ECOLOGICAL BIOCHEMISTRY - Study Guide - V. M. Isaenko 2005

Chapter 5. BIOCHEMICAL BASIS FOR THE FORMATION OF TASTE AND AROMA IN PLANTS CONSUMED BY MAMMALS

5.1. Formation of Taste and Aroma by Plants Consumed by Domestic and Wild Animals

Domestic animals. Studies on the palatability-based Selection of plant foods by domestic animals have been conducted using model systems. In such models, substances determining the primary tastes are added to drinking Water: sucrose or glucose (sweet taste), sodium chloride (salty taste), acetic or citric acid (sour taste), quinine (bitter taste), and tannic acid (astringent taste). These compounds are recognized by animals even at low concentrations. As the concentration of taste-imparting compounds in the drinking water increased, animals first rejected the bitter-tasting water, followed by the sour and astringent ones. At the same time, sweet water was consumed eagerly, even at high concentrations of sucrose or glucose.

In another model system, when animals were offered a choice of plants with different tastes, they generally preferred sweet plants. In this case, it was necessary to select plants that the animals consumed in their daily routine, i.e., those they were 'familiar' with.

In such a model system, when feed such as straw for cattle was moistened with water of various tastes, the animals also showed a clear preference for the sweet taste.

The range of model systems is not limited to those discussed above. Research conducted with domestic animals (cattle, sheep, and goats) convincingly demonstrates that these animals are capable of distinguishing the primary tastes of plant foods, but react to them differently. In most cases, cattle were the most sensitive to the palatability of plant foods, while sheep were the least sensitive; goats occupied an intermediate position (goats were more sensitive to bitter taste than sheep, whereas cattle were the least sensitive to this taste).

It should be noted that when testing animal taste responses, it is difficult to objectively assess their preferences, as animals cannot explicitly indicate them. Furthermore, model studies do not fully replicate natural conditions, where an animal has a wide selection of plants in the pasture, the taste and aroma of which are determined by numerous compounds.

Undoubtedly, the Nutritional Value of plants is very important, but animals appear to have a limited response to general parameters such as fiber content, total nitrogen, mineral salts, and others. As noted repeatedly, all herbivorous animals prefer sweet-tasting substances, though the specific sugar Structure influences their response. For instance, calves prefer sucrose over other sugars, whereas sheep prefer glucose. Cattle and sheep are capable of responding to the organic acid content in forages. A correlation has been identified between the selection of plants by ruminants and the content of citric and shikimic acids in them. Therefore, plant acidity is one of the key factors determining plant consumption by cattle and sheep.

It has been established that Tannins act as deterrents for cattle; at concentrations ranging from 4.8 to 12% of dry matter, voluntary intake of such plants by animals drops by 70%. At the same time, under certain conditions, tannins can elicit a positive response in ruminants. As is known, consuming certain forage legumes can lead to Metabolic Disorders resulting in bloat, which is caused by gas accumulation in the rumen of ruminants. During bloat, foam is formed due to the presence of specific Proteins in the leaves. However, when consuming forage legumes containing tannins, foam formation is prevented because tannins form complexes with the foam-producing proteins.

The pleasant sweetness of freshly mown hay for cattle depends on the presence of Coumarins in the plants, which are released when the grass is cut. For sheep, however, coumarins taste bitter; when their concentration in clover reaches 0.5 — 1.0 %, sheep refuse to eat it. Nevertheless, coumarins are volatile compounds and evaporate within an hour or two. The precursor of coumarin is o-hydroxycinnamic acid glucoside (in Melilotus), which, upon entering the bloodstream, exhibits anticoagulant properties and poses a certain hazard to cattle and sheep.

Cyanogenic Glycosides and HCN also act as feeding deterrents. For instance, sheep will browse on fern leaves that lack cyanogenic glycosides, yet they completely avoid ferns that accumulate prunasin and contain the Enzymes necessary for HCN production. The exact same behavior is observed in rabbits.

Essential Oils (volatile Terpenes) do not significantly affect the plant selection of domestic animals for foraging. However, some of them can pass into cow's milk, thereby reducing its quality. For example, milk can absorb the taste and odor of wormwood, among others. Monoterpenic alcohols such as linalool or a-terpineol inhibit Digestion in sheep.

As already noted (Chapter 4), isoflavonoids possess estrogenic activity and can negatively affect reproductive success in mammals that feed on clover rich in these compounds. At the same time, sheep do not avoid plants rich in isoflavonoids, which is likely due to the fact that these compounds do not act as deterrents for them.

In most cases, Alkaloids have a bitter taste and function as powerful deterrents. Both cattle and sheep consistently avoid consuming plants enriched with these substances.

It is worth noting that, whenever possible, domestic animals avoid consuming plants that contain high levels of tannins, cyanogens, and alkaloids. However, it is impossible to avoid all poisonous and dangerous plants growing in wild pastures. Therefore, grazing livestock in certain pastures requires a cautious and thoughtful approach.

Wild herbivores. The selection of plants for consumption by wild animals is also determined by factors that may differ from those of domestic livestock. In arid regions, for example, wild herbivores base their choice of plant food primarily on water availability, leading them to adapt to consuming almost any available vegetation.

Wild animals are generally better adapted to plant toxins than domestic ones and are capable of consuming many potentially toxic plants that still offer nutritional value. Such adaptation is characteristic, for instance, of hyraxes (order Hyracoidea), which feed on the leaves of Phytolacca decandra (rich in saponins), Solanaceae (rich in alkaloids), and others.

Studies on the foraging habits of hyraxes and mountain goats have shown that, under extreme conditions, animals apparently can adapt to most toxins produced by plants.

When vegetation is abundant, herbivores are guided by specific factors when choosing plants for consumption. For example, eastern gorillas in the Congo are strict vegetarians, feeding on the leaves, shoots, roots, fruits, and seeds of 29 plant species out of hundreds of angiosperms available. Up to 30% of these plants taste bitter—at least to humans—and contain deterrents, yet they lack tannins. As previously mentioned, tannins form indigestible complexes with proteins. Gorillas are presumably able to recognize plants with elevated tannin content, which gives them an astringent taste. The consumption of bitter-tasting plants by gorillas, much like by humans, is likely driven by habit, eventually leading them to acquire a taste for substances that are inherently repellent.