Ant-acacia mutualism:
In this relationship found most commonly in Central America savannas, the ant hollows out the large thorns of the plant for nests, feed on sweet secretions from the four nectaries at the base of each petiole and on the protein rich Beltian bodies found on the tips of the leaves, which together provide an almost complete diet for the ant. The ants in return protect these trees from invertebrate as well as vertebrate herbivores.With any movement of the branch, the ants emerge releasing a nasty odor as well as physically attacking the surprised herbivore. They are quite effective.
African ants and acacia trees get along great: The ants live in the acacia's special swollen thorns and pay the tree "rent" by attacking leaf-eating insects. But the ants steer clear of bees and other insects that pollinate the acacia's flowers, allowing the tree to reproduce, which in turn keeps alive the symbiotic relationship. Now scientists know why the ants turn up their feelers at pollinators: The tree exudes a chemical that tells ants to keep away. The findings, reported in Nature, show how a plant has evolved a way to thwart a potential conflict with a symbiotic insect. Studying acacia trees in Tanzania, ecologists Pat Willmer of theUniversity of St. Andrews in Fife, the United Kingdom, and Graham Stone of the University of Oxford observed that Crematogaster ants seem to avoid crawling over young, fresh flowers but not older ones that had already been pollinated. They were puzzled until they realized that on rainy days, "the effect seemed to disappear," Willmer recalls, and the ants would patrol new flowers as well. Thinking the young flowers might be making a water-soluble repellent, Willmer rubbed a young flower on an old one. The ants avoided that older flower. The researchers are still trying to identify the warning compound, although they speculate that pollen from the acacia blossom might be it. The bottom line, says Willmer, is that "the plants can manipulate the insects to do what they want."
The temporary repellent is particularly ingenious because it ends up maximizing the number of seeds the acacia can produce. After pollination, when the repellent wears off, the renewed presence of the ants protects the developing seeds from being eaten, says Ted Schultz, an entomologist at the Smithsonian Institution's National Museum of Natural History inWashington, D.C. This work is among the first to demonstrate conflict resolution in plant-animal interactions, he adds. "But there are probably all sorts of conflicts and controls [in such symbiotic relationships]. This is probably just the tip of the iceberg."