Woody plants of the Acacia genus thrive in Central America and Africa, boasting an impressive natural defense system. Fierce ant colonies are ready to attack any living creature that approaches. Yet, many local bird species still choose these trees to build their nests. Why is that?
A recent study published in the October issue of Animal Behaviour offers an explanation: when nesting, birds use a type of fungal thread that has the ability to repel ants and alter their behavior, preventing these tiny creatures from approaching the nests and harming the chicks.
Tolmomyias sulphurescens is one of the bird species that use fungal threads to build nests, protecting their chicks from ants – (Photo: Salvador Poot Villanueva/iNaturalist Mexico).
“What strikes me as odd is that the ants do not harm the chicks,” remarks biologist Rhayza Cortés-Romay from Universidad Mayor de San Andrés. “That’s why I took on the role of a bird to answer the question: How do these nests have this capability?”
In nature, there exists a symbiotic relationship between ants and the plant species Vachellia collinsii. Ants harvest the tree’s resin and nest in its thorns while patrolling around the tree to detect and eliminate invasive species, whether animal or plant. However, ants do not attack the chicks in the nests, which is a perplexing phenomenon that has puzzled zoologists.
In tropical and subtropical regions, many bird species utilize parts of fungi, such as rhizomorphs, to construct their nests. These structures are made up of millions of intertwined fungal threads that fungi use to seek nutrients in nature.
In tropical forests, rhizomorphs grow in the high canopies, forming a complex network to “capture” nutrients. Previous studies suggested that birds use rhizomorphs due to their strength, water resistance, and antimicrobial properties. The new research has opened up another possibility.
At Palo Verde National Park in Costa Rica, two bird species frequently nest on Vachellia collinsii trees, and most of their nesting materials consist of rhizomorphs from the Marasmius species (also known as horsehair fungi). It seems that this building material has made the birds’ nests immune to ant invasions.
Nests made from rhizomorph threads – (Photo: Sabrina Amador Vargas).
To conduct the experiment, researcher Rhayza Cortés-Romay and behavioral ecologist Sabrina Amador Vargas from the Smithsonian Tropical Research Institute in Balboa selected 30 acacia trees that were home to the symbiotic ant Pseudomyrmex spinicola.
They placed two types of threads on the trees for the birds to choose for nesting: one type was horsehair fungus and the other comprised non-fungal plant fibers of similar thickness, which birds also use for nesting. The researchers filmed the ants to record their reactions to the different threads.
Observations showed that ants that came into contact with the fungus tended to groom themselves more frequently and moved faster compared to those that only touched the non-fungal plant fibers; this behavior indicated that ants reacted strongly when interacting with the fungal threads, displaying heightened wariness. The researchers also noted that ants exhibited extreme behaviors after contact with the fungus, including biting fellow ants, spinning in place, and wandering aimlessly.
“When we first observed those behaviors, we were quite surprised,” said researcher Cortés-Romay. “These ants are typically very aggressive. It was a significant finding to see something affecting them like this.”
According to Fran Bonier, a behavioral ecologist at Queen’s University, the results provide compelling evidence that these fungi are harmful to ants. However, birds also use rhizomorphs to build nests on trees without ants, suggesting that this fungal thread may offer additional benefits.
In nature, at least 176 bird species utilize rhizomorphs for nesting. Given this frequency, it is not surprising that researchers have found evidence suggesting rhizomorphs may possess exceptional “guarding” capabilities.
“This raises many questions about the activity of the substances released, and whether it is specific to these ant and fungal species, or if similar situations are occurring in other ecosystems around the world,” commented Todd Elliott, a biologist at the University of New England in Armidale, Australia, regarding the study’s findings.
