Terrestrial snails and slugs are land-dwelling mollusks that lack the hard, protective skin necessary to prevent water loss. Their bodies are largely composed of water, making them highly susceptible to desiccation (drying out). This vulnerability is the primary factor dictating when these animals are active, as they must continuously seek environments that maintain their internal moisture levels. Their emergence is a direct response to prevailing environmental conditions, governed by the time of day, moisture availability, and seasonal changes.
Daily Rhythms: Nighttime and Dawn Activity
Snails and slugs are primarily nocturnal, active during the nighttime hours. This is a physiological adaptation to avoid the heat and direct sunlight of the day, which rapidly accelerate water loss. Peak activity often occurs around midnight, generally between 10:00 PM and 12:00 AM.
Activity also extends into the early morning and evening, a pattern known as crepuscular. During these periods, lower temperatures and higher humidity provide a safer window for foraging. Daytime hours, especially between 11:00 AM and 5:00 PM, see the lowest activity as the mollusks retreat to shaded, sheltered microhabitats to conserve water.
How Moisture Levels Influence Activity
Moisture is the most important factor determining whether a snail or slug emerges from shelter. Because their skin is permeable, they require a moist surface to breathe and move. Movement requires secreting a thick, water-based mucus trail, which facilitates gliding but costs the animal significant water.
Activity is strongly correlated with high environmental humidity, heavy dew, or recent rainfall. Snails often emerge immediately after a rainstorm, as wet conditions allow them to travel and forage with minimal risk of desiccation. If the air becomes too dry, the mucus used for locomotion quickly solidifies, making movement nearly impossible.
In arid environments, moisture is the overriding determinant of activity, sometimes more so than temperature. When conditions are sufficiently wet, a snail may be active regardless of the time of day. However, it will immediately retreat and seal itself away if conditions become too dry; many prefer a humidity range between 70% and 90% for optimal function.
Seasonal Dormancy: Hibernation and Estivation
Snails exhibit two types of seasonal dormancy to survive prolonged periods of unfavorable weather. The first is hibernation, a state of reduced metabolic activity triggered by cold temperatures, typically occurring during winter in colder climates. Snails burrow into the soil or find a sheltered spot before sealing the shell opening to protect against frost and dehydration.
The second type is estivation, often called “summer sleep,” induced by intense heat and drought. This state is common in arid climates where the primary threat is a lack of moisture. During both dormancy periods, the snail secretes a protective layer called an epiphragm, a hardened membrane made of mucus and calcium carbonate.
The epiphragm acts as a seal, significantly reducing water loss and creating a humid microclimate inside the shell. Metabolism slows drastically, allowing the snail to survive for months or years until favorable temperatures and moisture return. Activity resumes when the environment is sufficiently moist and warm.