BIOL 4362 Laboratory 3
29 February – 22 March 2016
Mammalian Locomotion in Snow
Note: The exercise described herein will be conducted in the field. Be sure to dress accordingly!
We will meet in 304 BSB at 1430 h sharp, and depart for the field at 1435 h. I would
appreciate several volunteers to have cars available to “chauffeur” people to the field site.
If there’s no snow, we’ll meet as scheduled and discuss behavioral adaptations to winter
conditions. Further, in the unlikely event of no snow, you’ll be provided with data from a
previous year to complete the exercise.
Winter conditions place harsh demands on animals. Given this powerful selective force, it
shouldn’t come as a surprise that a wide variety of strategies have evolved for “living in the
cold”. Some organisms, including many avian species, avoid the cold of winter altogether and
show a seasonal migration to warmer climes. Among non-migratory species, we can divide
organisms into those that essentially “shut down” over the winter months, entering a state of
torpor until favorable conditions again prevail, and those that remain active throughout
the winter.
In the former category, there are the true hibernators like the ground squirrels (tribe Marmotini).
Ground squirrels are inactive for up to 8 months of the year in underground hibernacula, living
off fat stores accumulated during the summer months. Although many of you probably think of
bears when you hear the term hibernation, bears are not true hibernators. Unlike the ground
squirrels, bears maintain a high basal metabolic rate throughout the denning season, but still
conserve energy by limiting activity, and sequestering themselves in a sheltered den site.
Poikilothermic organisms (like members of the phylum Arthropoda) also become inactive during
the winter. These species persist by producing cold-tolerant immature stages, huddling with
conspecifics in sheltered sites, and/or through the possession of so called “antifreeze” proteins
that prevent ice crystals from forming which would otherwise compromise cell integrity.
Among species that remain active, low temperatures impose a major energetic cost as body heat
is rapidly lost to the external environment. This cost may be particularly hard to endure as
suitable food items are often in short supply, or at least are more difficult to locate under the
snow. Some species, like red squirrels (Tamiasciuris hudsonicus) and black-capped chickadees
(Poecile atricapillus) cache food at points of peak availability, and thus ensure access to energy
rich food items throughout the winter.
With winter comes snow, which can be both a blessing and a curse for an organism's survival.
Snow provides an insulating “blanket”, under which various cavity nesting insects (e.g. ants) are
buffered from ambient temperatures. As the warmer soil below causes the bottom layer of snow
cover to melt, a pronounced “subnivean space” forms between the ground and the overlying
snow, supported by a lattice of icy pillars. Small mammals like voles, shrews, mice, and
lemmings are active in that space throughout the winter and thus benefit in occupying habitat that
is not only warmer than the ambient temperature, but also which provides cover, reducing their
detectability to avian predators. It should be noted, however, that these rodents are susceptible to
such predators at “ventilation shafts” constructed by the rodents themselves, presumably to
facilitate the removal of excess CO2 from the subnivean space (Fuller 1977).
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Snow characteristics affect both the distribution and abundance of animals that remain active on
the snow’s surface. Caribou migrate from the tundra to the taiga in winter, and settle in areas
where the snow is soft. Such conditions allow Caribou to reach food beneath the snow. Further,
snow can impede locomotion and is thus an important factor influencing habitat selection by
predator and prey species alike. From a study conducted in the southwestern Yukon, Murray and
Boutin (1991) reported that coyotes (Canis latrans) were more common at low elevations where
snow was shallower, while lynx (Lynx canadensis) were most common at higher elevations with
deeper snow. The authors of that study relate habitat choice to the differential “foot-loading” of
those species: Lynx, with a higher ratio of paw area to mass (lower foot-load) sink less in snow
than do coyotes, and thus can adequately obtain prey (like snowshoe hare, Lepus americanus)
even in deep snow. Coyotes on the other hand concentrate their hunting activities in snow that is
shallower, and more firmly packed than do lynx (Murray and Boutin 1991). Such patterns of
activity relative to snow conditions were not limited to hunting however. Coyotes traveled on
hard-packed trails more frequently than did lynx, and thereby reduced the average sinking depths
of their paws to levels similar to those recorded for lynx (Murray and Boutin 1991). In so doing,
coyotes would presumably reduce the energetic cost of moving through snow: as anyone who has
walked in snow can attest, less calories are burned when you don’t sink up to your waist with
each footstep!
