If a congress of naturalists were to gather to choose the seven wonders of the animal world, they would be compelled to include the bizarre and mighty civilizations of the attine leafcutters.
Bizarre and mighty indeed.
On jungle trails in the Osa Peninsula at Bosque del Cabo and Corcovado National Park, Magellan videoed these wonders of the animal world.
What a sight, a caravan of ants scurrying with such purpose and uniformity, one behind the other evenly spread apart on a path the width of a hiking boot, a leaf atop each of their tiny bodies, like rectangles of green on a conveyor belt.
Many animals create trails to their watering holes or food sources. But how many build ten-lane roads? And clear the roads of vegetation and debris?
Scurrying along on their miniature highways, the ants with cuttings atop their bodies look like they’re about to take off in leaf-green mini-parachutes. Take a closer look at the top of each leaf cutting and you’ll see hitchhikers, the smallest sub caste of these ants. They’re not just thumbing a free ride back to the colony. The hitchhikers are busy removing harmful microorganisms from the leaves. They’re also living flyswatters. Striking with their front legs and mandibles at parasitic flies trying to lay eggs on the transport ants, deadly eggs that hatch and consume their host.
And here’s something else—every one of these leafcutters is a female.
The gigantic queen, the sole reproductive individual in leafcutter society, is basically an egg-laying machine her entire life; she usually makes it to her tenth birthday. Surrounded by workers who groom and feed her, she lays about twenty eggs per minute—resulting in about 150 million daughters in her lifetime. A few grow up to be alate queens, able to mate and found a new colony. (Males are reared. But only seasonally, and only in a tiny minority to inseminate virgin queens during nuptial flights away from the nest after which, by genetic design, the males die.) Growth is slow in a new colony during the first two years, accelerates during the next three and tapers off when the queen starts to produce winged males and queens.
And what a metropolis the leafcutter colony is. A bear has a den. An owl, a nest. But leafcutters create an air-conditioned “city,” two-thousand rooms connected by a labyrinth of tunnels. To make a mold of a leafcutters’ nest, the size of a small human dwelling, you’d need 5.4 tonnes of cement! The weight of an ambulance.
From what scientists know, leafcutters have one of the most elaborate caste systems in the animal world. Seven castes are born with varying physical attributes qualifying them to work in a few of about twenty-five different jobs. The soldier caste, for example, has much larger, sharper mandibles for defense; the cutter caste has different body characteristics than the transport caste. Young and small workers toil inside the colony, older and bigger leafcutters labour outside.
Workers organize the gardening operation in the form of an assembly line. A caste system, based mostly on size, determines each worker’s role.
1) The largest workers are foragers that cut pieces of leaves and bring them back to the nest
2) Slightly smaller workers cut the leaf pieces brought by the foragers into smaller fragments
3) Still smaller ants crush the fragments into moist pellets and add fecal droplets
4) They then insert them into the fungal body
5) Next, smaller ants pluck loose strands of fungi from places of dense growth and plant them on a new surface, expanding the fungal colony
6) The smallest and most abundant workers search the beds of fungal strands for alien species of unwanted fungus, which they remove, and older individuals of this smallest caste might also “hitchhike” on leaf fragments being carried by the foragers to defend the leaf carrier from attacks by parasitic flies
Another example of operational units involving complex interactions of colony members is managing leaf-cutting.
Leafcutters, with their remarkably structured brains, have one of the most complex communication systems known in the animal world. Using their scent-guided behaviour and astounding odour sensitivity, they mark their trails with chemical secretions from their poison-gland sacs. For leafcutters, scent provides recruitment signals and orientation cues. To recognize one another in the colony, say differentiating a transport ant from a gardener ant, leafcutters secrete a specific blend of hydrocarbon compounds onto their tough exoskeletons. Their odour sensitivity is so adept they can sniff out the caste not just in their own colony, but between colonies. That’s beyond the olfactory genius of the world’s renowned perfumer, Lucia Turin.
