The Elements of Evolution (43-15-2) Fruit


Most woody tropical angiosperms produce fleshy fruits and rely on animals for seed dispersal. Fleshy fruits have evolved independently in more than half of extant angiosperm families. ~ Israeli evolutionary ecologist Omer Nevo et al

Fruit is a wily confection of plants: an ovary enclosing seeds, often full of sweet and succulent pulp, exquisitely designed to appeal to specific consumers. The seeds are hardened to survive the rigors of the animal digestive tract. The plan behind fruit is for an animal to eat it and deposit the seeds some distance away from the fruit-bearing plant; the deposit conveniently including some fertilizer.

The colors of fruits are optimized to contrast with the natural background according with the visual system of the intended seed disperser. The size, shape, form, smell, and taste of the fruit conforms with what primary seed dispensers like. Even the location of fruit and its presentation on the branch cater to the intended audience.

The simians in Uganda are trichromats (tricolor vision). The birds there have even better vision. By contrast, most lemurs in Madagascar are dichromats: red-green color-blind, only seeing yellow-blue light. Compensation for lemurs comes in a keen sense of smell.

Ugandan fruit eaten by the birds and simians there contrast with foliage in the red-green spectrum. In Madagascar, fruit for lemurs pops in yellow-blue contrasts. Further, the fruit smells right.

In plants that specialize on seed dispersal by lemurs – an olfactorily-oriented primate – fruits increase scent production and change their chemical composition significantly more than sympatric species whose seeds are largely dispersed by birds. Lemurs use these shifts in fruit scent to identify ripe fruits. ~ Omer Nevo et al

Balanites wilsoniana decided it liked elephants. This African tree produces a pungent odor detectable for kilometers. Elephants gobble up the large fallen fruit, not minding at all if it is a bit fermented. Only elephants can swallow the fruits and defecate the sizable seeds whole. This plant’s seeds won’t even germinate unless it passes through an elephant’s gut.

The traveler’s palm is not a true palm: instead, belonging in the genus of bird-of-paradise plants native to southern Africa. Bird-of-paradise plants produce red or yellow seeds easily detected by their avian consumers. In Madagascar, where traveler’s palms reside, the seeds are a brilliant blue: a color appreciated by aye-ayes, a lemur with ultraviolet sight.

Knowing that their fruit is intended to be eaten, many plants add a laxative to their seeds, easing their offsprings’ passage.

Fruit sweetly illustrates that plants somehow know what should be unknowable, and that an intelligent force is behind evolution.