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Hibernating snails

© Esther Wullschleger

Many small invertebrate animals of the northern hemisphere have a very restricted life span, such that an individual might never "experience" the different seasons of the year. Some insects, like flies or butterflies, live only a couple of weeks, if not just days. Snails, on the other hand, usually live for at least a few years, and therefore they experience the entire range of seasonal variation in environmental conditions. In the northern regions, they have to find ways to cope with the harshness of cold winters.

If day temperatures fall to 8 degrees Celsius or below, Helix pomatia, the "French escargot" which is often kept in breeding farms for gastronomic purposes, becomes more sluggish and does not eat much food any more. Depending on region and climate, this happens in Europe around September or at the latest in October. In these first cold days, plants begin to reduce their green parts above ground, and to put nutrient contents into their roots and tubers below ground. Therefore, the snails won't find as much food any more.

In this situation, Helix pomatia snails begin to search for suitable places where they can bury themselves into the ground for hibernation. Often, several individuals gather at a suitable place, which is mostly covered with moss, grasses or leaves. If this cover of plant material is sufficient, they do not even bury a hole into the ground, and simply hibernate under the plant material. When buried, the snail turns itself around so that its shell opening shows upwards, and builds a solid lid made of chalk to close its shell. This requires a lot of chalk - the snails start to accumulate this material for the lid in their body tissue already in August. Usually, they form a second epiphragm lid below the solid chalk lid, and even more of these weaker lids, each time retreating deeper into the shell. With these, a hibernating snail constructs air chambers which are an excellent isolation against the cold.

Amazingly, Helix pomatia snails often seem to build a thicker lid before strong winters than before mild winters. It is unknown how these snails anticipate the intensity of winter. Actually, they should make their decision about the lid already in the late summer months, when they use to start accumulating chalk material for the hibernation lid.

Healthy individuals of Helix pomatia, which form a solid lid, should be in a good position to survive winter. However, it gets dangerous if winters are extremely mild. Warm days in the midst of winter will wake the snails up, and if the cold returns, they do not have enough time to build a new lid. Therefore, large or irregular temperature variations, as they might come with climate change, could be dangerous for hibernating snails.

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