Ant Bodyguards Get Exclusive Contract from Trees
If animals and plants can’t defend themselves, they often form partnerships with bodyguards. Wasps use zombified caterpillars. Corals recruit goby fish. And acacia trees hire ants. The ants defend the trees against hungry mouths by biting and stinging any invading plant-eaters. Some are so ferocious that they can deter elephants. In return, the trees pay their bodyguards by providing shelter in the form of swollen thorns, and food in the form of nectar or nutritious parcels called “food bodies”.
This alliance between ants and acacias is a staple of textbooks, but it’s even more intimate than anyone suspected. Some acacias don’t just supply their ants with any old food. They offer the biological equivalent of a cheque—a reward that only the ants can cash.
Every partnership is vulnerable to thieves. The acacia’s bright, nutritious food bodies could easily be pilfered by any insect quick enough to avoid the patrolling ants. But insects that steal them are in for a poor and possibly dangerous meal.
Domancar Orona-Tamayo from CINVESTAV-Irapuato in Mexico and Natalie Wielsch from the Max Planck Institute for Chemical Ecology in Germany found that the food bodies of two acacia species are loaded with molecules called protease inhibitors. As their name suggests, these block enzymes called proteases, which animals use to digest the protein in their food.
These acacia enzymes were extremely good at neutralising the proteases of two species of seed-eating beetles, slashing their protein-busting abilities by more than 98 percent.
Pseudomyrmex ferruginea—one of the ants that guards the acacia—has no such problems. Its guts are dominated by a special protease called chymotrypsin-1, which the acacia’s protease inhibitors do not inhibit. When these bodyguards eat the food bodies, they get a nutritious reward. When beetles try to do the same, they get indigestion.
The protease inhibitors aren’t found throughout the acacia, just in the food bodies. They are security measures that protect the tree’s rewards by harming would-be thieves. Only the ants can bypass these defences, and only the right ants at that.
Orona-Tamayo and Wielsch found that Pseudomyrmex gracilis—a species that exploits the acacia’s rewards without ever lifting a mandible to defend it—isn’t quite as well-equipped as the P.ferrugineus. It has some chymotrypsin-1, but also plenty of other proteases that are inactivated by the acacia’s neutralising enzymes. It gets something out of the food bodies, but not as much as the tree’s true partner.
There are other examples in the natural world of alliances where partners lock each other into exclusive contracts. Some do it physically. Many flowers hide their nectar at the bottom of long tubes that only the right pollinators can reach them, whether they’re long-billed hummingbirds or long-tongued flies.
In these cases, it’s clear that the flowers and their pollinators evolved alongside one another. As nectar tubes got longer, so did bills and tongues, until both fit together like locks and keys. Is the same true for the acacia and the ant? It’s possible, but the team suspects that both partners came prepared for exclusivity.
The acacia uses the same protease inhibitors as many other related plants, and many ants and spiders* have chymotrypsin-1 in their guts. The tree eventually concentrated its inhibitors into its food bodies, while its ant partners emphasised chymotrypsin-1 and downplayed other proteases. They were already a good match from the start. They just became closer over time.
*This might be why the world’s only vegetarian spider, Bagheera kiplingithe world’s only vegetarian spider, Bagheera kiplingi, can get away with eating acacia food bodies.
Reference: Orona-Tamayo, Wielsch, Blanco-Labra, Svatos, Farias-Rodriguez & Heil. 2013. Exclusive rewards in mutualisms: ant proteases and plant protease inhibitors create a lock–key system to protect Acacia food bodies from exploitation. Molecular Ecology http://dx.doi.org/10.1111/mec.12320
More on ants and acacias:
- Slackers and parasites can sometimes make the best partners
- Threatened by elephants? Try recruiting ants
Related Topics
Go Further
Animals
- Cougar travels 1,000 miles in one of longest recorded treksCougar travels 1,000 miles in one of longest recorded treks
- Rare gray whale spotted in the Atlantic—and it's only the beginningRare gray whale spotted in the Atlantic—and it's only the beginning
- Why 'funga' is just as important as flora and faunaWhy 'funga' is just as important as flora and fauna
- Termite fossils prove mating hasn't changed in 38 million yearsTermite fossils prove mating hasn't changed in 38 million years
Environment
- Why the 2024 hurricane season could be especially activeWhy the 2024 hurricane season could be especially active
- Mushroom leather? The future of fashion is closer than you think.Mushroom leather? The future of fashion is closer than you think.
- This deadly fungus is hitchhiking its way across the worldThis deadly fungus is hitchhiking its way across the world
- Why 'funga' is just as important as flora and faunaWhy 'funga' is just as important as flora and fauna
- This exploding mine holds a treasure that may change the worldThis exploding mine holds a treasure that may change the world
History & Culture
- See the story of Jonah and the whale like never beforeSee the story of Jonah and the whale like never before
- This ancient mosaic offers extraordinary insights into the pastThis ancient mosaic offers extraordinary insights into the past
- These are the real dunes that inspired Dune—and you can visit themThese are the real dunes that inspired Dune—and you can visit them
- Meet the only woman privy to the plot to kill Julius CaesarMeet the only woman privy to the plot to kill Julius Caesar
Science
- Women’s bodies are understudied—but that’s starting to changeWomen’s bodies are understudied—but that’s starting to change
- Hundreds of tiny arachnids are likely on your face right nowHundreds of tiny arachnids are likely on your face right now
- What's worse than a hangover? Hangxiety. Here's why it happens.What's worse than a hangover? Hangxiety. Here's why it happens.
Travel
- A guide to Gdansk, Poland's regenerated maritime cityA guide to Gdansk, Poland's regenerated maritime city
- A taste of West Bengal, from curries to Kolkata street foodA taste of West Bengal, from curries to Kolkata street food