Relationship Provides Nutrients, Housing, Protection

small ants climb on a green tree with large red horns

Biological Strategy

Bullhorn wattle

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Protect From Animals

Animals–organisms that range from microscopic to larger than a bus–embody a wide variety of harms to living systems, including other animals. They threaten through predation, herbivory, defense, and parasitism, and they compete for resources such as water, nutrients, and space. Any given living organism commonly faces threats from a variety of animals, requiring strategies that effectively defend from each. Trout and other bony fish, for example, escape predators by having scales made of very thin, flake-like pieces of bone covered with slippery mucus. They also have behavioral strategies such as camouflage, fast swimming, and twisting and turning to achieve release from a predator’s grip.

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Cooperate/Compete Between Different Species

From parasitism to mutualisms, there are endless interactions between organisms in nature. Interactions between species that lead to a negative outcome for one species are known as competition. Cooperation occurs when organisms work together for the benefit of each organism or species. While competition often occurs over resources or mates, the more biologists have begun to look for examples of cooperation in nature, the more they have found. For example, around 90% of plants have a beneficial partnership with fungi. Fungi provide the plant with nutrients such as nitrogen and phosphorus, in exchange for sugars from the plant.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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

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.

Image: eakspeasy /

Image: Alexander Wild / CC BY - Creative Commons Attribution alone

Top photos show features of this ant/acacia mutualism.

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.

Video

In this video from The Field Museum, Dr. Stefanie Kautz examines the unusual relationships acacia ants form with their host plant—the acacia tree.

Last Updated May 15, 2020

References

“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 B | Ward, PS, Branstetter MG

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Reference

“Some types of acacia trees have evolved special structures to support acacia ant colonies…The 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.

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