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
Related Topics
Go Further
Animals
- Octopuses have a lot of secrets. Can you guess 8 of them?
- Animals
- Feature
Octopuses have a lot of secrets. Can you guess 8 of them? - This biologist and her rescue dog help protect bears in the AndesThis biologist and her rescue dog help protect bears in the Andes
- An octopus invited this writer into her tank—and her secret worldAn octopus invited this writer into her tank—and her secret world
- Peace-loving bonobos are more aggressive than we thoughtPeace-loving bonobos are more aggressive than we thought
Environment
- This ancient society tried to stop El Niño—with child sacrificeThis ancient society tried to stop El Niño—with child sacrifice
- U.S. plans to clean its drinking water. What does that mean?U.S. plans to clean its drinking water. What does that mean?
- Food systems: supporting the triangle of food security, Video Story
- Paid Content
Food systems: supporting the triangle of food security - Will we ever solve the mystery of the Mima mounds?Will we ever solve the mystery of the Mima mounds?
- Are synthetic diamonds really better for the planet?Are synthetic diamonds really better for the planet?
- This year's cherry blossom peak bloom was a warning signThis year's cherry blossom peak bloom was a warning sign
History & Culture
- Strange clues in a Maya temple reveal a fiery political dramaStrange clues in a Maya temple reveal a fiery political drama
- How technology is revealing secrets in these ancient scrollsHow technology is revealing secrets in these ancient scrolls
- Pilgrimages aren’t just spiritual anymore. They’re a workout.Pilgrimages aren’t just spiritual anymore. They’re a workout.
- This ancient society tried to stop El Niño—with child sacrificeThis ancient society tried to stop El Niño—with child sacrifice
- This ancient cure was just revived in a lab. Does it work?This ancient cure was just revived in a lab. Does it work?
- See how ancient Indigenous artists left their markSee how ancient Indigenous artists left their mark
Science
- Jupiter’s volcanic moon Io has been erupting for billions of yearsJupiter’s volcanic moon Io has been erupting for billions of years
- This 80-foot-long sea monster was the killer whale of its timeThis 80-foot-long sea monster was the killer whale of its time
- Every 80 years, this star appears in the sky—and it’s almost timeEvery 80 years, this star appears in the sky—and it’s almost time
- How do you create your own ‘Blue Zone’? Here are 6 tipsHow do you create your own ‘Blue Zone’? Here are 6 tips
- Why outdoor adventure is important for women as they ageWhy outdoor adventure is important for women as they age
Travel
- This royal city lies in the shadow of Kuala LumpurThis royal city lies in the shadow of Kuala Lumpur
- This author tells the story of crypto-trading Mongolian nomadsThis author tells the story of crypto-trading Mongolian nomads
- Slow-roasted meats and fluffy dumplings in the Czech capitalSlow-roasted meats and fluffy dumplings in the Czech capital