ANIMAL SNOW COPING

Teacher Background

When snow is deep and soft, large mammals, such as deer, drag their bellies in it as they travel.  To overcome such snows, they move by either plowing through it or by bounding, both energy-consuming modes of locomotion.  Therefore, the depth and characteristics of snow affects the movements and activities of large mammals and may influence their winter distribution profoundly.  For example, the Canadian winter ecologist William Pruitt (1959) noted that caribou seek out soft, light, thin snows because of the ease of feeding and of predator escape in it.  Snow cover over 45 cm limits the movement of mule deer, and elk are generally limited by snowpack greater than 40-70 cm in depth. Furthermore, white-tailed deer are severely limited by snow depth greater than 40 cm, whereas moose are limited in movements when depths exceed 70 cm.  When snow depth exceeds 24 cm, deer move away from the upland Nova Scotian habitats, which they share with moose in the summer, toward lowland regions of lesser snow.  Thus the depth and characteristics of the snowpack determines where large mammals travel and feed in the winter.

Telfer and Kelsall (1984), Canadian wildlife biologists, proposed that some large mammals are adapted to cope with snowpack morphologically and behaviorally.

o Morphological snow coping is best illustrated in animals with long legs, large foot surface area, and light body mass.  Two useful measures of an animal's morphological ability to snow cope are its standing chest height, essentially its leg length, and its weight load on track.  The latter measure is determined by dividing the animal's live mass (weight) by the total area of its four feet.  If either snow hardness exceeds weight load on track, or if the snow is very dense, large mammals can walk on top of it and not incur a great energy loss due to locomotion.

o Behavioral snow coping methods include (1) migration away from unsuitable snow conditions, (2) ability to utilize (i.e. eat and digest) food above the snow, (3) pawing or rooting into snow for food, (4) specialized locomotion, (5) following behavior and trail making (i.e. yarding), and (6) within the winter range selection of habitat with little or with suitable snow.

Formozov (1946), a Russian winter ecologist, classified animals in terms of their overall adaptations to snow cover.  His classification included chinophobes, chineuphores, and chinophiles.

o Chinophobes (snow avoiders) are animals with no adaptations to snow such as antelope, bison, opossum, bobcat, turkey and bobwhite.  Most chinophobes are able to withstand cold temperatures associated with winter, but are unable to cope with the snowpack and the resulting difficulty of finding food.  Thus chinophobes inhabit or migrate into areas of lesser snow, or hibernate.

o Chineuphores (snow tolerators) are species able to withstand considerable snow and possess some adaptations to it.  These include moose, elk, deer, cottontail rabbit, fox, wolverine, wolf, voles, moles, shrews, otter, and ruffed grouse.  Chineuphores may migrate within their range to habitats with preferred snow conditions, but they do not as a rule migrate out of their range as do many chinophobes.

o Chinophiles (snow lovers) are species with specific adaptations to snow and include lynx, caribou, arctic fox, snowshoe hare, arctic hare, collared lemming, and willow ptarmigan.

Chinophiles and chineuphores should include species which inhabit regions of deeper and persistent snows because they are better snow copers, whereas chinophobes should be species existing in regions of less snow.  Telfer and Kelsall (1984) developed an overall snow coping index for various large mammal species which they have been able to relate to the nature of the snowpack in the winter range. 

Telfer and Kelsall's (1984) snow coping index is the sum of a species' behavioral and morphological snow coping abilities.  The index is unitless and is intended to reflect a species' relative degree of adaptation to snow.  They measured behavioral snow coping as the sum of the scores for the six kinds of behaviors mentioned above, which they considered important adaptations to survival in snow.  Each kind of behavior was arbitrarily scaled from 0 (absent) to 5 (very good).  Thus the highest behavioral snow coping index a species could score would be 30.  Morphological snow coping was measured as the sum of a species' leg length (chest height) and foot loading, scaled so that foot loading matched leg length in magnitude, and also that higher values represented better morphological snow coping. Thus:

Snow Coping Morphological Index = Leg length + (100 - body mass/foot area)/10 (1)

A more intuitive measure of morphological snow coping is the Snow Coping Body Index which relates an animal's body mass to its leg volume.:

Snow Coping Body Index = body mass / (leg length x foot area) (2)

This measure, the units of which are gm/cm³, incorporates the same three important parameters of morphological snow coping: leg length (chest height), body mass and foot surface area.  This index correlates with that of Telfer and Kelsall (1984), but does not require any scaling.  Unlike the morphological index, a small Snow Coping Body Index means an animal copes well with snow.  Table 1 gives the effective Snow Coping Body Index of some large mammals as well as humans on foot and on snow shoes.

Table 1.  Leg length, foot loading and Snow Coping Body Index for various large mammals including man (mammalian data adapted from Telfer and Kelsall 1984).

*Data from 12 people varying in age and footwear.

A combination of large feet, long legs and light mass gives a small Snow Coping Body Index indicating a superior morphological snow coping ability.  Note that caribou, true chinophiles, possess the smallest index, whereas bison and prong-horned antelope, more southerly ranging chinophobes, possess the highest values, implying poor snow coping.

Telfer & Kelsall (1984) point out that in addition to species differences, within a species there are sexual as well as age differences in morphological snow coping.  As they note, these differences could have considerable survival value in allowing the use of deeper or softer snow by different sex and age groups.

The purpose of the student activity is to demonstrate how morphological parameters could be evolutionarily adjusted to enable an above-snow mammal species to better cope with deep snowpack.  Data from humans on boots and snowshoes are used to demonstrate the morphological characteristics that may be adaptive in deep snows.

Formozov, A. N. 1946.  Snow cover as an integral factor of the environment and its importance in the ecology of mammals and birds.  Occasional Publication 1. Boreal Institute, University of Alberta, Edmonton, Canada. 176 pp.

Kelsall, J. P.  1969.  Structural adaptations of moose and deer for snow.  J. Mammal.  50:302-310.

Pruitt, W. O.  1959.  Snow as a factor in the winter ecology of the barren ground caribou (Rangifer arcticus).  Arctic. 12:159-179.

Telfer, E. S. & J. P. Kelsall 1984.  Adaptations of some large North American mammals for survival in snow.  Ecology, 65:1828-1834.