Strange Ants: Zombie, Dracula, Herders, and Farmers
Ants are impressive animals that have intricate colonies. Some have developed very interesting lifestyles. In tropical rainforests, carpenter ants are invaded by a fungus that controls their behaviour, turning them into zombies. The queen of another type of ant feeds on the blood of her larvae, reminding scientists of the Dracula legend.
Ants may also be farmers. Some species herd caterpillars. They “milk” their charges by stimulating them to release a sweet secretion. Other ant species take care of aphid colonies, stimulating the aphids to release honeydew for food. Leafcutter ants bite off pieces of leaves and take them to underground nests. Here the leaves serve as food for a fungal colony, which the ants harvest and eat.
Ants live on all continents except Antarctica. The greatest variety of the insects is found in tropical regions. All known species live in colonies in a home known as a nest. An ant nest is often a complex underground construction with many tunnels and chambers. There may be a raised anthill above the nest, which also has tunnels.
Ant colonies contain a queen who lays eggs, males to fertilize the queen, and non-breeding females known as workers. The workers collect food, maintain and protect the nest, rear the young, and take care of the queen. The males have one job—to fertilize the queen—and don’t live for very long.
Some ants form supercolonies. When ants from different nests meet, they are normally aggressive towards each other. Scientists have noticed that in a few species, ants from different nests are not only unaggressive when they meet but actually cooperate with each other as though they belong to the same colony. These collections of ants are sometimes huge and have been dubbed "supercolonies". Evidence suggests that the ants in a supercolony are closely related genetically.
The creation of a zombie ant begins when spores of a fungus named Ophiocordyceps unilateralis enter certain species of tropical carpenter ants. (The fungus was formerly known as Cordyceps unilateralis.) The spores geminate inside the ants, producing the thread-like mycelium which makes up the body of the fungus. The mycelium spreads through the body of an infected ant, releasing chemicals that affect its behaviour. There is some debate about whether the fungus controls the ant's behaviour by affecting the insect's brain, its muscles, or the muscles and the brain, as described below.
Infected ants leave their nest in the trees and fall to the forest floor. The temperature and humidity in this area are ideal for fungal survival and reproduction. Under the influence of the fungus, the ant moves to the underside of a leaf which is about ten inches above the ground. The ant then bites down hard on a leaf vein, attaching itself to the underside of the leaf.
The ant eventually dies but maintains its grip on the leaf. The fungal mycelium produces a reproductive structure that sprouts from the ant's head and releases spores from a spore case. The spores then infect more carpenter ants. Researchers have found that many fungus-controlled ants attach themselves to leaves at the same time, forming an ant graveyard.
Zombie ants are carpenter ants whose behaviour has been affected by a fungus. Carpenter ants belong to the genus Camponotus. They build their nests in the wood of trees or buildings, but they don't eat wood. They feed mainly on dead insects and honeydew.
Zombie Ants in North America
New fungi that can produce zombie ants have recently been discovered. These discoveries should enable scientists to learn more about the intriguing relationship between the two organisms.
One species of zombie fungus was found in the United States in 2014. The fungus has been studied by scientists at Pennsylvania State University. They've discovered that it releases a complex mixture of behaviour-altering chemicals when it enters an ant belonging to its host species. When the fungus infects an ant of a different species, however, it doesn't release the chemicals and doesn't alter the ant's behaviour, even though it may kill the insect.
Somehow the zombie fungus "knows" when it's inside an ant that it can control. As David Hughes, an assistant professor of entomology at Penn State says, "the one without the brain controls the one with the brain".
The scientific name of the North American zombie fungus hasn't been finalized. It's believed to be a type of Ophiocordyceps unilateralis. This species appears to be a complex of similar but not identical insects rather than a species in the usual sense. It's sometimes referred to as Ophiocordyceps unilateralis sensu lato. The last words in the name mean "in the broad sense".
