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Subnivean Layer

Spiders Crawling On The Snow

2-12-18 snow spider 049A2557Most adult spiders (as many as 85% of temperate zone species) are dormant during the winter, seeking shelter beneath the leaf litter. Their metabolism slows and their need for food is greatly reduced. Other species die at the end of the summer, and their eggs overwinter, protected inside silken sacs. A third, even smaller, group of spiders remains active through the winter.

Spiders’ body temperatures vary significantly, heavily influenced by their environment. Many spiders that remain active year round seek shelter in the subnivean layer between the ground and snow, where the temperature (+/-32°F.) is often warmer than the air. Occasionally, however, they do appear on the surface of the snow, where they are exposed to the wintery blasts of cold air.

Scientists don’t know exactly how these active spiders survive the cold. Some species can tolerate temperatures as low as -4° F.°. Glycerol acts as a type of anti-freeze for these arachnids, but its effect is marginal. In order to survive, some species bask in the sun and derive energy from their diet of snow fleas (a type of springtail) and other small prey, but these strategies don’t totally explain their ability to survive a New England winter. Species of spiders in the families Linyphiidae and Tetragnathidae (see photo) are often what you see crawling on top of the snow.

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What Goes On Beneath Our Feet In Winter

Alfred's mouse2 2018-01-16 20.39.08 (003)It’s fairly unusual to see Deer or White-footed Mice, especially in the winter – they are nocturnal and they spend much of their time in the airspace under the snow next to the ground known as the subnivean layer. (A blanket of snow traps the earth’s heat, which melts the bottom of the snow, creating this layer of space.) Here both Deer Mice and White-footed Mice travel extensively, protected from both the cold (it stays within a degree or two of 32 F. regardless of outside temperature) and the eyes of predators. On cold winter days, groups including both species of mice keep warm by huddling in a common nest. (Photo of White-footed/Deer Mouse – extremely difficult to tell the difference by sight – by Alfred Balch.)

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Tunnel Vents

12-20-16-vole-hole-049a2261

Frequently you find a hole about an inch wide in the snow in the middle of a field, with no tracks going in or coming out of it. Logic tells you it leads to the subnivean layer – where the snow, warmed by the ground, sublimates into water vapor, creating a small space between the surface of the ground and the snow where the temperatures is relatively stable at 32 F. It is here that small rodents such as mice and voles create a maze of tunnels through which they travel from their sleeping quarters to feeding stations, undetected by many predators.

However, the lack of tracks into and out of this hole indicates that it is not an exit or entrance to the subnivean layer, but rather, it is a vent leading from the subnivean tunnels to the surface of the snow. Carbon dioxide from animal respiration as well as carbon dioxide released from the ground builds up to an unhealthy level in these tunnels. The holes we see in the snow are ventilation shafts, allowing the carbon dioxide to escape from the tunnels. The formation of crystals around the edge of the pictured vent indicates that warm, moist air from rodent lungs is rising up out of it.

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