The European grass snake, scientifically known as Natrix natrix, is a fascinating and widely distributed non-venomous snake. Also referred to as the ringed snake or water snake, this colubrid species occupies a significant ecological niche across Eurasia. This article delves into the dietary habits of the grass snake, exploring its preferred prey, hunting strategies, and the broader ecological implications of its feeding behavior. The grass snake is also used in the United Kingdom to refer to the barred grass snake (N. helvetica) and in North America to refer to the smooth green snake (Opheodrys vernalis) and the rough green snake (O. aestivus).
Identifying the Grass Snake
The grass snake (Natrix natrix) is typically dark green or brown in color with a characteristic yellow or whitish collar behind the head, which explains the alternative name ringed snake. The color 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 whitish with irregular blocks of black, which are useful in recognising individuals. Fossils of N. natrix have been found dating back to the late early Pleistocene. Grass snakes can grow quite large, with adults typically reaching lengths of 3 to 5 feet (0.91 to 1.52 meters), though some grow even longer.
It is important to note the distinction between Natrix natrix and Natrix helvetica. Grass snakes in Britain were thought to belong to the subspecies N. n. helvetica but have been reclassified as the barred grass snake Natrix helvetica. Natrix natrix helvetica (Lacépède, 1789) was formerly treated as a subspecies, but following genetic analysis it was recognised in August 2017 as a separate species, Natrix helvetica, the barred grass snake. Therefore, any records of N. natrix from Britain now refer to Natrix helvetica.
Distribution and Habitat
The grass snake is widely distributed in mainland Europe, ranging from mid Scandinavia to southern Italy. Grass snakes are also found in the Middle East and northwestern Africa. These snakes are strong swimmers and may be found close to fresh water. 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 thermoregulation through basking. They also favor areas near ponds, streams, and lakes. Grass snakes can also live in heavily-modified landscapes if suitable water sources are present and can be found in gardens and urban parks.
Hunting and Feeding Behavior
Grass snakes are carnivores. They prey mainly on amphibians, especially the Common toad and the Common frog, although they may also occasionally eat ants and larvae. Grass snakes mainly prey on amphibians, especially the common toad and the common frog, although they may also occasionally eat ants and larvae. Adult snakes more often eat metamorphosed frogs than tadpoles. The maximum proportion of tadpoles in the grass snake’s diet (33.3%) was reported by Klenina [3]. Usually, the diet of grass snakes is dominated by the marsh frog Pelophylax ridibundus (Pallas, 1771), the moor frog Rana arvalis Nilsson, 1842, and the common frog Rana temporaria Linneaus, 1758. Captive snakes have been observed taking earthworms offered by hand, but dead prey items are never taken. As Grass snakes are not venomous their main defense strategy is to produce a garlic-smelling fluid from the anal glands or to "play dead" by becoming completely limp.
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Grass snakes are solitary creatures and are active during the day and search actively for prey, often on the edges of the water, using sight and sense of smell (using Jacobson's organ). After breeding in summer, snakes tend to hunt and may range widely during this time, moving up to several hundred metres in a day. Prey items tend to be large compared to the size of the snake, and this impairs the movement ability of the snake. Snakes that 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 two or three significant prey items throughout an entire season. Grass snakes aren't constrictors, so they don't subdue their prey by wrapping their bodies around it.
Seasonal Variations in Diet and Activity
As spring approaches, the males emerge first and spend much of the day basking in an effort to raise body temperature and thereby metabolism. The mating season starts up to two weeks later in April, or earlier if environmental temperatures are favorable. Grass snakes, like most reptiles, are at the mercy of the thermal environment and need to overwinter in areas which are not subject to freezing. These snakes typically hibernate during the winter underground, usually from about October to March, depending on the climate.
Role in Ecosystem and Transfer of Fatty Acids
The grass snake plays a crucial role in the transfer of polyunsaturated fatty acids from the aquatic environment to land. It was shown for the first time that the high total content of eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids in the biomass of the snakes indicates its important role in the transfer of these essential substances from aquatic ecosystems to land. The composition of polyunsaturated fatty acids (PUFAs) in vertebrates almost entirely depends on the food consumed from water bodies, since these substances are synthesized mainly by aquatic microorganisms. For this reason, PUFAs are often used as biomarkers in assessing the transfer of biomass from aquatic to terrestrial ecosystems [1]. This transfer occurs when aquatic organisms are eaten by terrestrial ones, or when the habitat of amphibiotic animals with aquatic larvae-amphibians and insects-changes.
The diet of the grass snake itself is dominated by amphibians. For instance, in the Volga basin, according to different studies, they account for 70.8 [3] to 92.8% [4] of the diet. Cases of eating of mammals, reptiles, and birds by grass snakes were reported in many studies (for example, [5, 6]); however, in total they account for no more than 2% of the diet by biomass [7]. In the Penza Region, the grass snake’s diet was dominated by the moor frog (61%) and the marsh frog (20%) [8].
