Herbivores only eat things that need photosynthesis to live; therefore, they do not consume any animals, fish, insects, etc.
A herbivore only eats plants such as vegetables, fruits, bulbs, grasses, roots, berries, and wood.
Some herbivores’ diets are adventurous, and they eat various types of plants, while others may seem to have quite a dull meal routine as they always have the same food on their menu. For instance, certain herbivores only eat fruit and are called frugivores (flying foxes, Jamaican fruit bats, and fish from the Characidae family). There are others, called folivores, that only eat leaves (e.g., caterpillars, koalas, and sloths), and then there are the ones that choose to eat wood solely (e.g., termites).
Herbivore characteristics
They often have broad molars that help them crush leaves and grasses.
Some herbivores have four chambers in their stomachs; they are called ruminants. Each chamber has its functionality. The first is a place for the plants to soak and soften up. Once the food is ready for the next step, the animal vomits it and chews it again before it travels down to the second chamber. In the second and third chambers, the food is digested further and ends up in the fourth chamber, which resembles the human stomach.
Have you ever noticed how cows and sheep eat all day? It is because herbivores tend to have high energy needs, resulting in them having to eat a lot.
Many herbivores are ungulates, which means they have hooves. However, it’s not always the case, as they come from various species.
They have alkaline saliva, so they don’t start digesting their food as soon as it enters their mouths. Instead, it goes through stages of preparation first. However, meat-eating carnivorous have acidic saliva, so they immediately start the digesting process. So, would it surprise you if you were told humans have alkaline saliva?
Examples of herbivores
- sheep;
- elephant;
- panda;
- goat;
- parrotfish;
- wildebeest;
- cow;
- koala;
- horse;
- manatee;
- rhino;
- deer;
- kakapo;
- rabbit;
- caterpillar;
- iguana;
- green sea turtle.
Some dinosaurs used to be herbivores, such as the diplodocus, triceratops, moschops, and dracorex. The extinct woolly mammoth was also a herbivore.
Herbivore Adaptations
Like all animals, herbivores have adapted slowly to survive and fit into their environment better. The major herbivore adaptations can be broken down into four categories:
- Herbivore adaptations and plant defense
- Mechanical adaptations
- Biochemical adaptations
- Behavioral adaptations
Herbivore Adaptations and Plant Defence
Animals are not the only things that have evolved; plants have formed a variety of defense adaptations to protect themselves from herbivorous animals. Plant defense falls into two categories: tolerance and resistance. Tolerance is when a plant adapts to withstand damage without compromising its health. There are several ways that plants can form this tolerance. For instance, it can be achieved by redirecting herbivorous animals to other non-essential plants or compensatory growth, where plants increase their biomass in response to damage.
On the other hand, resistance is when a plant adapts to reduce the damage it receives from herbivores. It can be achieved through a variety of physical and chemical defenses. Physical defenses are barriers that plants develop to deter herbivores from eating them. Common examples of these defenses include thorns on roses and spikes on cacti. Another less visible example of physical defense is the tiny hairs called trichomes that can cover leaves or stems. Moreover, certain plants develop waxes that alter their texture and make them difficult for herbivores to eat.
Chemical defenses are when plants produce different chemical molecules to deter herbivores from eating them. One plant can have hundreds of chemical defenses to guard itself against herbivores. Chemical defenses fall into two main categories; they are either carbon-based and nitrogen-based defenses. These defenses can include producing a poison like cyanide and developing an increased gene defense expression.
In response to these plant defenses, herbivores have evolved to bypass them and continue eating the plants. The relationship between plants and herbivores is that they have co-evolved and adapted together. For example, when a herbivore eats a particular plant, it triggers that plant to develop a defensive response. In this way, the evolution of plants and herbivores is reciprocal.
Mechanical Herbivore Adaptations
Mechanical adaptations, also known as physical adaptations, are structures herbivores have developed to allow them to eat plant materials. For instance, herbivores have evolved specific teeth depending on the types of plants they consume. For example, frugivores, whose diet consists mainly of fruit, and herbivores that eat soft foliage, have low-crowned teeth. These teeth are designed for grinding foliage and seeds. On the flip side, grazing animals whose diet consists of a lot of hard, silica-rich grasses have high-crowned teeth. These teeth enable them to grind tough plant tissues and are durable so they won’t wear down as quickly as low-crowned teeth.
Over time, insect herbivores have developed various adaptations to facilitate their feeding. These adaptations are specific to each insect’s diet. For example, caterpillars that eat soft leaves have adapted to have incisors that are excellent at tearing and chewing. Contrastingly, caterpillars whose diet consists mainly of mature leaves and grasses cut through them with toothless snipping mandibles (the upper part of insects’ jaws)
Biochemical Herbivore Adaptations
Biochemical adaptations are where herbivores produce enzymes that combat and reduce the effectiveness of defensive toxic secondary metabolic products that plants have. One of the enzyme groups that herbivores make is mixed-function oxidases (MFOs). Mixed-function oxidases are designed to detoxify damaging plant compounds by catalyzing oxidative reactions.
