Plants have evolved a variety of ways to distribute their seeds. Some simply drop to the ground, others blow through the air on tiny, delicate "wings", others are moved through water, and still others are encased in fruits and have to survive a trip through an animal's digestive system before they sprout.

If a large quantity of seeds fall in one location instead of being spread around, there is a greater chance of attracting "seed predators", or mammals that consume the entire seed, thereby preventing it from sprouting.

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.

In the tropical rainforests, the fruit supply is relatively constant all year. The fruits of fig trees are especially popular with many species of monkey. Fruit makes up over 50% of the diet of a large proportion of tropical birds, mammals, reptiles, insects, and even some fish. Since birds have relatively poor senses of smell and taste, fruits do not need to develop a sweet taste and odor in order to attract them.

One particularly unusual tropical fruit-eater is the oilbird (Steatornis caripensis). These birds are nocturnal and inhabit the forests of northern South America. They live in colonies, spending the day in caves, much like bats. Also like bats, they use echolocation to keep from bumping into things as they fly about in the dark. They are the only birds known to use echolocation, a technique where the birds' click-like vocalizations work like sonar signals, and bounce off nearby objects, helping the birds to "see" where it is safe to fly. At night, these large birds, with wingspans of nearly three feet, come out to feed on the fruits of palms and laurels. They often fly long distances to locate fruiting plants, and they feed while hovering at the trees and use their hook-like beaks to pick off the fruits.

Above and below: Palm fruits growing in clumps near the top of the tree. Photos by Maya Walters.

In the early fall, some temperate zone trees including dogwoods and magnolias produce fruits that taste sour and actually have a high fat content. Because of their taste, they are not attractive to mammals. Their ripening is timed perfectly for migrating songbirds, which need fuel for their long flight south, and high-fat fruits are a much more efficient source of energy than those which are high in sugar. In the temperate forests, seeds and nuts are an even more important food source than fruit to many animals. Fruit is only available at certain times of the year and can not be stored over the winter.

Above: Squirrels eat large quantities of coniferous tree seeds, often eating in the same spot many times, and eventually leaving a substantial pile of empty cone husks, called a "midden". Below: Typical coniferous tree cone and the seeds it contains (Douglas-fir). Photos by Maya Walters.

Nuts from trees, such as acorns, and seeds from many conifers can usually be found scattered over the forest floor, even when the crop of nuts and seeds still on the trees are not ripe. The fact that they store well makes them an ideal winter food source for small mammals such as squirrels. The populations of squirrels actually rise and fall with the abundance of seeds and nuts each year. These animals do destroy some of the seeds by eating them. The squirrels distribute them widely and bury them in caches great distances from the parent plant, and since many caches of seeds are forgotten and left to sprout, the squirrels help the trees to spread to new areas.