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  Behavioral Education for Human, Animal,
Vegetation & Ecosystem Management

Stories of Applied Animal Behavior
Created by members of a graduate Foraging Ecology Class
     at the University of Idaho and Washington State University
     under the direction of Drs. Karen Launchbaugh and Lisa Shipley

Surviving the Winter: The Importance of Snowshoe Hare Foraging Behavior

By Ethan Ellsworth

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 Consumption

One 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/

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Learn more about the Foraging Ecology Class by visiting http://www.cnr.uidaho.edu/range556/