Last week, I took a break from hauling wood to sit in the sun on the front porch. Looking down at a patch of snow left by Winter Storm Jonas, I saw a tiny spider making its way across it.
I don’t think I’ve ever seen a spider on snow, and it wasn’t exactly a warm day — low 40s at best — so I had to admire the pluck of this little critter and also wonder what the heck it was doing out in the first place. I went inside to grab my camera to record the moment, but, by the time I’d returned, the spider had disappeared under my porch.
Seeing the little spider got me thinking about how arachnids survive winter here in this subtropical climate. In adapting to the cold, some animals have developed specialized overwintering strategies. Prime among them is reducing activity and slowing internal processes to limit the need to eat at a time when food is normally scarce. How animals achieve winter torpor (sometimes referred to as “hibernation,” although the meaning of that term is in flux these days) can be quite complex, as I wrote about in a couple of articles in April 2014 on how black bears do it.
Among the few studies about how spiders overwinter, the most cited is by German researcher M. Schaefer, “Winter Ecology of Spiders (Araneida).” Depending on the species, spiders can spend their winters in temperate areas in any of their three life-cycle stages: eggs, juveniles (spiderlings) or adults.
Schaefer found that most (45 percent) reproduce in the spring and summer and hibernate as spiderlings. The next most-common group (23 percent) can hibernate in any of the three life stages. The rest either remain active and reproduce in winter (9 percent), reproduce in autumn and hibernate in the egg stage (7 percent), or overwinter mainly as adults and reproduce in the spring (3 percent).
Since I didn’t know the species of the spider near my porch, I wasn’t sure which group it fell into. It looked like a spiderling, but it could just be a species that is small. And was the spider out because it normally is active in winter, or did it come from indoors, hitching a ride in my car (a foot away and in the direction the spider was coming from) among the groceries I’d purchased earlier in the day and just fall out when I unloaded them?
So where do spiders that normally live outside overwinter? With rare exceptions, not inside our houses. According to spider specialist Rod Crawford, in a post to his blog, “Spider Myths”, “generally fewer than 5 percent of the spiders you see indoors have ever been outdoors” because the environment — “constant climate, poor food supply, very poor water supply” — is far from the conditions they have evolved to survive in. (See more on this in the sidebar.)
Rainer Foelix, in his book “Biology of Spiders,” writes that 85 percent of the spider fauna overwinter in soil, “mainly in leaf litter.” The moist litter not only serves as protection from freezing but also from desiccation. According Richard Headstrom, in his book “Spiders of the United States,” some spiders also shelter “among the roots of mosses” and “many adult spiders escape the rigors of winter in a special shelter such as a cave, a deep crevice, a burrow in the ground, or under the scales of bark or beneath stones.”
While some animals can actually survive freezing, spiders cannot, Schaefer found. Death from cold comes to them through the formation of ice crystals in cells, especially those that serve critical functions. Species that overwinter in a dormant state have evolved several strategies for keeping that from happening.
One way to keep from freezing is to find shelter in a place that remains above freezing. Leaf litter is a good place for that, as the organic materials not only insulate animals but, as the litter continues to decay, it releases some heat that keeps it warmer than other substrates (rock, for example). Snow can also serve to insulate leaf litter and soil from subfreezing temperatures.
Some animals survive temperatures below freezing through “supercooling,” maintaining body temperatures below the freezing point of water while keeping out ice crystals. In some spiders, this is achieved through producing antifreeze proteins, which bind to ice crystals to prevent them from spreading.
Some other spiders produce other antifreeze substances, such as glycerol (a sugar alcohol compound) or simple sugars (such as sorbitol), lowering the freezing point by increasing the concentration of solutes (materials that are not yet dissolved) in bodily fluids. According to Crawford “most temperate zone spiders have enough ‘antifreeze’ in their bodies that they won’t freeze at any temperature down to –5 degrees C. [23 degrees F.]” and “some can get colder.”
To limit the surface area that is exposed to cold, spiders in winter torpor draw their legs close to their bodies. Eggs that overwinter usually do so in egg sacs that the mother has attached to plant stems, stones, structures and other sturdy objects and covered in silk, creating a cocoon that helps protect the eggs from the cold. Slowing or stopping the development of the embryos or ovaries, which delays reproduction, are other adaptations to cold that some spider species have developed.
The activity of adult or immature spiders in winter depends on the ambient temperature and the particular species’ ability to keep from freezing. While winter cold can endanger spiders, only a few of the spider species Schaefer studied suffered a high rate of winter mortality.
© 2016 Pam Owen
Indoor vs. outdoor spiders
Years ago, when I rented a house in Huntly, I experienced an invasion of wolf spiders — big, hairy spiders that are quite common in our area. It was early fall and, within minutes, at least a dozen came in through a gap under the front door.
According to Rod Crawford, these arachnid visitors probably were not looking for shelter but rather for mates. Many spiders breed in late summer and early fall, including those in the wolf (Lycosidae) and fishing (Pisauridae) spider families.
In Virginia, our native spiders have evolved over many thousands, even millions, of years to living in the outdoors. Their life history is built around seasonal change, and winter is an important part of that. As Crawford points out, should one of our native spiders find itself inside, it likely would die, or at least not reproduce (for reasons described above, in the column): “Any North American spider that needed artificial shelter for the winter would have been extinct long before Europeans arrived!”
The few spider species that do normally live in our houses, including the most common species — house spiders (a common name that covers several species) and the common cellar spider (in the Pholcidae family) — do not come from North America and adapted to living inside with us long ago. According to Crawford, house spiders have been occupying houses since at least the days of the Roman empire, and both house and cellar spiders are seldom found outside.
While these harmless and nonaggressive spiders are quite happy sharing the dry warmth of our living areas, a few other spiders, including the black widow and the less-common brown recluse, sometimes also take up residence in structures. These spiders usually choose dark, cool areas, such as basements, garages, and outbuildings, that mimic the outdoor environment they’ve evolved to inhabit and can come in at any time of the year.