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Introduction Recently declining mule deer populations have alarmed biologists,
managers and citizens. Although the causes of declines in mule deer remain unclear,
western biologists agree that nutrition may have contributed to population declines.
If nutrition is in fact a cause of decline, managers need to understand the foraging
behavior of mule deer in various habitats. Mule deer are ruminants; that is, they
have a multi-chambered digestive tract that includes a rumen. In the rumen millions
of microbes digest plant material through a process called fermentation. Mule deer,
like other ruminants, are unable to digest plants on their own, so they use the microbes
to do the work for them. Ecologists consider mule deer browsers (i.e., they eat
shrubs, trees and forbs) yet they often eat grasses also. How are mule deer, which
are so well adapted to foraging on tree and shrub material, also able to handle digestion
of grass?
Forage Types
Grasses have the following characteristics:
1. Single leafed plants that grow in uniform distribution, making it easy to collect.
2. Provides a uniform and consistent food source.
3. Thick cell walls containing high levels of cellulose, a sugar produced by plants
that requires lengthy fermentation to thoroughly digest.
4. Small amount of nutritious material within each plant cell.
Browses have the following characteristics:
1. Woody and leafy plants that grow in discrete, widely dispersed patches.
2. Grow in a branching manner where different plant parts vary in level of
nutrition.
3. Includes forbs (leafy, green plants), tree and shrub leaves and stems.
4. More cell contents, which are highly digestible.
5. The cell wall, albeit thinner, is actually more difficult to digest because of
the high levels of lignin, a compound found in some plants that is completely
indigestible.
Defining Feeding Strategy
Ruminant grazers, have a large subdivided rumen that can hold large quantities of
food. Because of its large rumen, a grazer can keep food in the rumen longer for
extensive digestion of cellulose. Browsers, on the other hand, do not hold food in
the rumen long; their rumens are smaller and hold less food. To compensate for a
short retention time browsers have a high intake rate of easily digested food that
requires less fermentation.
The size and shape of muzzles and teeth of ruminants also affect feeding strategy.
Grazers have wider muzzles and a wider row of teeth that are suitable for clipping a
continuous plot of grass but make it very difficult to select specific parts of a
plant. Browsers on the other hand, have narrower teeth and narrower muzzles that
allow for greater manipulation of plant parts and selection of specific plant parts.
Overall body size, rather than specific body characteristics, may also explain feeding
behavior. For example, fermentation rate in the rumen decreases with increasing body
mass. In other words, a large ruminant, like a cow, will hold forage in the rumen
longer than, say, a small antelope. And we now know that grass, a staple in a
cow’s diet, takes longer to digest. In addition, smaller animals have
inherently smaller mouths that aptly select specific browse plants and plant parts but
have difficulty biting large portions of grass. Thus, larger mouths and teeth may
result because of a larger body size rather than grazing strategy. Using each of
these ideas, we can attempt to classify mule deer.
Where Do Mule Deer Fit?
Mule deer (Odocoileus hemionus) span a variety of habitats on the western coast
of North America and thus, must be able to adapt to a variety of habitat types. If
mule deer are so adaptable, how do they maintain a browser status? Are they truly
browsers? If not, why don’t they eat more grasses?
A mule deer's body is adapted to browsing in the following ways:
1. A row of teeth shaped like chisels allow clipping food off stems or forbs close to the
ground.
2. Long face with narrow muzzles that allow them to clip specific plant parts.
3. High intake rate, which accommodates for the small rumen, short retention time and high
fermentation rate of browsers.
All these characteristics of mule deer point in the direction of a browser. They are
able to eat the leaves and needles form trees and shrubs, small plants like alfalfa and
even stems. However, research indicates that mule deer are not always so particular
in what they eat. Instead, they adapt to a changing environment.
Why Do Mule Deer Eat Grass?
Availability seems to drive a mule deer’s foraging behavior. Forage selection
can change depending on location and time of year, both within and among variable
habitats. Deer prefer green, leafy plants first and then shrub and tree leaves and
stems. However, in the spring, when the availability of immature grasses increases
as snow cover desists, mule deer will increase their grass consumption. Finally, if
agriculture presides in the habitat, mule deer will often eat in wheat and rye fields,
taking advantage of an abundant and constant food source.
How Can Mule Deer Eat Grass?
Although the body characteristics point in the direction of browsing, there are other
possible factors that might allow mule deer to digest grass:
1. Immature grasses that emerge in early spring do not contain as much of the cellulose
that is difficult for mule deer to digest
2. Moderate size of mule deer indicates a moderate gut capacity, perhaps accommodating
some grass material that requires longer retention time.
3. Lack of other options in a variable environment.
While further research is needed to define the feeding strategy of mule deer, we cannot
pigeonhole mule deer into one category. If nutrition is the cause of declining
populations, by understanding which foods mule deer eat and in which situations they eat
them, managers may be able to reverse the downward trend in mule deer populations across
the west.
Additional Reading
Shipley, L. A. 1999. Grazers and browsers: How digestive morphology
affects diet selection. In: Proceedings of Grazing Behavior of Livestock and
Wildlife 70:20-27. Available online at: http://www.cnr.uidaho.edu/
range/behavior.htm.
Wallmo, O. C., editor. 1981. Mule and black-tailed deer of North
America. University Nebraska Press, Lincoln, Nebraska, USA.
Desert USA Home Page. "Mule deer." http://www.desertusa.com/feb97/du_muledeer.html.
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