Bullhorn acacias provide nutrients and housing for ants in return for protection from herbivores thanks to a mutualistic relationship.

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In the lowlands of Mexico and Central America, the bullhorn acacia tree and a species of ant (Pseudomyrmex ferruginea) help each other to survive. This is known as a mutualistic relationship, where two or more species work together to provide what the other needs. The ant aggressively defends the acacia from plant-eating insects, neighboring plants, and disease-causing microorganisms. The acacia provides the ant with plenty of food and a place to live and raise its young.

The bullhorn acacia has odd-looking thorns that look like bull horns. The tiny ants cut entrance holes into the thorns, and that’s where they care for their eggs and larvae. In three years, the population of ants can grow from the lone queen ant laying her first eggs to a tree teeming with 16,000 worker ants. The thorns are waterproof and hold in moisture, which protects the eggs and larvae during dry periods. Their sharp tips keep birds from going after the eggs inside.

In addition to providing housing, the acacia  provides food in the form of nectar from special glands. The nectar is a thick syrup full of sugars. Even more valuable to the ants are tiny sacs full of proteins, fatty nutrients, and vitamins called Beltian bodies that grow on the end of acacia leaves. The ants   feed these Beltian bodies to their larvae. These two food sources provide most of the food for the ants.

So what does the ant do for the acacia? The fast and agile worker ants race around the tree defending it from plant-eaters such as insects (including other ants) and larger animals like rodents. The ant that finds an invader sends out an odor alarm that causes other ants to attack it. The ants kill any foreign plants, such as vines, that touch the trees. They also aggressively kill any vegetation growing around the base of the tree. Because of this, the acacia does not have to compete for soil nutrients, water, and sunlight with other trees. Recently, scientists have also discovered that the ants also spread bacteria from their feet onto the acacia leaves. These bacteria can kill fungi and other disease-carrying bacteria that have infected the leaves.

Mutualistic relationships like this evolve over millions of generations. The relationship helps both the tree and ant survive difficult times like droughts. In human societies, we know that acting alone is less effective than finding ways to work together by sharing resources and ideas. The acacia and the ant have it figured out by meeting each other’s needs in just the right ways.

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Here we examine the evolutionary history of a classic ant/plant mutualism: the association of Central American ants in the Pseudomyrmex ferrugineus group with swollen-thorn acacias (Vachellia species). In this system, the ants receive nesting space in the form of swollen stipular thorns, and food from extra-floral nectaries and specialized leaf-tip food bodies (Beltian bodies). The ants in return protect their host plant from herbivores and competing plants, by patrolling aggressively, removing or repelling intruders and clipping competing vegetation…other species of Vachellia do not display the constellation of mutualism-associated traits—inhabitable swollen thorns, food bodies and enlarged extrafloral nectaries—shown by the Mesoamerican ant-acacias.” (Ward and Branstetter 2017:1-2)

Journal article
Ward, PS, Branstetter MG. 2017. The acacia ants revisited: Convergent evolution and biogeographic context in an iconic ant/plant mutualism. Royal Society of London B 284:20162569.Royal Society of London BWard, PS, Branstetter MG

“Some types of acacia trees have evolved special structures to support acacia ant coloniesThe orange bodies on the tips of new leaflets are used only to feed the ants. The trees also excrete nectar for ant food and have swollen hollow thorns that can be used for nest sites. In return, the ants, which are equipped with powerful stingers, attack caterpillars and deer and any other animals that try to eat the acacia leaves.

“Some ant-plant partnerships are so strong that the ants live their entire lives within one kind of tree. Bulls horn acacia trees have large hollow thorns that are occupied by skinny, rust-coloured acacia ants. The trees produce not only nectar but also special oil- and protein-rich nodules on their young leaf tips. These provide all the nutrition the ant colony will ever need. To allow the tree better growing opportunities, the ants even trim away weeds from its base.” (Forsyth 1992:52)
Note: Similar relationships occur with other species of acacia, including Acacia drepanolobium.

Forsyth A. Exploring the World of Insects: The Equinox Guide to Insect BehaviourExploring the World of Insects: The Equinox Guide to Insect BehaviourOctober 30, 1992
Adrian Forsyth

Journal article
Janzen DH. 1966 Coevolution of mutualism between ants and acacias in Central America. Evolution 20(3):249-275. EvolutionJanzen DH

Journal article
Chomicki G, Ward PS, Renner SS. 2015. Macroevolutionary assembly of ant/plant symbioses: Pseudomyrmex ants and their ant-housing plants in the Neotropics. Royal Society of London B 282:20152200.Royal Society of London BChomicki G, Ward PS, Renner SS

Journal article
Heil M, González-Tauber M, Clement LW, Kautz S, Verhaagh M, Bueno JCS. 2009. Divergent investment strategies of Acacia myrmecophytes and the coexistence of mutualists and exploiters. PNAS 106(43):18091-18096.PNASHeil M, González-Tauber M, Clement LW, Kautz S, Verhaagh M, Bueno JCS

Journal article
Heil M, Orona-Tamayo D, Eilmus S, Kautz S, González-Tauber M. 2010 Chemical communication and coevolution in an ant – plant mutualism. Chemoecology 20, 63 – 74. ChemoecologyHeil M, Orona-Tamayo D, Eilmus S, Kautz S, González-Tauber M

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