Family: Colubridae (Natricinae) - Water Snakes
Order: Squamata (Serpentes) - Snakes
Phylum: Chordata - Vertebrates
Red List status: Lower Risk/least concern
Grass snake (Natrix natrix helvetica) in captivity. Buckinghamshire, United Kingdom
Total length: 1,300 mm, occasionally to 2,000 mm. Females are larger than males.
Mainland Europe north to southern Scandinavia and Russia, and south to northwest Africa; also throughout temperate Asia to China and Mongolia. The species is absent from many European islands, including Ireland, but occurs throughout England and Wales, and may range into southern Scotland. The grass snake ranges further north than any other egg-laying European snake.
Among Europe's larger snakes, the grass snake is also heavily-built with a distinct, rounded head. The eyes are prominent and the pupils rounded, and the nostrils are at the sides of the snout (not towards the top). Body scales are keeled, but those of the tail may not be, and there are 19 scale rows at mid-body. Body colour is typically olive-green but ranges through black, brown and grey. Patterning is highly variable but generally takes the form of irregular, squarish black blotches or vertical bars along the flanks (absent in some populations), often spotting on the upper body, and most distinctively a light collar with a thick black border. This border generally takes the form of two dark triangular patches, one either side of the head. The collar is typically yellow, but is occasionally white, orange or red. The upper lip is pale and each labial scale has a thick black border. Stripes are sometimes present, although this varies with subspecies (see Taxonomy). The underside is typically pale with black patches, but may be entirely black in some specimens.
Similar species: Other European water snakes typically have more pronounced dorsal patterning, and generally lack a complete, pale collar edged in black. The dice snake (N. tessellata) has nostrils facing upwards, and a more pointed snout. The viperine snake (N. maura) normally has 21 mid-body scale rows. These species also have two preocular scales, while the grass snake has a single preocular. The young of several other snake species look superficially similar to grass snakes, particularly the Aesculapian snake (Elaphe longissima) which has a pale collar. This species can be distinguished by its smooth rather than keeled scales and more slender appearance. Grass snakes can easily be distinguished from members of the viper family by their round (not vertical) pupils, rounded rather than triangular heads and differences in patterning. All snakes have forked tongues and lack either eyelids or visible ears, features which distinguish them from legless lizards.
Grass snakes are wide-ranging in their habitat requirements. Animals preferentially occur close to water, usually rivers but sometimes ponds or coastlines, or in damp woods or wet grassland, but have large home ranges (up to 120 hectares) and can be found in nearly any habitat within this area as vagrants. In some areas, snakes may be resident in drier woodland, meadows and hedges.
Reproductive habitat: The grass snake is known for laying eggs in compost heaps, where the warmth given off during decomposition serves to incubate the eggs. In natural situations, snakes often lay eggs in dung heaps, under leaf litter or within clumps of seaweed, which achieves a similar result. Females may also take advantage of artificial heat sources when choosing oviposition sites. However, eggs are often laid in other situations, such as under logs and stones, within mammal burrows or in tree holes. Animals have been known to lay in communal oviposition sites containing up to 4,000 eggs.
Elevation: 0 - 2,400 m
The grass snake is day-active in most of its range, though in warmer weather and some southern parts of its range it may be active mainly at dawn and dusk, or occasionally at night. It is a strong swimmer and is semi-aquatic, often foraging in water. Uncommonly, animals may climb to forage in low shrubs or trees. Grass snakes are highly mobile; animals have been recorded covering 4 km in a year, and may travel up to 300 m in a day.
Male grass snakes shed their skin twice each year as adults; females once.
Diet: Amphibians form the major part of the diet throughout most of the grass snake's range, with animals feeding principally on adult frogs and toads but occasionally also on newts and larval amphibians. Fish are also taken in water; on land, snakes may prey on small mammals, juvenile birds, reptiles and large invertebrates. The Cyclades population is reported to feed primarily on lizards.
Courtship behaviour: Males aggregate around females, forming 'mating balls' of up to 22 animals around a single female. Males in the ball compete with one another through tail-wrestling for access to the female, and rub her with their chins. A successful male may spend up to three hours in copulation, and females typically mate only once in a season. In the UK the grass snake usually makes in April and May, soon after emergence from hibernation, but may breed as late as September.
Defensive behaviour: Animals often bask along the banks of watercourses, and slide into the water when approached. Grass snakes typically respond aggressively to threats, striking as a warning, usually with the mouth closed. Rather than biting, if this fails animals will often resort to a characteristic 'feigned death' display, lying upside down with its mouth open and tongue out. This display is accompanied by the release of foul-smelling secretions from the anal glands. Not all snakes will display this behaviour, and its prevalence differs between populations.
Breeding biology: The grass snake is oviparous, laying clutches of between 2 and 105 white eggs (typically about 30) between 2 and 5 weeks after copulation. Eggs range from 20 to 40 mm in diameter, tending to expand and stick together as they develop.
Life cycle: Incubation lasts 6-10 weeks in southern parts of the grass snake's range, though is dependent on temperature. In the UK, they hatch as late as October, towards the end of the snake's annual activity period. Sexual maturity is reached at a length of 400-600 mm, occurring at three years in males and five in females. Newly-emerged hatchlings are 140-220 mm long.
Lifespan: Wild grass snakes have been known to live up to 28 years.
The grass snake has four formally-recognised subspecies, delineated on the basis of patterning and geographical range. In this scheme, N. n. helvetica (patterned as described above, with a white or yellow collar and dark vertical bars present on the flanks) occurs throughout mainland Western Europe from northern Italy, with populations east of this region assigned to N. n. natrix (lacking bars, sometimes with an orange collar, and typically more slender). Corsica and Sardinia each have their own subspecies, respectively N. n. corsa (typically lacks a collar, narrow bars fairly short) and N. n. cetti (slender, with a short tail and dark bars that often reach the midline of the body)
Arnold & Ovenden (2004) identify seven additional subspecific forms N. n. astretophora is recognised from the Iberian peninsula, and identifiable by a blood-red eye, very robust body, and often the lack of patterning on the green or grey body. N. n. lanzai from the Italian peninsula has dark-edged head scales, bars along the flanks and occasionally dark barring on the back. The remaining subspecies are island forms; see Arnold & Ovenden for further details.
Arnold, N. and Ovenden, D. (2004) Collins Field Guide: Reptiles and Amphibians of Britain and Europe HarperCollins, London: 288pp
Wareham, D.C. (2008) The Reptiles and Amphibians of Dorset British Herpetological Society: 120pp
Natrix natrix helvetica, Buckinghamshire, United Kingdom