Whereas most gymnosperms are pollinated by wind, most angiosperms are pollinated by animals. The many different *mutualistic pollination relationships between plants and animals are vital to both parties.

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Many flowers entice animals to visit them by providing food rewards. Pollen grains themselves sometimes serve as food for animals. In addition, some flowers produce a sugary fluid called nectar, and some of these flowers have specialized structures to store and distribute it. In the process of visiting flowers to obtain nectar or pollen, animals often carry pollen from one flower to another or from one plant to another. Thus, in their quest for food, the animals contribute to the genetic diversity of the plant population. Insects, especially bees, are among the most important pollinators. Other major pollinators include some species of birds and bats.

For more than 150 million years, angiosperms and their animal pollinators have coevolved in the terrestrial environment. The animals have affected the evolution of the plants, and the plants have affected the evolution of the animals. Flower structure has become incredibly diverse under these selection pressures. Some of the products of coevolution are highly specific. For example, the flowers of some yucca species are pollinated by only one species of yucca moth, and that moth may exclusively pollinate just one species of yucca. Such specific relationships provide plants with a reliable mechanism for transferring pollen only to members of their own species.

Most plant–pollinator interactions are much less specific. In most cases, many different animal species pollinate the same plant species, and the same animal species pollinates many different plant species. However, even these less specific interactions have developed some specialization. Bird-pollinated flowers are often red and odorless. Many insect-pollinated flowers have characteristic odors. Bees see ultraviolet light well, so many bee-pollinated flowers have nectar guides that are visible in this region of the light spectrum (Figure 28.15).

The fruits of some plants that are still around today originally evolved to attract large frugivorous animals—many of which went extinct in the Pleistocene. For example, the large round fruits of the osage orange tree attracted wooly mammoths, which ate the fruits and dispersed the seeds. After the extinction of the mammoths, osage orange trees survived in large part because early humans used the wood of this species for making bows for hunting, dispersing the fruits to new areas in the process. Some other species that depended on now-extinct mammals to disperse their seeds, however, may be slowly declining toward extinction.