1. Why Deer, Though Mostly Grass-Eaters, Eat Tree Bark and Other Plants in Winter
Deer primarily eat grass. However, materials from the Ministry of the Environment explain that they will eat almost any plant. They consume leaves, fruits, tree bark, bamboo leaves, and fallen leaves [1]. So, even though they are called "herbivores," they do not eat only grass.
What they eat depends on the forest they live in and the season. In deciduous broadleaf forests, which lose their leaves from autumn to winter, deer eat grasses from the rice family, bamboo leaves, tree leaves, hard nuts like acorns, and tree bark. Deer living in evergreen broadleaf forests, which keep their leaves year-round, eat tree leaves throughout the year [2].
In areas with deep snow, ground plants are covered. As a result, deer begin eating accessible tree bark [2]. The Ezo deer, a subspecies of the Japanese deer found in Hokkaido, is also introduced as eating a wide variety of foods: grass and pasture in summer, and bamboo or tree bark in winter when food is scarce [3]. So, how does their body manage on just grass, leaves, and bark? The key lies in their stomach.
2. What Deer in Nara Park Mainly Eat: Grass, Not Deer Crackers
When you think of Nara Park, you might picture deer begging for "deer crackers." However, an article introducing a survey of deer populations in Nara states that the deer’s main food is plants such as Japanese lawn grass. The deer crackers, made from rice bran and wheat flour, are treated as "snacks" for the deer [7].
According to introduction materials from Nara Prefecture, deer eat the growing lawn grass in spring. Because branches and leaves on park trees are eaten completely, they also eat branches and leaves that have fallen due to wind [8].
This shows that "what they eat changes by season" happens even within a single park. However, the materials do not provide specific numbers showing the percentage of grass or crackers in their total diet.
3. How Microbes in a Deer's Stomach Break Down Tough Grass Fibers
Grass and leaves contain a lot of a hard fiber called cellulose. The stomachs of deer and cattle (ruminants) are usually divided into four chambers: the rumen, the reticulum, the omasum, and the abomasum [4].
Dictionary explanations state that swallowed food is first stored in the first two chambers (the rumen and reticulum). Symbiotic microbes living there break down the cellulose [4]. In other words, deer get help digesting fibers from microbes in their stomachs.
Unlike humans, who have one stomach and cannot digest cellulose well, the deer’s stomach has chambers prepared for microbes to work. However, the specific types and numbers of microbes in a deer’s stomach are not mentioned in these materials. Please read this as a general explanation of ruminant animals.
4. What Rumination Is: How Deer Chew Their Food a Second Time
Have you ever seen a deer sitting and moving its mouth? This is called rumination. It is the process of returning food swallowed earlier from the stomach back to the mouth to chew it again.
The finer the food becomes, the more surface area there is for microbes to work on, which aids digestion. Materials from the Oze National Park by the Ministry of the Environment state that deer repeat a rhythm of eating for 2 to 3 hours and ruminating for 2 to 4 hours, day and night [5]. These times are rough estimates provided by the materials.
How much do they eat in a day? One report cited in the materials suggests that female deer may eat more than 5 kg (about 11 lbs) per day. On the other hand, materials from Gunma Prefecture state about 3 kg (about 6.6 lbs) [6]. The reason for the difference in numbers is not clear from the materials. It is safer to view the amount as a range from about 3 kg to over 5 kg, rather than a fixed number.
5. How the Large Amounts That Deer Eat Change the Look of Forests and Lawns
When deer eat a lot, it changes the scenery. Materials from Nara City explain that the lawn in Nara Park remains beautiful without mowing because deer are eating the grass [9]. The landscape is maintained without mowing because the deer consume the grass.
In forests where deer populations have grown too large, this same eating habit leads to different results. According to the Kyushu Forest Management Bureau of the Forestry Agency, low-lying grass and branches are eaten completely, resulting in vegetation disappearing below about 1.5 m (about 5 ft) from the ground. The line marking this boundary is called the "deer line" [10].
It is also introduced that plants deer do not eat, such as pampas grass and bracken ferns, tend to remain. However, this example is based on separate research. Whether it is a park with neat grass or a forest with no undergrowth, the amount eaten determines how plants grow. The variety of food and the quantity eaten can be seen in the landscape.
6. What to Look For When You See a Deer: Its Mouth and Its Feeding Traces
If you see a deer at a zoo or park, observe it from a distance. It is best not to feed, approach, or touch wild deer.
There are three points to look for. First, watch the mouth movements. If it is sitting and moving its mouth, it might be ruminating. Second, look for feeding traces. Are grass or tree leaves missing only below about 1.5 m (about 5 ft) from the ground? Third, check the season. See if what they are eating changes in spring, autumn, or winter.
If you visit the same place in different seasons, you might see that the contents of their "eat anything" diet change little by little.