Dice snake



The dice snake (Natrix tessellata) is a Eurasian nonvenomous snake belonging to the family Colubridae, subfamily Natricinae. It is also called water snake.

Brief description
Females are bigger than males. Their typical size is 1.0 - 1.3 m long. Their color may vary from greyish green to brownish or almost black, with dark spots on the back. The belly is sometimes vividly coloured in yellow or orange, with black spots, very similar to dice, hence the name.

Biology
Living mainly near rivers, streams and lakes, it frequently feeds on fish. Sometimes, it feeds also on amphibians such as frogs, toads, and tadpoles.

Classified as nonvenomous, N. tessellata produces a potent antihemorrhagin in its serum and has been said to produce a neurotoxin through a gland in its mouth. As a defence, it spreads a very bad-smelling secretion from its cloaca. Another defence mechanism is thanatosis, playing dead.

During the mating season (March–May), they congregate in large groups. Egg-laying is usually in July, and one clutch consists of 10–30 eggs. The young snakes hatch in early September.

Dice snakes hibernate from October to April in dry holes near the water.

Distribution
The dice snake is found throughout much of Eurasia: Afghanistan, Albania, Armenia, Austria, Azerbaijan, Bosnia and Herzegovina, Bulgaria, China, Croatia, Cyprus, Czech Republic, France, Georgia, Germany, Greece, Hungary, India, Iran, Iraq, Israel, Italy, Jordan, Kazakhstan, Kyrgyzstan, Lebanon, Montenegro, North Macedonia, Pakistan, Poland, Romania, Russia, Serbia, Slovakia, Slovenia, Switzerland, Syria, Tajikistan, Turkmenistan, Turkey, Ukraine, Uzbekistan, and Yemen. The species is also present in Egypt.

Czech Nature Conservation Agency
The dice snake is considered Critically Endangered in the Czech Republic, mostly due to destruction of habitats and the introduction of an invasive species, the American mink. In order to combat this, the Czech Nature Conservation Agency began a project in 2007 to figure out what factors determine the distribution of the dice snake. The predictive model suggested that the most influential factors were watercourses and bodies, average annual temperatures, altitude, slope inclination, and precipitation seasonality. At the conclusion of the study, researchers proposed that the next route of study should be focused on areas that should but don't contain dice snake populations based on the predictive model. These areas might become the focus for conservation/migration efforts.

Fluctuating asymmetry in urban dice snake populations
Urbanization is one of the greatest reasons that habitats for dice snake populations are on the decline. As a result, some dice snake populations have been forced to live in artificial lakeside habitats. A study published in 2023 researched the effects of urban living on the dice snake and found that urban environmental factors influence the fluctuating asymmetry, body condition, and size of dice snake populations.

Fluctuating asymmetry (FA), a measure of developmental instability, was found to be significantly influenced by local environmental variables. Specifically, larger harbors were associated with poorer body condition in dice snakes. This correlation suggests that larger harbors may harbor higher pollutant concentrations or increased human disturbance, leading to adverse effects on snake health and fitness. Proximity to main roads was identified as a significant factor influencing the level of fluctuating asymmetry in dice snake populations. Snakes living closer to main roads exhibited higher levels of asymmetry, indicating potential negative effects of road-related factors on snake development and health.

The study concluded that conservation efforts should focus on mitigating the negative impacts of urbanization on dice snake populations. Strategies may include reducing pollutant release into the environment, exploring alternative road materials to minimize the adverse effects of roads, and implementing measures to reduce human disturbance in snake habitats.

Population bottlenecks and resulting loss of genetic variation
Another study in 2001 studied the effect of bottlenecks on the dice snake population when dice snakes were introduced into several lakes in Switzerland. A bottleneck on a population is "an event that drastically reduces the size of a population"

The study followed two introduced populations, one that was serially bottlenecked and one that was only bottlenecked once. Both populations had much less allelic diversity and resulting heterozygosity (due to inbreeding) than populations that had never been bottlenecked, but the one that was serially bottlenecked displayed a greater severity of these conditions.

Scale anomalies were another factor that was studied during this project. Scale anomalies are often a sign of developmental stress, and the introduced populations had a greater frequency of scale anomalies than normal. The occurrence of scale anomalies correlated with the degree of bottlenecking and individual heterozygosity. Developmental stability, reflected in the ability to withstand environmental and genetic perturbations, is indirectly linked to individual fitness. Studies on other snake species indicate a negative relationship between scale anomalies and traits such as locomotion speed and growth rate, which affect survival.

Researchers argue that population decline should be prevented and large populations maintained in order to conserve allelic diversity and genetic variability. But the root problem of habitat destruction must be addressed as well. Habitat protection and restoration are crucial for protecting natural populations of dice snake. Demographic and genetic monitoring of populations can aid in detecting declines and assessing genetic variability. Introduction of new genes from different populations can enhance genetic variability, but careful selection of source populations is essential to avoid harming existing populations and outbreeding depression.

Parasitic threat
One of the most numerous populations lives in the vicinity of the ruins of Histria, in the Dobruja region, Romania. This population has been recently discovered to be threatened by a parasitic nematode of the genus Eustrongylides. Since 2005, the population from Histria has been receiving researchers' attention. For example, a joint Romanian–Swedish–Czech research program is focused on population biology studies and parasitic threats of this unique coastal population.