Herbivores use mechanical, biochemical, behavioral, microbial, and host manipulation strategies to consume plants despite their defenses. Plants have physical, chemical, and other defenses to deter herbivores. Herbivores have adapted teeth, enzymes, and stomachs to cope with plant defenses, and some manipulate plants to maximize nutrition.
Herbivores use five main categories of strategies to evade plant defenses and consume the plant: mechanical adaptations (such as teeth), biochemical adaptations, behavioral adaptations, microbial symbionts, and host manipulation. All are generally used at the same time, although to a greater or lesser extent depending on the species. All are generally characterized as offensive adaptations, in that the herbivore launches the attack on the plant, except in the rare case of carnivorous plants.
Much like the interaction between predators and prey, the interaction between herbivores and the plants they eat is an evolutionary arms race. Plants use various defenses to discourage herbivores from eating them: physical defenses such as thorns, efforts to make themselves less palatable, harmful chemicals (called metabolic by-products) designed to stop the herbivore, and other strategies. At the same time, herbivores develop various strategies to bypass plant defenses. Modern plants have batteries of defenses to use against both invertebrate and vertebrate invaders.
The most universal adaptation used by herbivores to cope with plant defense is mechanical. Herbivores must have a feeding mechanism, teeth or jaws, to tear off parts of a plant and consume it. This is reflected in thousands of different variations on the simple concept of a tooth, from the tens of thousands of “teeth” in a snail’s radula, to 32 permanent human teeth. Depending on whether an animal is an exclusive herbivore or an omnivore, its teeth will be more or less sharp or curved, of different shapes to adapt to its personal diet.
Another class of plant defense adaptations is the chemical one. Many herbivores produce enzymes that neutralize poisonous chemicals released by the plant when it is under attack. These negate the immediate defenses and allow the herbivore to consume the plant. Then, another class of adaptations takes over in the stomach: Different herbivores have different stomachs with different bacteria customized to digest the molecules in their favorite foods. One of the most impressive evolutionary innovations among Cenozoic land animals was the evolution of the multi-chambered stomach, which evolved to digest nutrient-poor grasses.
Some of the most interesting herbivorous adaptations to plant defense are the behavioral adaptations of host manipulation. By eating a plant at a certain time or in a certain season, a herbivore can maximize the nutrition it gets while minimizing the presence of unwanted chemicals such as tannins. Some of the more clever adaptations are host manipulation, where the feeder somehow tricks the plant into giving up its nutrients. This is seen both in the case of root-knot insects and in human agriculture.
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