All seeds are important food sources for animals and birds. During the annual floods in the Amazon rainforest, some fish even survive on a diet of seeds. Fruits contain the seeds of many trees and bushes, and their purpose is to attract animals which eat the fruit, but then disperse these seeds. Fruits are meant to be eaten, and they are usually brightly colored and highly visible to hungry mammals and birds. Right: Some seeds are very well protected from seed predators, with thick shells and spines. Below: Papaya, an edible fruit, changes color as it ripens. Photos by Maya Walters.

Plants do not want their unripe fruit to be consumed, because the seeds inside are still immature and therefore unable to grow into new plants. There are usually a series of signals which indicate, with color and odor variations, the ripeness of the fruit. For example, most berries begin as a dull green color, then gradually darken to red or purple. Many unripe fruits contain small amounts of toxic substances that become harmless once the fruit ripens. Some specialized fruit-eating animals (called "frugivores") have special adaptations that allow them to eat certain amounts of unripe fruit without any ill-effects from the toxins. While it is an obvious advantage to be able to use a food source that other animals find poisonous, large amounts of any toxin can still be harmful, and therefore most animals try to consume a variety of different fruits.

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