While many people inside Panama were at home and spending time during the pandemic, five children shot a clay ball at trees with a slingshot. They wanted to give themselves a challenge, so they focused on slender cecropia trees in particular. One of the children aimed for its broad, flat leaves, but instead hit the stem, leaving a clean, gaping hole, from one side to the other.
William Wcislo, an entomologist at the Smithsonian Tropical Research Institute and father of two of the children, had cared for this tree and feared it had suffered irreparable damage. But when he came back later, he was in for a surprise. The holes were plugged.
Closer inspection revealed who was responsible: Azteca Alfaric, small, sand-colored ants known to build nests in cecropia trees, which are widespread in Central and South America. Wcislo watched them swarm to the site of the injury, collecting small, fibrous plant pieces from the stem, combining them with a sap-like substance and quickly starting to close the hole.
Scientists already knew that Aztecs ants live in a with cecropia: The trees have hollow stems in which the insects build complex multi-storey houses. In return, the ants repel caterpillars, birds, and other herbivores that like to eat fruits, leaves, and flowers. But what Wsiclo and the kids observed, which was eventually replicated and published in a recent in the Journal of Hymenoptera Researchwas the first time the ants were seen actually repairing damage to their home.
“I think it’s really exciting to know there’s a potential new dimension to that beneficial relationship,” said Jane Lucas, an assistant scientist at the Cary Institute of Ecosystem Studies in Millbrook, New York, who was not part of the study. was involved. Knowing this, she says, doesn’t mean the behavior is necessarily mutually beneficial — after all, the ants could be doing this purely to protect themselves and their colony — but it does suggest that the interaction between the ants and the trees might be more complex. than previously thought.
To determine whether this behavior extended beyond the first, unfortunate target tree, the team decided to conduct an experiment on 22 small cecropias growing on the edge of the Bosque Urbano de Cárdenas, a tropical forest in Panama. Their method was simple: they drilled holes in the trees and waited.
Wsiclo and his team found that in about half of the trees Aztecs ants ran to the site of the wound and began to sew up the holes. Within a few hours, the holes had narrowed noticeably. After 24 hours, the diameter of the holes was almost half of what it had originally been. And that was just average. In four of the trees the holes were completely closed
“The speed at which the ants are closing these holes is pretty amazing,” Lucas says.
When holes were drilled near or in the nest in the stem, the ants sprang into action, evacuating their brood to nearby chambers first. Then they usually worked in groups of about 10, both inside and out, using fibrous plant pieces and that sticky substance.
It seems clear that plugging the holes helps the ants. Their immature, vulnerable brood would be at risk of predation or disease if exposed. But it’s less clear whether it benefits the tree at all, Lucas says, since cecropia naturally heals their own wounds anyway.
“[The ants] it probably doesn’t matter that their host tree is injured. They make sure that their nest is disturbed,” says Lucas. Indeed, as Wsiclo’s team wrote in the paper: “It is unlikely that the restorative behavior of ants directly benefits the plant.
But Lucas thinks there could be more to it, at least in theory. The ants may add antimicrobial agents as they close the holes. This, along with the speed at which the repairs are taking place, could also mean positive things for the plants, she says. “The [speed] could certainly help a plant by not getting pathogens in it because it happens so quickly.”
The observation is fascinating, but Eugene Schupp, a professor of plant ecology at Utah State University who was not involved in the study, isn’t entirely surprised. Azteca amalfi are great architects: their nests are made the same way, with wood material glued together with secretions from their mouthparts. In fact, when these ants first start building their nests, they carve holes in the tree, only to patch them up. “So they definitely need to have the ability to work with building materials to repair damage,” he says.
But there are some important questions, Lucas says. Why are about half of the trees not patched up? Wsiclo and his team could only speculate about this, but it could be due to a colony’s health, size, or even “personality.” Some colonies, Schupp says, are large, dynamic, and respond to a simple tap on the trunk. Others are small, slow and completely unresponsive. “This one [differences] could easily explain why there are big differences in recovery from colony to colony, with some closing the wound quickly and others not at all,” he says.
Another question is how drastic the behavior can be, says Lucas. The research focused on small, relatively young cecropia trees in a specific region of Panama. “I think it’s really important that you can show that it’s a species-level trait and not just, oddly enough, this one forest in Panama,” she says. Of course, that kind of local adaptation would also be interesting.
Wsiclo’s team plans to dig further into this intimate natural interaction. Specifically, they want to test whether the ants really secrete antimicrobial chemicals that further protect the tree — yet another amazing example of how complex and multi-layered ecosystems can be, Lucas says.
“It would be this kind of crazy interaction, where it’s like there are microbes in the ant helping the plant,” she says, “like a three-way connection of organisms, which would be really exciting to see.”