<P> Herbivores are dependent on plants for food, and have coevolved mechanisms to obtain this food despite the evolution of a diverse arsenal of plant defenses against herbivory . Herbivore adaptations to plant defense have been likened to "offensive traits" and consist of those traits that allow for increased feeding and use of a host . Plants, on the other hand, protect their resources for use in growth and reproduction, by limiting the ability of herbivores to eat them . Relationships between herbivores and their host plants often results in reciprocal evolutionary change . When a herbivore eats a plant it selects for plants that can mount a defensive response, whether the response is incorporated biochemically or physically, or induced as a counterattack . In cases where this relationship demonstrates "specificity" (the evolution of each trait is due to the other), and "reciprocity" (both traits must evolve), the species are thought to have coevolved . The escape and radiation mechanisms for coevolution, presents the idea that adaptations in herbivores and their host plants, has been the driving force behind speciation . The coevolution that occurs between plants and herbivores that ultimately results in the speciation of both can be further explained by the Red Queen hypothesis . This hypothesis states that competitive success and failure evolve back and forth through organizational learning . The act of an organism facing competition with another organism ultimately leads to an increase in the organism's performance due to selection . This increase in competitive success then forces the competing organism to increase its performance through selection as well, thus creating an "arms race" between the two species . Herbivores evolve due to plant defenses because plants must increase their competitive performance first due to herbivore competitive success . </P> <P> 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 . </P> <P> 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 (sphinx moths), it has been observed that the caterpillars of 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). </P>

State two adaptations of herbivores which enable them to digest cellulose