Spiders are ectotherms, meaning their body temperature is regulated by the external environment. This presents a significant challenge when winter arrives and temperatures drop. To counteract the cold, spiders employ a combination of complex physiological changes and strategic behavioral shifts. Their survival relies on internal biochemistry and the selection of insulated microclimates to persist through periods of environmental stress.
The Winter State: Are Spiders Active?
Most spiders in temperate climates are not active during the winter, instead entering a state of significantly reduced metabolic activity known as diapause. This is a genetically programmed developmental arrest typically triggered by environmental cues like decreasing daylight hours and falling temperatures. Diapause differs from true hibernation, as it involves a profound biochemical reorganization rather than simply a long period of sleep. During this time, the spider’s oxygen consumption can be drastically reduced, allowing for energy conservation when food is scarce. The spider essentially puts its growth, development, and reproduction on hold until more favorable conditions return in the spring.
While the vast majority of species enter this dormant phase, a small percentage (estimated to be around 9%) remain active throughout the winter months. These winter-active species, often belonging to families like Linyphiidae or Cobweb weavers, can continue to move, feed, and even reproduce at temperatures near or slightly below freezing.
Seeking Shelter: Where Spiders Overwinter
The survival strategy of almost all spiders involves finding protected microclimates that shield them from the most extreme external cold. They seek out insulated spaces where temperature fluctuations are minimal and freezing is less likely to occur. The most common outdoor refuge is within leaf litter, where as many as 84% of spiders may overwinter, benefiting from the insulating layer of decaying vegetation.
Spiders also commonly seek shelter under loose tree bark, beneath rocks, or deep within soil crevices. Some species, like certain jumping spiders, utilize empty snail shells, sealing themselves inside with silk to create a small, insulated hibernaculum. This careful selection of a sheltered site is often referred to as behavioral thermoregulation.
A few species, often house spiders, may accidentally find themselves overwintering inside human dwellings such as attics, basements, or wall voids. These indoor locations offer consistently mild temperatures compared to the outdoors, though the spider still enters a state of diapause or greatly decreased activity. Spiders that are native to a region are well-equipped to survive outside and do not rely on moving indoors to persist.
Physiological Adaptations for Cold Survival
Once a spider has found its sheltered spot, its internal biochemistry takes over to prevent its bodily fluids from freezing. The primary physiological defense mechanism employed by most spiders is freeze-avoidance, which centers on the process of supercooling. Supercooling involves lowering the freezing point of the spider’s hemolymph, or “blood,” below zero degrees Celsius without the formation of damaging ice crystals.
Spiders achieve this by producing cryoprotectants, which are antifreeze-like compounds such as glycerol or specific sugars (poly-ols). These compounds accumulate in the body fluids as winter approaches, often triggered by the onset of diapause. The cryoprotectants bind to water molecules, which lowers the temperature at which ice nucleation can begin, allowing the spider to survive temperatures as low as -4°C or even -20°C in some Arctic species.
In addition to cryoprotectants, the spider’s body clears out substances that could act as ice nucleators, which are particles that promote ice formation at warmer sub-zero temperatures. The combination of these biochemical changes allows the spider to maintain its hemolymph in a liquid, supercooled state. Only a few specialized species have developed the rare strategy of freeze-tolerance, where they can survive the controlled formation of ice in their extracellular spaces, protecting the cells from rupture.
The Role of Egg Sacs in Winter Perpetuation
For many species, the survival of the adult spider is less important than the perpetuation of the species through the wintering of the egg sac. Many adult spiders, particularly orb weavers, mate in the fall, lay their eggs inside a protective sac, and then die as the first hard frosts arrive. The eggs themselves are often intolerant of freezing temperatures and cannot survive the winter unprotected.
To ensure survival, the female encases hundreds of eggs in a thick, multi-layered silk sac that is then strategically hidden under bark or attached to sheltered structures. In many cases, the eggs hatch shortly after being laid in late fall, and the newly emerged spiderlings remain dormant inside the insulating sac throughout the winter. This silk structure acts as a communal shelter that insulates the young spiders until spring, when they emerge and disperse.