Charles Darwin worked hard to figure out how cooperation within a species—self-sacrifice among worker bees, for example—could have evolved. But he was stumped when it came to understanding cooperation between species. In his book, On the Origin of Species, he wrote, “Natural selection cannot possibly produce any modification in a species for the good of another species.” If that could be proved to have happened, “it would annihilate my theory, for such could not have been produced through natural selection.” And he scoffed at supposed examples perpetuated by some natural historians, such as a rattler using its rattle to warn prey. “I would almost as soon believe that the cat curls the end of its tail when preparing to spring, in order to warn the doomed mouse.” Instead, he argued that the rattle was meant to scare off birds and other potential predators.
Nonetheless, countless examples of cooperation between species exist—albeit many perhaps outside Darwin’s knowledge. Many long-standing partnerships are strengthened by specialized structures or traits in one species that benefit the other. Many of these relationships are dynamic, shifting back and forth over evolutionary time between exploitative and mutualistic.
Take the ant plants and their ants (see right). In tropical forests, certain types of trees make a home for ants that inhabit them, providing hollow stems or leaf pouches where the insects can roost and raise young. In return, the ants keep hungry herbivores at bay and sometimes kill off surrounding vegetation, creating a clearing around the trees.
Nineteenth century naturalists fiercely debated whether these ants forced their way into trees as parasites, wounding trees to make their nests, or whether they had a more benign relationship with the plants they lived in. In 1873, botanist Richard Spruce likened the ants to fleas on a dog—a nuisance. But others contended that some plants, acacias in particular, provided hollow thorns and food rewards to keep ants around for protection from herbivores. Many studies have supported this hypothesis over the past 4 decades. It seems the ants bite and poison surrounding vegetation, reducing the competition for space, water, and sunlight.
In the November issue of American Naturalist, David Edwards of the University of Leeds, U.K., and his colleagues describe how sometimes these ants get carried away. “I think most [researchers] believed that ants’ relationship with their host plants had been pretty well defined,” says John Tooker, a chemical ecologist at Pennsylvania State University, University Park. But in 1996, scientists observed a new, peculiar ant behavior. While exploring the jungles of southeastern Peru, a team of ethnobotanists came across a number of “devil’s gardens,” what many locals call the ant-made clearings around trees. Although they had seen such clearings before, the researchers were surprised by what the natives showed them next: Trees of other species on the outside of the clearing were scarred and swollen (below, right) with networks of cavities filled with worker ants, queens, brood, and mealy bugs. “These galls made up a large percentage of the swollen trunk volume,” says Edwards. Sometimes the internal excavations were so extensive that the tree had collapsed. The locals blame the scars on forest spirits.
The researchers think the ants are attacking these other trees because there aren’t enough ant plants to house ever-expanding colonies. “It suggests a level of ecosystem engineering [by the ants] not previously recognized,” says Tooker. And at this point, the relationship seems anything but mutual. “I expect this to be an antagonistic relationship because of the range of tree species that are galled by the ants,” Tooker notes. These are trees not typically associated with ants, and so there’s been little opportunity for a partnership to evolve. But if the ants patrol the area and ward off herbivores, then perhaps there is some payback by the ant to the tree, he adds.
The ants involved belong to the genus Myrmelachista, which typically nest in stems. Some species in the genus do not form associations to particular species, says John Longino, an entomologist at Evergreen State College in Olympia, Washington. He suggests that devil’s garden ants coevolved with new queens, gradually evolving a preference for certain plant species as the best nesting spots. Their targets eventually provided housing rather than chance having irregular holes chewed into their stems. Then, “perhaps it doesn’t take much to turn a mild-mannered and inconspicuous stem-nesting ant into a ferocious devil’s gardener. Maybe just the right kind of plant can encourage and manipulate those latent talents,” he says.
These enticements can lead to trouble, however, as Edwards’s collaborator, Megan Frederickson of the University of Toronto in Canada, has discovered. Ever in need of more room, the ant Allomerus octoarticulatus takes a devious step to promote its host tree’s growth. It destroys any flower buds that the host produces. When Frederickson measured the growth rates of sterilized and reproductive plants, she found that the ant’s drastic maneuver did encourage more vegetative growth—and more living space for the ants. She reported those findings in the May issue of American Naturalist.
Darwin did not have the benefit of these experiments that show reciprocated benefits and the dynamic balance between giving and taking. But if he had, “he would have found ant-plant symbioses a real hoot,” says Mike Kaspari, an ecologist at the University of Oklahoma, Norman.