Mutualism is a biological interaction that is beneficial to both parties. Most mutualisms are facultative, meaning the partners can successfully live apart. However, some mutualisms are so intimate that the interacting species can no longer live without each other; they have a mutually obligate interdependence. Many mutualisms are fascinating in their intricacies and reciprocal usefulness as is apparent in the examples described below.
Many species of angiosperm plants require the services of an animal vector to achieve pollination. The animal benefits in this mutualism from access to a source of food of nectar or pollen, while the plant benefits through having its gametes fertilized. Some pollination systems involve a remarkably tight co-evolution of the plant and animal species, with morphological and behavioral adaptations that make the mutualistic interaction more efficient and more interdependent. Some species of orchids, for example the genus Ophrys, have a floral structure and coloration that closely mimics that of the female of their pollinating species of wasp so that the plant becomes pollinated when the male wasp is tricked and attempts pseudo-copulations with its flowers. There are numerous other examples of similarly extraordinary co-adaptations of plant-animal pollination systems.
Another plant-insect mutualism involves a tropical ant (Pseudomyrmex ferruginea) and a shrub known as the bull's horn acacia (Acacia cornigera). This acacia has evolved hollow thorns that are used by the ants as protected nesting sites. In addition, the ants feed on protein-rich exudates at the tips of the leaflets of the acacia. In return, the ants protect the acacia from competition with other plants by weeding the vicinity of their host of encroaching foliage. The ants also protect the acacia from many defoliators by killing herbivorous insects and attacking larger herbivores such as grazing mammals. Many other species of ants have developed mutualistic interactions with plant species. This fact is evidenced by the independent evolution of extrafloral nectaries by various families of plants. These organs exude nectar from leaves, stems, or branches, attracting ants that help to protect the plants from competition and herbivory.
A marine mutualism involves unicellular algae known as zooxanthellae and corals, a type of coelenterate animal. In this mutualism the coral provides the algae with shelter and inorganic nutrients, while the pigmented algae provide photosynthate. Sometimes this mutualism is upset by environmental stresses associated with unusually warm or cool water temperatures, a change in salinity, or excessive exposure to sunlight or shading. This leads to expulsion of the zooxanthellae by the coral, a phenomenon known as "bleaching," which may lead to death of the coral unless it can re-establish another algal mutualism.
Another interesting mutualism involves two animals, an African bird known as the honey guide (Indicator indicator) and a mammal known as the honey badger (Melliovora capensis). In this relationship the honey guide finds bee hives and then actively leads the honey badger to its discovery. The badger tears the hive apart with its strong claws and then feeds on the honey, while the bird feeds on the bee larvae and pupae.
See also Symbiosis.