Nectaries at the base of the leaf’s petiole provide a rich source of energy for busy Pseudomyrmex ants.
No, this week’s episode is not a revival of the classic Costner-meets-Houston romance, but it is a story about a different type of symbiotic relationship as curious as previous weeks’ episodes where we met leafcutter ants and their fungus gardens and nozzle-headed termites protecting their colony. This week we explore the fascinating relationship between tropical plants that partner with ants, and the feisty ants which serve as their bodyguards.
The bullhorn acacia is armed with a variety of defenses against hungry animals and competitive plants.
While walking along a dusty road in Belize with a medicine woman, a class of students in a tropical biology class encountered a brilliant green acacia tree near a pasture. Our local guide noted that this remarkable tree was completely unmolested by any type of leaf-munching caterpillar, sucking insect, or large mammal such as the horses or cows that grazed nearby. A closer inspection revealed fearsome looking thorns arising from nodes of the branches. Surely these thorns, locally known as cockspurs or bullhorns, helped explain why large grazing mammals avoided the otherwise delectable looking leaves of the acacia. A mouthful of thorns would be a painful experience indeed. However, as I fondled the foliage of the acacia, I was instantly attacked by a furious band of ants that bit and stung my hand with extreme prejudice. Their sting was memorable and my swollen hand throbbed and itched for hours after the encounter.
Nosey humans will receive memorable bites and stings from ants guarding acacias.
Small holes in the Acacia thorn allow ants to enter and exit.
The secret weapon of the bullhorn acacia, ant bodyguards, is part of a symbiotic deal struck eons ago by acacias and ants in the genus Pseudomyrmex. The deal works like this: A newly mated Pseudomyrmex queen lands on the bullhorn acacia and locates a large thorn. Either by chewing a new hole or by using an existing one, she enters the hollow thorn and lays eggs. Eggs hatch and develop into sterile workers. Workers, the queen, and subsequent broods subsist on carbohydrate rich nectar produced by specialized glands called extrafloral nectaries found near the bases of many leaves.
Nutrient rich orange Beltian bodies at the tips of leaflets are harvested by acacia ants and fed to the queen and colony.
But ants, like people, cannot live by sugar alone. At their tips, some leaves also produce specialized structures called Beltian bodies. These detachable tidbits are rich in nutrients such as proteins and lipids. Ants harvest and consume Beltian bodies to round out their diet. By providing room and board, acacia plays the generous host for Pseudomyrmex. In return, fearless worker ants provide maniacal protection of the acacia from caterpillars, sap-sucking insects, probably large herbivores, and, certainly, nosey entomologists.
One additional benefit of the bodyguards is their role as vegetation managers. Not only was the acacia tree devoid of leaf-eating insects, choking vines that ascended and engulfed other rainforest plants nearby were notably missing from the acacia. Why were so many other plants cloaked in vegetation while the acacia remained free of clinging vines? In a clever series of studies, rainforest guru Dan Janzen demonstrated that in addition to protecting acacias from herbivores, Pseudomyrmex also aggressively remove tender tips of encroaching plants that might compete with acacia for sunlight. Room and board in exchange for protection from herbivores and competing plants - all deals should be so good!
Acacia ants vigilantly patrol the leaves and stems of their host plant.
References
We thank the hearty crew of ‘BSCI 339M, Mayan Culture and the Interface Between Tropical Rainforests and Coral Reefs’ for providing the inspiration for this Bug of the Week. The wonderful book "The Insect Societies" by Edward O. Wilson, and the fascinating articles “Interaction of the bull's-horn acacia (Acacia cornigera L.) with an ant inhabitant (Pseudomyrmex ferruginea F. Smith) in Eastern Mexico” and “Coevolution of mutualism between ants and acacias in Central America” by Dan Janzen, were used as references for this episode.