A nose full of biting ants can really spoil your appetite. Especially if your nose is 3 meters long. African bush elephants (Loxodonta africana) avoid this discomfort by refusing to munch on acacia trees that house swarming ant colonies. Their aversion, a new study suggests, helps maintain the savanna's delicate balance between forest and prairie. Trees and grasses constantly vie for control of the savanna, but wildfires, drought, variable soil chemistry, and giant herbivores prevent either plant from taking over. Not enough fire to keep the trees in check, and the canopy will close in; too many elephants eating the trees, and the savanna would become grassland. Or so scientists thought. They seem to have underestimated the acacia's ability to defend itself.
Unlike many acacia trees that are stripped bare by elephants, whistling thorn trees (Acacia drepanolobium) seemed immune. The trees bristle with the 5-centimeter-long thorns typical of many acacias, but some of the spikes also swell into hollow bulbs the size of ping pong balls. Crematogaster ants colonize the empty thorns and feed on nectar secreted from the plant's leaves. That makes a whistling thorn tree the ants' territory—which they defend against intruders. Todd Palmer, an ecologist at the University of Florida in Gainesville, wondered whether the tiny bodyguards could really protect trees from the world's largest land animal.