From early spring thru late fall, there will be some slugs and isopods underneath almost every large enough rock in my backyard (for example, see this post). But one day last January when the air temperature was -13 °C, I looked under several rocks and could not see any animals, active or frozen. Of course, that wasn't too surprising, for the soil underneath the rocks was frozen solid and the undersides of the rocks were covered with ice crystals. In fact, I had to use a small shovel to pry the rocks off the ground to which they were glued by ice.
I am pretty sure I will find the slugs and the isopods again in the spring. So then, where are they now? If they all died during the first frost, where will the new ones come from in the spring? If some are surviving, where are they hiding now?
One likely answer is that the slugs and the isopods migrate deep into the soil thru the cracks and the holes (earthworm burrows?) and wait out the winter in a semi-dormant state; hibernation may be a suitable description of their state.
Soil has 2 characteristics that could definitely contribute to the winter survival of soil-dwelling animals: 1. the diurnal temperature fluctuations that take place on the surface dampen with depth; 2. temperature rises with increasing depth. The following figure shows the variation of average temperature with depth and time under bare soil in St. Paul, Minnesota in January (the number above each curve is the depth).
Figure 2.2 from Microclimate: The Biological Environment by Norman J. Rosenberg, Blaine L. Blad, Shashi B. Verma, 2nd ed., Wiley-Interscience, 1983. (Google Books)
The lowest temperature (-10 °C) and the widest fluctuations were at the surface, while the highest temperature (about -2.5 °C) was at a depth of 80 cm and stayed more or less constant during the day.
I wanted to determine if something similar was taking place in my backyard. So this morning I took some soil temperature measurements. The procedure was simple: Using a long screw driver and a hammer I drilled a vertical hole in the ground under a rock, stuck a long, glass thermometer in the hole, waited for about 10 minutes, recorded the temperature and the depth. And then repeated this for different depths in soil under 2 rocks. Here are the results.
Between 10 and 11 in the morning, air temperature about 1 cm above the soil was -2 °C. The soil under the rocks was frozen and, as before, there were no animals under the rocks. However, at a depth of 23 cm below the 1st rock, the temperature was 1 °C. Interestingly, at comparable depths the soil under the 2nd rock was slightly colder than that under the 1st one.
Although these data do not demonstrate that the missing animals are hiding deep in the soil, it does indicate that if they could migrate to depths more than about 20 cm, they would avoid freezing temperatures. Both the thermometer and the screw driver were coming back up with mud on them, which showed that at those depths the soil was not dry and therefore, there would be no danger of dehydration for the animals.