One of the most essential enzymes that herbivorous insects produce is a protease. The protease enzyme is a unique protein that lives in the gut and helps insects digest plant tissue, their primary food source. Unfortunately, many plants defend themselves against insects by producing protease inhibitors, which inactivate the protease enzyme. When protease is inactivated, it can have a very harmful effect on insects and can lead to issues like reduced feeding, prolonged larval development time, and weight gain. However, several insects possess mechanisms that allow them to avoid the effects of protein inhibitors. These mechanisms include developing protease enzymes that aren’t affected by the plant protease inhibitors. They can also include developing the ability to break down protease inhibitors and obtaining mutations that permit the digestion of plant tissue without its destructive effects.
More essential enzymes that herbivores produce are salivary enzymes. These enzymes are designed to reduce the defense level of host plants. For example, the enzyme glucose oxidase, one of the components of saliva for the caterpillar Helicoverpa zea, combats the production of induced defenses in tobacco plants.
Behavioral Herbivore Adaptations
Behavioral herbivore adaptations are another common way for herbivores to avoid plant defenses. Behavioral adaptations often involve animals eating plants selectively in space and time. Take the winter moth as an example; it eats oak leave early in the season. It is when winter moths can get the maximum amount of protein and nutrients and the minimum amount of tannins produced by the tree (one of the oak tree’s plants’ defenses).
Furthermore, in terms of avoiding specific spaces to avoid plant fences, we can look at caterpillars as an example. Many species of caterpillars eat maple leaves by selectively feeding on some regions of the leaves and staying away from more challenging areas and those with a high lignin concentration. Likewise, the cotton leaf perforator, a moth, avoids consuming its host plants’ epidermis and pigment glands. Again, it is because those areas contain harmful defensive terpenoid aldehydes.
Beetles are another example of spatially selective animals with the plants they eat. Beetles feed on plants that produce small amounts of toxins and send them to vital areas when attacked. To counteract this, some beetles by attacking target plants in groups. Because they do it in large groups, each beetle is prevented from ingesting too much toxin.
On the other hand, some animals eat by ingesting large amounts of toxins in their food but follow this by eating clay and other minerals to neutralize the poisons.
Here are some other herbivore adaptations that have developed over time:
- Microbial Symbionts
The purpose of microbial symbionts is to detoxify secondary plant metabolites and thus allow herbivores to eat plants that would otherwise be inedible. Certain plants are inedible for herbivores because they cannot digest complex cellulose and rely instead on mutualistic, internal symbiotic bacteria, fungi, or protozoa to break down cellulose.
Microbial symbionts also help herbivores to obtain plant materials by weakening the host plant’s defenses. For instance, several species of bark beetles introduce blue stain fungi of the genera Ceratocystis and Ophiostoma into trees before they feed. This blue stain fungus causes lesions that, in turn, reduce the trees’ defensive mechanisms and allow the bark beetles to feed.
- Host Manipulation
Another way that herbivores combat plant defenses and get the most out of their host plants is through host manipulation. Host manipulation is when insects modify the microhabitat in which the herbivore feeds to counteract existing plant defenses and use them to their advantage. Let’s retake caterpillars for an example. Caterpillars from the Pyralidae and Ctenuchidae families roll mature shrub leaves, Psychotria horizontalis, around an expanding bud that they eat. The caterpillars can drastically reduce the amount of light reaching the bud by moving the leaves. This shading stops the leaf from becoming tough and prevents leaf tannin concentrations in the expanding bud while maintaining the amount of nutritional gain of nitrogen.
Another way that herbivores manipulate their microhabitat is by creating galls, which are plant structures made of plant tissue controlled by the herbivore. Galls have many functions. For example, galls operate as housing for the gall maker. They also act as food sources, as the inside wall of a gall is made up of edible, nutritious tissue. In addition, aphid galls, which are found in narrow-leaf cottonwood, act as what is known as ‘physiological sinks.’ These ‘physiological sinks’ concentrate the resources in the gall from the surrounding parts of the plant. Another function of galls is that they can protect the herbivore against their predators.
Moreover, there are several feeding behaviors that herbivores can use to disarm the defenses of their host plants. One of the defensive mechanisms that these feeding behaviors can disarm is the use of latex and resin canals, which contain sticky toxins and digestibility reducers. These canal systems contain fluids under pressure, and when they are ruptured by herbivores eating the plant, secondary metabolic products flow to the release point. Herbivores have adapted to avoid this defense mechanism by damaging the leaf veins. By harming the leaf veins, herbivores can minimize the outflow of latex or resin beyond the cut, thus allowing them to eat safely above the damaged section of the leaf.
