Herbivores have developed a diverse range of physical structures to facilitate the consumption of plant material. To break up intact plant tissues, mammals have developed teeth structures that reflect their feeding preferences. For instance, frugivores (animals that feed primarily on fruit) and herbivores that feed on soft foliage have low-crowned teeth specialized for grinding foliage and seeds. Grazing animals that tend to eat hard, silica-rich grasses, have high-crowned teeth, which are capable of grinding tough plant tissues and do not wear down as quickly as low-crowned teeth. Birds grind plant material or crush seeds using their beaks and gizzards.
Insect herbivores have evolved a wide range of tools to facilitate feeding. Often these tools reflect an individual’s feeding strategy and its preferred food type. Within the family Sphingidae (moths), it has been observed that species which eat relatively soft leaves are equipped with incisors for tearing and chewing, while the species that feed on mature leaves and grasses cut them with toothless snipping mandibles (the uppermost pair of jaws in insects, used for feeding).
An herbivore’s diet often shapes its feeding adaptations. Grasshopper head size, and thus chewing power, was demonstrated to be greater for individuals raised on rye grass (a relatively hard grass) when compared to individuals raised on red clover (a soft diet). Larval lepidoptera that feed on plants with high levels of condensed tannins (as in trees) have more alkaline midguts when compared to lepidoptera that feed on herbs and forbs (pH of 8.67 vs. 8.29 respectively). This morphological difference can be explained by the fact that insoluble tannin-protein complexes can be broken down and absorbed as nutrients at alkaline pH levels.
Herbivores may also produce salivary enzymes that reduce the degree of defense generated by a host plant. The enzyme glucose oxidase, a component of saliva for the caterpillar Helicoverpa zea, counteracts the production of induced defenses in tobacco. Similarly, aphid saliva reduces its host’s induced response by forming a barrier between the aphid’s stylet and the plant cells.
Plant defense may explain, in part, why herbivores employ different life history strategies. Monophagous species (animals that eat plants from a single genus) must produce specialized enzymes to detoxify their food, or develop specialized structures to deal with sequestered chemicals. Polyphagous species (animals that eat plants from many different families), on the other hand, produce more detoxyfying enzymes (specifically MFO) to deal with a range of plant chemical defenses. Polyphagy often develops when an herbivore’s host plants are rare as a necessity to gain enough food. Monophagy is favored when there is interspecific competition for food, where specialization often increases an animals’ competitive ability to use a resource.
Herbivores are unable to digest complex cellulose and rely on mutualistic, internal symbiotic bacteria, fungi, or protozoa to break down cellulose so it can be used by the herbivore. Microbial symbionts also allow herbivores to eat plants that would otherwise be inedible by detoxifying plant secondary metabolites. For example, fungal symbionts of cigarette beetles (Lasioderma serricorne) use certain plant allelochemicals as their source of carbon, in addition to producing detoxification enzymes (esterases) to get rid of other toxins. Microbial symbionts also assist in the acquisition of plant material by weakening a host plant’s defenses. Some herbivores are more successful at feeding on damaged hosts. As an example, several species of bark beetle introduce blue stain fungi of the genera Ceratocystis and Ophiostoma into trees before feeding. The blue stain fungi cause lesions that reduce the trees’ defensive mechanisms and allow the bark beetles to feed.
Herbivores often manipulate their host plants to use them better as resources. Herbivorous insects favorably alter the microhabitat in which the herbivore feeds to counter existing plant defenses. For example, caterpillars from the families Pyralidae and Ctenuchidae roll mature leaves of the neotropical shrub Psychotria horizontalis around an expanding bud that they consume. By rolling the leaves, the insects reduce the amount of light reaching the bud by 95%, and this shading prevents leaf toughness and leaf tannin concentrations in the expanding bud, while maintaining the amount of nutritional gain of nitrogen. Lepidoptera larvae also tie leaves together and feed on the inside of the leaves to decrease the effectiveness of the phototoxin hypericin in St. John’s-wort. Herbivores also manipulate their microhabitat by forming galls, plant structures comprised of plant tissue but controlled by the herbivore. Galls act as both domatia (housing), and food sources for the gall maker. The interior of a gall is composed of edible nutritious tissue. Aphid galls in narrow leaf cottonwood (Populus angustifolia) act as “physiologic sinks,” concentrating resources in the gall from the surrounding plant parts. Galls may also provide the herbivore protection from predators.
Some herbivores use feeding behaviors that are capable of disarming the defenses of their host plants. One such plant defensive strategy is the use of latex and resin canals that contain sticky toxins and digestibility reducers. These canal systems store fluids under pressure, and when ruptured (i.e. from herbivory) secondary metabolic products flow to the release point. Herbivores can evade this defense, however, by damaging the leaf veins. This technique minimizes the outflow of latex or resin beyond the cut and allows herbivores to freely feed above the damaged section. Several strategies are employed by herbivores to relieve canal pressure, including vein cutting and trenching. The technique used by the herbivore corresponds to the architecture of the canal system. Dussourd and Denno examined the behavior of 33 species of insect herbivores on 10 families of plants with canals and found that herbivores on plants with branching canal systems used vein cutting, while herbivores found on plants with net-like canal systems employed trenching to evade plant defenses.
Plant chemical defenses can be used by herbivores, by storing eaten plant chemicals, and using them in defense against predators. To be effective defensive agents, the sequestered chemicals cannot be metabolized into inactive products. Using plant chemicals can be costly to herbivores because it often requires specialized handling, storage, and modification. This cost can be seen when plants that use chemical defenses are compared to those plants that do not, in situations when herbivores are excluded. Caterpillar and adult monarch butterflies store cardiac glycosides from milkweed, making these organisms distasteful. After eating a monarch caterpillar or butterfly, the bird predator will usually vomit, leading the bird to avoid eating similar looking butterflies in the future. Two different species of milkweed bug in the family Hemiptera, Lygaeus kalmii and Oncopeltus fasciatus, are colored with bright orange and black, and are said to be aposematically colored, in that they “advertise” their distastefulness by being brightly colored.
Secondary metabolic products can also be useful to herbivores due to the antibiotic properties of the toxins, which can protect herbivores against pathogens. Additionally, secondary metabolic products can act as cues to identify a plant for feeding or oviposition (egg laying) by herbivores.