1. Effect of winter phases on Survival of Subnivean Mammals
Kevin Kohler
Winter Ecology 2008
Mountain Research Station, University of Colorado, Boulder

2. Redefining “Winter”
Pre-Nival
From onset of freezing temperatures until establishment of permanent snowpack
Nival
From establishment of a permanent snowpack until precipitation no longer falls as snow
Thaw
The period of snowmelt
The calender defines winter as a 90 day period between December 21st and March 21st, however it does not make sense to think of winter in this way in regard to organisms. Climatic factors rather than the position of the sun dictate how organisms experience winter.
Post-Nival
From the end of snow melt until the chance of frost has passed

3. What types of conditions and threats do these different phases of winter pose upon overwintering subnivean mammals?
Mean Temperature vs. Height
So, given these different phases of winter… Something we have already gone over But I would like to refresh is how temperature changes with depth in the snowpack…
Colder
Warmer

4. Meadow Vole Distribution
So I took a look at the effect of these winter phases on the meadow vole, a small overwintering mammal known to make subnivean tunnels.

5. Pre-Nival Conditions Threats Re-radiation frosts
Sub-freezing temperatures
Precipitation in the form of rain
Threats
Surface frosts present barrier to burrowing
Saturation of soil threaten burrows
Vulnerability to predation
Reradiation frosts are when cold clear windless nights radiate off a lot of heat into space and can freeze the ground. Saturation of soil can flood burrows either drowning the subnivean creatures or forcing them to the surface exposing them to other threats like cold temperatures, wind, and predation.

6. Effect of min temps and precip on meadow voles
Two Deaths
Precip
Five Deaths
This graph shows very nicely the effect of temperature and precipitation in the form of rain upon meadow vole mortaility. In Ontario Canada a total of 7 voles were placed in a cage and exposed to these conditions during a 7 day period in October. During a period of heavy rains and temperatures around zero two voles died. Two days later when the temp dropped to -6 celcius there was only a trace amount of pricip but the remaining five voles died

7. Nival Conditions Threats are reduced Precipitation primarily snow
Temperatures primarily below freezing
Threats are reduced
Snow cover provides protection from cold
Temps in subnivean zone stable
Snow density relatively low
Fewer potential preditors
Snow density is directly related to its ability to act as an insilator which I will explain further in just a second.

8. Deaths Thermal Index Snow Depth
It is seen here that the thermal index, which is a combination of snow depth and snow density, has a direct effect on vole mortality. We can see that a thermal index below 100 is unsuitable for vole survival. Mortailities occur frequently when the thermal index is below 100 and not at all when it is above 100.

9. Thaw Conditions Threats Precip primarily rain
Lowest portion of snowpack often saturated
Depth of snowpack reduced
Snowpack undergoes firnification
Threats
Threats resemble pre-nival conditions
Saturation at depth endangers tunnels
Soil very likely to be saturated preventing burrowing
Increased vulnerability to predation
Firnification is basically a process of freeze thaw that metamorphoses snow grains into larger granuals. This not only allows more light into the subnivean, increasing further melting but also increases infiltration into the snow due to the larger grain sizes. This has the effect of threatening the subnivean tunnels of the voles.

10. Post-Nival Conditions
Precip mostly rain
Reradiation frost associated with clear nights
Soils wet or saturated
Threats
Protection of subnivean tunnels gone
Burrowing difficult due to frost and saturation
Increased vulnerability to predation
This period again has the effect of making it difficult for the voles to find shelter.

11. Weasels!
Weasels shown to detect location of prey quite accurately in subnivean
Long thin shape ideal for hunting in subnivean but poor for conserving heat
Weasels are not well adapted to cold variable temps outside of subnivean
Weasels are more sucseptable to avian predators like owls when forced out of subnivean
Weasels are a primary predator of meadow voles and also overwinter in the subnivean. Also in a study involving the choice of subnivean entrance point it was shown that weasels will choose entrance points with higher prey density. (and of course their primary prey during the winter are voles). It is uncertain how the weasels locate the voles but it is thought to be auditory, olfactory, or both. Either way it has been shown that they are excellent at locating prey in the subnivean.

12. Hypotheses Weasels are not as effective of hunters outside of tunnels.
Weasel mortality is higher during pre-nival, thaw, and post-nival, than during nival.
Given this information I would like to propose a few possibilities that could be looked into in the future.

13. Citations
1. Arctic and Alpine Research Effect pf Abiotic Factors on the Overwintering Success of the Meadow Vole vol. 23 pp
2. Corn J and M. G. Raphael Predicting Marten Winter Habitat Requirements Laramie Wyo. pp21
3. S.C. Forrest, M.G Raphael Winter Resting Site Ecology of Marten in Central Rocky Mountains J Wildl. 53 pp
4. Sherbern and Bissonette Marten Access Point Use J. Wldl 58(3)