Monkeying with dogwood evolution.

Monkeying with dogwood evolution

The soft, sweet flesh of an Asian dogwood's red fruit poses a puzzle in evolution. Among a group of closely related dogwoods, all the Asian species have a compound fruit that looks like a fat strawberry, while all the species native to the Western Hemisphere have bunches of smaller, olive-type fruits. What could have caused the evolution of the different fruits? Richard H. Eyde of the Smithsonian Institution in Washington, D.C., now suggests the distinguishing force was the appetite of Old World monkeys.

The bean-sized simple fruits, common to the majority of dogwoods, appear to be the ancestral type. Eyde is studying a subgroup of dogwoods that includes the common North American flowering dogwood, which bears the simple fruits, and the Asian dogwoods, which bear the sweet compound fruits. While trees in this subgroup, called big-bracted dogwoods, appear to have a large showy flower, it is actually four large white, pink or yellow petal-like structures, called bracts, beneath a tight cluster of small, true flowers.

Fossils of simple- and compound-dogwood fruits, collected at European sites, can be distinguished by their shape: The stone of the simple fruit is ovoid whereas the stone of a compound fruit is asymmetrical and tapered. The simple-fruit fossils are found at sites dated earlier than those of the complex fruits.

The change from a simple to a compound fruit must be linked to the dispersal of the seeds, Eyde says. "Why are dogwoods with such fruits found only in the Old World?" he asks. Because both the simple and compound fruits of big-bracted dogwoods have red seeds, Eyde believes they are dispersed primarily by animals with good color vision. The simple fruits, which are bitter or tart, are known to be dispersed by robins and other migratory birds. These birds also peck at the compound fruits, but do not seem to prefer them. Eyde has considered which animals might be preferentially attracted to a compound fruit.

Among the mammals, monkeys are the most likely candidates. Eyde says that macaques, an Old World species, can distinguish colors. These monkeys eat dogwood fruits and spit or void the seeds. The larger fruit is both more conspicuous and more manageable to a monkey. "They get more goodies per grab," Eyde says. When he mapped their natural habitats, he found that macaques once ranged almost everywhere where compound-fruited dogwoods grow. In addition, one fossil site has been found to contain both compound dogwood fruit and the bones of a macaque.

"Big-bracted dogwoods formerly extended round the Northern Hemisphere, and all had ordinary fruits until monkeys came in contact with them about 5 million years ago," Eyde proposes in ARNOLDIA, the magazine of the Arnold Arboretum (Vol. 45, No. 4). "Selection for a better monkey meal meant better scattering of seeds; so compound fruits replaced the simple ones, but only in Eurasia. America retains the older kind because the New World monkeys, blind to red and tied to warmer regions, never took up foraging on dogwoods."