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Hare Physical Attributes and The
Winter Environment
Snowshoe hares are renowned for their ability to move and hide in snow-covered woodlands,
as they use their enormous hind feet and white winter coat to avoid an array of predators.
Yet, surviving a winter is no easy task in the northern latitudes of North America. In the
Rocky Mountains, for example, snow piles high and buries the most nutritious browse,
unreachable for perhaps as many as 6 or 7 months. All that is left for hares to eat, then,
is that which protrudes a few feet above the snow surface in the form of coarse woody
stems, conifer needles, and even tree bark if they get desperate. Indeed, this woody
browse is notably poor in nutrients and it often contains a suite of chemical compounds
that makes the browse toxic or inedible.
Acquiring nutritional sustenance in such an environment is
difficult, to say the least, and only a handful of animals in North America can subsist on
such a diet. In fact, moose and caribou are often the only other mammalian herbivore to
forage in some of the same habitats that hares do during the winter months within vast
areas of the Rocky Mountains and boreal forests of Canada and Alaska. Most other plant
eaters, like deer and elk, vacate deep snowy areas and migrate to the lowlands, river
bottoms, or south-facing slopes that are relatively snow free. Movement through snow, of
course, is not nearly so energetically costly to hares as it is to large ungulates, and
hares are able to remain in snow-covered woodlands year-round.
Digestive Anatomy and Physiology
Hares can move about in deep snows, but how do hares persist on such nutritionally poor
diets? Importantly, the gastrointestinal (g.i.) tract of a hare can handle all sorts of
difficult to digest plant material. Food is primarily digested in the hindgut (after food
moves through the stomach) rather than in the foregut like some herbivores (cows, deer,
and moose for example). Hares also have gastrointestinal bacteria that live in a
pouch called the cecum that extends from their small intestine. These bacteria digest
plant carbohydrates such as cellulose. Hares also pass food quickly through their
digestive system, void fibrous fecal pellets, and eat soft cecal pellets that never hit
the ground as hares consume them straight from their anus. Although it may sound
unpleasant, cecal pellets are concentrated packets of valuable protein, and hares try to
assimilate this into their body the second time through.
Winter Foraging Behavior
In addition to internal physiological adaptations that enable hares to feed off such rough
winter forage, it is also believed that hares behave in a way that makes them efficient
winter browsers. For example, there is good evidence that hares do little in the winter
besides eating and hiding, a behavioral strategy that helps them conserve energy. Hares
also seem to know exactly which plant parts to consume to give them the most energy and
protein. Several researchers have found that hares select the most nutritious parts of
plants such as winter buds and small diameter twigs. Winter buds typically have more
nutritional value than other plant parts in the winter. Likewise, the ends of branches
less than 4 mm in diameter are preferred by hares, probably because they have more protein
and other nutrients than large branches. In small branches the ratio of the more
nutritious bark to woody material is greater than in large branches. Thus, hares avoid
eating large branches because the proportion of woody, difficult to digest material will
fill their guts. Not surprisingly then, hares consume larger twigs only when smaller
ones become limited in number. This usually only happens when hare densities are so
high that small branches are overeaten and hares are forced to consume larger branches.
Hares also have been shown to be selective in the species of browse they will consume,
with a preference for the most nutritious foods. For example, in my study area in
the Selkirk Mountains of northern Idaho, hares preferred lodgepole pine branches over all
other conifers in the winter. As expected, lodgepole pine was the most nutritious conifer,
and it had the highest protein and energy content of all the species I tested. In fact,
hare consumption of browse in the field and in the laboratory directly reflected the
amount of protein contained in the plant species. The two species with the lowest protein
and energy values, hemlock and grand fir, were almost entirely avoided in the field, and
in the lab they would only resort to feeding on these species when they had consumed all
other browse species.
Avoidance and Dilution of Plant Anti-Quality Chemicals
However, the nutrient content (protein in particular) is not necessarily indicative of the
value of a browse species because plants often have anti-quality chemicals that interfere
with nutrient digestion. Hares must contend with a wide variety of these
anti-quality chemicals across their broad geographic distribution, such as tannins,
alkaloids and terpenoids. Although many of these compounds are thought to be produced in
the plant for reasons other than preventing herbivory, there are plants in Canada and
Alaska that respond to increasing population densities of hares and increased browsing by
producing higher levels of anti-quality compounds. Hares avoid these chemicals
because they can cause a nitrogen deficit due to the metabolic costs associated with
detoxifying certain secondary compounds. In addition, some chemicals obstruct the ability
of gut microbes to help digest protein, while yet others chemically bind with protein,
reducing the ability of hares to digest protein. Combined with the fact that hares are
already faced with reduced protein availability in browse species, the presence of these
compounds can have substantial detrimental effects on hares. It is thought that
hares in such low protein situations might lose muscle mass during the winter, have
reduced reproductive capabilities, or be unable to escape predators.
Thus, hares avoid eating plants with high loads of anti- quality
chemicals if possible. Hares instead may select those species that do not contain
many detrimental chemicals or they may select plants of certain age or growing condition
that have fewer such compounds. In other situations where avoiding plants with many
chemical defenses is impossible, hares increase the breadth of their diet by consuming a
variety of plant species with different chemical compounds, diluting, in effect, the
amount of any one anti-quality compound.
Limits on ConsumptionOne additional
behavioral strategy that hares use to acquire enough energy is to consume as much food as
possible during nocturnal feedings. In other words, they can make up for low
nutritional value in their food by eating lots of it. Such a strategy has
limitations, however. First, hares do not usually feed during the day; instead they
hide from predators in deep cover where often food supplies are extremely low.
Second, hares are constrained in their intake by how much browse that their g.i. tract can
hold and process. This makes hares unable to adjust their consumption of food based
on nutritional value because they simply cannot fit any more in. Thus, hares are
constrained by the number of hours that they can feed and by the amount of food they can
hold in their guts.
Winter Wonders
Although living off a diet of marginally nutritious woody browse in the winter seems
difficult at best, hares have "solved" this foraging problem. As a testament to
their success, hares are one of the most abundant and well-distributed mammals in North
America. Hares also rarely starve, do not lose much weight during the winter, and one
study showed that hares contained the same muscle mass in the winter as they did in the
summer. Clearly, hares have physical attributes and a digestive system that allow hares to
forage in such a restricted environment as a northern latitude winter. There is good
evidence, in addition, that the ability of hares to select the most nutritious and
beneficial plants and plant parts is equally as key to hare survival and persistence in
these snowy, woodland habitats.
To Learn More...
Murray, D.L. 2003. Snowshoe hare and other hares. Wild Mammals of North
America. Vol II. (G.A. Feldhamer and B. Thompson, eds.) Johns Hopkins University
Press.
Krebs, C.J., R. Boonstra, S. Boutin, and A.R.E. Sinclair. 2001. What drives the 10-year
cycle of snowshoe hares? Bioscience 51:25-35.
Canadian Wildlife Service Web Site: http://www.cws-scf.ec.gc.ca/ |