In this “lab” we will visit the Assiniboine Forest in Winnipeg. There, we will examine animal
tracks and collect data on snow compaction and snow depth relative to animal activity. Hand in
your data sheets at the end of the lab so that we can collate the data for distribution via the
course web site. Your assignment will be completed using those class data. (note: if snow
conditions preclude data collection, class data from a previous year will be distributed in lab)
Methods
**Note: If at any time during this exercise you find yourself getting too cold, retreat to a vehicle
to warm up! We do not want anyone ending up in the hospital with frostbite!!
Obtain a ruler, penetrometer, and coin. Identify tracks of at least 2 different mammalian species.
Some likely candidates are snowshoe hare, red squirrel and grey squirrel (which we will not
discern between, but simply regard as “tree squirrels”), coyote and domestic dog (which we will
record as “canids”), and white-tailed deer. If you are stuck for a second species, you can use
humans (a perfectly valid mammal!) to complete the exercise.
Follow the tracks you have located for a distance of 20 m. At 1 m intervals, measure the snow
depth and snow compaction using the ruler and penetrometer respectively. At each sample
interval, take both “on-track” and “off-track” measurements. On-track measurements will be
made on the path the animal travelled, while off-track measurements will be taken at a distance
of 1 m directly perpendicular to the animal’s path. Which side of the path to sample from should
be determined by a coin toss. At each interval, also record the sinking depth of the track, and at
the end of the track-sampling session (the 20th interval), estimate the surface area of the
animal’s “foot” by outlining the smallest rectangle that would enclose the track. Record all
of your data on the sheet provided.
Hand in your data sheets at the end of the lab!
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Assignment (due 22 March 2016: data available on the course web site by 3 March)
Although measurements within tracking sessions are not independent (and thus should be pooled
to give an average value for each individual animal), we will wantonly commit the “pooling
fallacy” (given time constraints) and treat each sample as an independent measure.
1. Using paired-sample t-tests (see lab 2 appendix), determine whether snow depth or snow
compaction differed for on-track and off-track measurements for each species.
2. Is there evidence that animals restrict their movement to more shallow or more densely
packed snow? Present your results in a bar graph, showing the mean ± SE depth (&
compaction) on and off-track for each species. For which species was such selective
activity apparent?
3. Explain the presence or absence of selective activity documented in question 2 with
reference to the foot loading (mass/area), and the mean sinking depth of each species.
While individuals vary in mass, we’ll use an average mass in the calculation of foot loading
for each species as follows: deer 70 kg, hare 1.5 kg, canid 16 kg, tree squirrel 520 g,
white-footed mouse 55g, human 75 kg. Further, we’ll make the simplifying assumption that
the entire mass is borne on a single foot with each footfall. In support of your general
explanation, plot figures of the relationship between snow depth and sinking depth, and
snow compaction and sinking depth for the species considered.
4. Describe any one morphological adaptation of one of the species considered that facilitates
individuals of that species movement through deep snow.
5. Explain whether or not the patterns of selectivity documented constitute evidence of animal
intellectual ability (i.e. animal thinking).
Literature Cited
Fuller, W.A. (1977). Demography of a subarctic population of Clethrionomys gapperi: numbers
and survival. Can. J. Zool., 55, 42-51.
Murray, D.L., & Boutin, S. (1991). The influence of snow on lynx and coyote movements: does
morphology affect behaviour? Oecologia, 88, 463-469.
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