.

Behaviour and Sociality: Hunt and often found basking solitarily but will bask communally with other grassies and other species, including adders and slowworms. Shed skin to grow, adult males shedding twice per year and females once. Highly sensitive to disturbance, fleeing readily, although may hiss and occasionally lunge (mouth closed) when cornered or handled; rarely bite. When disturbed will emit an unpleasant (garlic) smelling liquid from paired cloacal glands, which are larger in females than males, and feign death. Feigning death appears to be adult response (captive-born hatchings do not exhibit it) that includes tongue hanging free and sometimes going limp - not all animals exhibit the behaviour. Appear to have keen sense of eyesight, hearing, and smell - scent trails followed to prey and mates with aid of vomeronasal organ in roof of mouth.

Threats and Conservation: Predated by foxes, badgers, dogs, cats, and some birds, such as corvids, storks, and birds of prey. Striking nuchal spot in youngsters, which appears correlated with locomotion speed and aggressiveness, seems to reduce bird predation (possible aposematism). Apparently declining in much of central and western Wales and large swathes of northern England, based on sightings reported to Amphibian and Reptile Conservation, despite protection from Wildlife and Countryside Act (1981) and being designated a Priority Species under the UK Post-2010 Biodiversity Framework. In Jersey it's protected under the Conservation of Wildlife (Jersey) Law 2000. Currently illegal in UK to deliberately kill, injure or sell grass snakes, and decline appears to be driven by habitat destruction, agricultural intensification, and pollution.

Barred grass snake in detail


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