More intel on one of the seven wonders of the animal world. How do leafcutters choose what to harvest? The density of the leaf—they prefer drought-stressed plants because they have higher concentrations of amino acids and proteins. How many leaves are at the site. The energetic cost of cutting. How easy it is to communicate foraging information to the colony to prepare the inside workers—yes they do this! They produce stridulatory vibrations to communicate leaf quality to their nearby mates, which is “telegraphed” down the line to the colony. Transportation—a short travel time appears to be an asset in the foraging system, but sometimes they even establish caches along the trail.
When a harvest site is chosen, an assembly line gets into production, resulting in the transport caravan we saw so frequently.
There’s no keyhole for us to telescope into the nest after vegetation is “trucked” on down. But here’s another surprise: although they eat a small bit of plant sap, leafcutter ants don’t actually eat leaf cuttings—instead they have one of the most successful symbioses of all time. They cultivate fresh vegetation into a fluffy gray mass that hosts a fungus, which they in turn feed upon. (Attine means fungus-growing.)
When the transport caste delivers leaf cuttings to the metropolis, other castes of workers cut the leaves into small pieces, treat it with their fecal liquid and insert it into the substratum garden(s), the precious fungus about three metres down shown—sometimes as deep as eight metres. The gardener caste plucks out alien fungi, secretes antibiotics to depress competing fungi, fertilizes it with its own growth hormones and fecal droplets, and inoculates the grey mass to prevent disease. Mature colonies have multiple entrances ensuring a continuous supply of oxygen and efficient gas exchange.
Leafcutters also practice good hygiene. They dispose of their waste outside the nest and partition the task to avoid the spread of disease—waste is left in caches by one set of workers and then picked up by another caste, primarily by jubilado leafcutters, who must take on the jobs with greater risks.
Found only in the New World, attine leafcutters in Costa Rica are called Zampopo. They belong to the two percent of known insects (all ant species) that are Eusocial, identified by taking cooperative care of their young, having a population overlap of at least two generations and coexisting with reproductive and non-reproductive members.
As Magellan and I watched these bizarre and mighty ants, we pondered the sheer weight of leaves they were transporting to the colony over the course of a day. We found the answer in the fantastic book referenced below.
A full-grown colony consumes about the same daily quantity of plant material as what other animal? Can you guess?
What can we say but, “Holy Cow!!!”
UPDATE: September 22, 2021. From the book Entangled Life by Merlin Sheldrake, 2020. “After humans, the animals that form some of the oldest and most complex societies on Earth are leaf-cutter ants. Colonies can reach sizes of more than eight million individuals with underground nests that grow larger than thirty metres across.” He also says it is uncertain whether leaf-cutter ants domesticated the fungus they depend on or the fungus domesticated the ants. “The researchers suspect that the fungus is able to puppeteer the ants’ movements by secreting chemicals that act on their muscles and central nervous system even if the fungus does not have physical presence in their brains”—essentially manipulating the ants’ behaviour with ergot alkaloids. He writes that “leaf-cutter ants have been observed to actively forage for a certain type of psilocybin mushroom, carrying them back to their nests in one piece.”
Hölldobler, Bert and Wilson, Edward O. The Leafcutter Ants. New York: W.W. Norton & Company, 2011. When two authors with three Pulitzers, the National Medal of Science, and the Körber Prize for the European Sciences illuminate, in laywoman terms, the fascinating topic of leafcutter ants, you have a mighty fine little book. The quote we kicked off with comes from their book.
The still images in this post were copied from Alexander Wild’s website. He has an incredible collection of macrophotography of ants from around the world. His sharing policy for personal social media use is extremely generous. “People acting in a personal capacity are welcome to share my photographs on blogs, web pages, and social media accounts without prior permission, provided that all images are accompanied by a link back to www.alexanderwild.com.”