Multivariate correspondence analysis of the FA composition of the studied animals revealed significant differences in the first (largest) dimension between tadpoles of both species and P. vespertinus metamorps, on the one hand, and R. arvalis metamorps and grass snakes, on the other hand. The differences between the samples of these species were largely due to differences in the levels of 18:4n-3 FA and ∑16PUFA (the sum of 16-atom PUFAs) (markers of microalgae [10]), on the one hand, and 24:1 (markers of higher plants [10]) and C18-20 monounsaturated FAs (de novo synthesis products in animals [10]), on the other hand. In addition, in the second dimension, significant differences were found between R. arvalis tadpoles, on the one hand, and P. vespertinus tadpoles and metamorps, on the other hand, which were mostly due to the differences in the levels of 12:0 and 18:4n-3 FAs in their biomass (Fig. 1). The 18:4n-3 FA, which was present in relatively high amounts in P. vespertinus tadpoles, is a marker of food of aquatic origin (microalgae [10]).
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Comparison of the statistical significance of differences in the levels of individual acids (Table 1) made it possible to clarify the results of multivariate analysis. Tadpoles of both amphibian species and P. vespertinus metamorps had significantly higher levels of 18:4n-3 FA and ∑16PUFA than R. arvalis metamorps and grass snakes (Table 1). In turn, R. arvalis metamorps and grass snakes had significantly higher levels of essential linoleic acid 18:2n-6, a marker of food of terrestrial origin, than tadpoles of both species and P. vespertinus metamorps (Table 1). In addition, R. arvalis metamorps and grass snakes had significantly higher levels of 20:2n-6, which is synthesized from the essential FA 18:2n-6, than tadpoles of both species and P. vespertinus metamorps (Table 1). In R. arvalis metamorps and grass snakes, the level of 22:6n-3 (DHA) was significantly higher than that in tadpoles of both species and P. vespertinus metamorps (Table 1).
Measurements of the content of physiologically significant PUFAs, EPA, and DHA, in biomass (mg/g wet weight), characterizing the quality of organic matter transmitted on food webs, gave the following results. The maximum EPA content was found in P. vespertinus metamorps (Fig. 2). The maximum DHA content was detected in R. arvalis metamorps and grass snakes (Fig. 2). The total content of EPA + DHA in metamorps of both species and grass snakes was significantly higher than in tadpoles (Fig.2). The significantly higher level of DHA in muscle tissue probably indicates that R. arvalis metamorps and grass snakes are more active: they move more intensely (faster and/or longer) than P. vespertinus metamorps, which is consistent with observations of the lifestyle of these species. Moreover, the levels of the biochemical precursor, EPA, in R. arvalis metamorps and grass snakes are lower than those in tadpoles of both amphibian species and P. vespertinus metamorps.
Conservation Status and Threats
While grass snakes are not currently endangered, they face threats from habitat loss, pollution and persecution by humans. The main threat to these snakes is the loss of grassland and wetland habitats through human activity. They have various predators, including corvids, storks, owls and perhaps other birds of prey, foxes, and the domestic cat. In England, Grass snakes are protected and cannot be harmed or traded without a license, although they may legally be captured and kept in captivity.
Defense Mechanisms
Grass snakes are shy and will often flee when threatened. They rarely bite in defense and lack venomous fangs. However, if cornered, they might play dead as a defense mechanism. Playing dead involves the snake flipping onto its back, sometimes emitting a foul odor to deter predators. In defence they can produce a garlic-smelling fluid from the anal glands, and feign death (thanatosis) by becoming completely limp when they may also secrete blood (autohaemorrhage) from the mouth and nose. They may also perform an aggressive display in defence, hissing and striking without opening the mouth. When caught they often regurgitate the contents of their stomachs. Grass snakes may also display a rare but known defensive behavior involving raising the front of the body and flattening the head and neck so that it resembles a cobra's hood, although the geographic ranges of grass snakes and of cobras overlap very little.
Cultural Significance
In the Baltic mythology, the grass snake (Lithuanian: žaltys, Latvian: zalktis) is seen as a sacred animal. It was frequently kept as a pet, living under a married couple's bed or in a special place near the hearth. After the Christianization of Lithuania and Latvia, the grass snake still retained some mythological significance. In spite of the serpent's symbolic meaning as a symbol of evil in Christianity, in Latvia and Lithuania there were various folk beliefs, dating even to the late 19th century, that killing grass snakes might bring grave misfortune or that an injured snake will take revenge on the offender. Well-known literary works based on these traditions include Lithuanian folk tale Eglė the Queen of Serpents (Eglė žalčių karalienė) and the Latvian folk fairytale "The grass snake's bride" (Zalkša līgava). These works include another common theme in Baltic mythology: that grass snakes wear crowns (note grass snake's yellow spots) and that there is a king of snakes who wears a golden crown. Today grass snakes hold a meaning of house blessing among many Latvians and Lithuanians. One tradition is to put a bowl of milk near a snake's place of residence, although there is no evidence of a grass snake ever drinking milk. Driven by late 19th century and 20th century Romantic nationalism, grass snake motifs in Latvia have gained a meaning of education and wisdom, and are common ornaments in the military, folk dance groups and education logos and insignia.
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