Roe deer are herbivorous, which means that they only eat plants 

Now, to be absolutely accurate, that’s not strictly true; deer will occasionally eat some animal matter as they unintentionally consume tiny bugs, etc…which are attached to the leaves and stems of the plants they eat. 

Mother does will also consume their fawn’s waste during their early days, to prevent their baby’s scent drawing the attention of a predator, such as a red fox. But the overall, the amount of animal matter taken in by a roe deer is negligible.

Roe deer are browsers

This means they eat foods, such as leaves, shoots, stems, etc., above ground level.

Now, clearly roe will take food items at ground level, since they can sometimes be seen feeding on the grass in fields. But generally speaking about 85% of their feeding takes place between 0 and 75 cm above the ground. The maximum height they can reach is around 120 cm. (Red deer and fallow deer are predominantly grazing animals, meaning that mainly (but not solely) feed at ground level on grass.)

The roe’s relatively large mouth opening of 5 to 7 cm (equating to about 30% of the length of their head) and very manipulable lips, allow them to be particularly selective for eating leaves, by stripping them off plant stems, etc…

Why do roe deer browse, gather than graze?

Picking off fresh shoots, leaves and stems, which are commonly found above ground level, yields a higher energy intake than they would get from grazing purely on grass, etc.

Roe deer have a relatively high energy expenditure because of their modest size and, in the case of mothering does, the exceptional level of care they invest in raising their young.

The volume of a deer’s digestive system is directly proportional to the animal’s weight, therefore, with the European roe having a maximum body weight of only 25 kg (35 kg for Siberian and Chinese roe), their digestive tract will have a relatively small capacity. So, they can only satisfy their daily energy requirements by consuming plants which have a high level of soluble carbohydrates, and they do that by being selective.

Types of plant eaten by roe deer

More than one thousand species have been recorded being eaten by roe deer, including ferns, deciduous trees, shrubs, agricultural plants, grasses, moss, fungi, lichens and conifers. And not only that, all parts of a plant are known to be consumed, too; seeds, stems, bark, roots, flowers and leaves.

What deer actually choose to eat at any one time is largely determined by what is available. But given the choice, the fresh green leaves of deciduous plants will  make up at least half of their total food intake. In agricultural regions, the roe will be drawn to the energy rich crops and therefore, a high proportion of their diet will consist of grains, fruits, nuts, seeds and roots.

Selection of food

One reason why roe deer have been so successful over the last few decades is because they are able to consume such a large number of different plant species, as well as fungi, etc.... Having said that though, they are normally are very selective over their choice of food.

Roe deer are very attracted to plants that are especially high in easily digestible, soluble carbohydrates, as long as the plant doesn’t contain irritants to the digestive system or carcinogens.

So, although they can consume a very large variety of plants, etc… they actually choose to eat quite a small range at any one time. Therefore, they are often called concentrate browsers because of their tendency to select foods with a high nutritional density, rather than just eating all the vegetation that is available to them.

Rather unexpectantly though, roe show a distinct preference to plants high in tannins.

Tannins are found in oak, willow, maple, pine and birch (and other plants), and are known to cause  serious problems in the liver and stomach to many animals including sheep, cattle and horses.  However, a roe’s saliva contains a high concentration of a specific protein fraction that deactivates the free tannins, making them harmless to the deer.

Another food that be eaten by roe without harmful effects on the animal is yew,  even though it contains a toxic taxine alkaloid, called taxol – as reported by Mysterud and Ostbye in the journal Wildlife Biology, in 1995. (Taxine is used in the manufacture of chemotherapy drugs.)

Plants that are actively avoided (assuming there are more nutritionally rich alternatives available) are those high in cellulose and lignin (which is essentially insoluble fibre), as well as those high in silica. Such plants include grasses and mosses.

How do deer learn what foods to eat and which ones to avoid?

Within the first couple of weeks of a fawn’s life, it will start to eat vegetation between feeding sessions with its mother. At first, the youngster will taste all types of soft plants that are in close range of its hidey hole. And, like many animals when testing food items for the first time, instantaneous reactions to the taste and any possible irritation in the mouth, will immediately tell the young deer which plants to seek out and which ones to avoid in the future.

Plants that are particularly sweet are very much preferred, buttercups are particularly favoured, as are maple and hornbeam.

Certain plants contain toxins that cause digestive problems a short time after they have been ingested, and fawns are able to develop aversions from that feedback, too.

The young deer then carry their preferences on into their adult life.

Only baby bites first

Just thought I’d mention one important point here, when it comes to dealing with toxins in plants:

Although a plant may contain a potential deadly toxin, it’s important to remember that it’s the dose that makes a toxin dangerous.

So, that’s that the advantage of the deer only consuming a small amount of any one new food item at a time; they are unlikely to get poisoned and suffer any serious ill effects from ingesting such a small amount of a given food item.

How does the deer’s body deal with toxins in plants

The organ responsible for ridding the body of potentially poisonous substances is the liver. And because of the relatively high number of toxins ingested by roe, the deer’s liver is comparatively large, in fact, about 2.6% of its total body weight. 

Their high metabolic rate also requires a large liver, too, since their daily turnover of proteins and other nutrients is high.

When it comes the dealing with the high levels of tannic acid found in the roes’ diet, the deer produce a large volume of salvia, which deactivates their harmful effects.