We found that a high percentage of the cells in a host were fungal cells. In essence, these manipulated animals were a fungus in ants' clothing.— David Hughes, Pennsylvania State University
Muscle and/or Brain Control
It has long been said that the fungus affects the behaviour of zombie ants by entering and controlling the brain of its host. A November 2017 report from Pennsylvania State University made an interesting announcement in relation to this idea. According to the scientists, the fungal threads form a connected 3D network over many of the muscles in an ant's body but don't enter its brain. The fungus invades the muscles in addition to surrounding them.
The researchers say that Ophiocordyceps unilateralis controls the muscle action of its host. They don't refute the idea that the fungus may also be producing chemicals that affect the ant's brain, however. They did notice that fungal cells were concentrated outside the brain, even though they didn't enter it. The relationship between the host and the parasite is interesting and seems to be complex. Scientists have found that the fungus actually controls gene expression (activity) in the ant.
A Dracula Ant
Dracula ants are believed to be related to the first ants, which are thought to have evolved from wasps. Their name was derived from one of their feeding methods, which resembles that of Count Dracula, the vampire in Bram Stoker's famous novel.
Adetomyrma venatrix is a Dracula ant found in Madagascar. It lives in rotting logs or leaf litter and is yellow to orange in colour. Workers hunt for prey and bring them back to the colony to give to the larvae. Colony members have another method of feeding, however. Both the queen and the workers bite holes in the larvae and then feed on their blood. Although this process sounds dramatic, the wounds and blood loss generally don't kill the larvae. Researchers refer to this feeding method as "non-destructive cannibalism". An ant’s blood is technically known as hemolymph and is colourless.
The feeding behaviour of Dracula ants may have been the precursor to the trophallaxis seen in some other ants. In trophallaxis, food or fluid is passed from one member of a community to another by mouth-to-mouth feeding or anus-to-mouth feeding.
The ant shown above has a similar appearance to the six recently discovered species in the genus Prionopelta but is not one of them.
Dracula Ants in the Genus Prionopelta
In 2015, researchers found six new species of Dracula ants in Madagascar and Seychelles. They have been classified in the genus Prionopelta. They are tiny creatures around 1.55 mm in length. They live underground or in deep leaf litter and are rarely noticed.
The ants have been described as "fierce" predators. They catch tiny invertebrates as prey. Like other Dracula ants, at least some members of the colony obtain blood (hemolymph) from the larvae. One of the six species is appropriately known as Prionopelta vampira.
Herding and Milking Caterpillars
The caterpillars (or larvae) of many butterflies in the Lycaenid family have a special relationship with ants. Lycaenid butterflies are sometimes known as blues, coppers, or hairstreaks. The relationship between the caterpillars and ants takes several forms, but it’s often beneficial for each insect. The ants crawl over the caterpillars and trigger them to release a sugary solution. They do this by touching glands on various parts of the caterpillars’ bodies with their antennae. The ants then drink the secreted solution. The insects are often said to be “milking” the caterpillars.
Some species of ants build shelters for their caterpillars. At night the ants protect the caterpillars in the shelters. At the start of the day the ants herd the larvae up a tree so that the caterpillars can feed on leaves. The ants guard their herd during the day.
Dopamine and Ant Behaviour
One factor about the ant and Lyaecinid butterfly larvae relationship has puzzled researchers. The ants could survive on a different food source from the larval secretion if necessary, so it seems strange that they would maintain the relationship with the caterpillars. From the caterpillars' point of view, however, the relationship is important because the ants protect them.
In 2015, a multi-university research team found that the caterpillar secretion decreased the amount of dopamine in the ants' brains. This resulted in a decrease in the ants' movements and caused them to stay close to the caterpillars. When ants without caterpillars were given a drug that suppressed dopamine production, the same decrease in locomotion was observed.
Honeydew From Aphids
Aphids are small insects found around the world. They are most common in temperate areas, where they are major plant pests. They have a wide range of colours. They may be colourless, pink, red, yellow, green, brown, or black. The green forms are sometimes known as greenflies.
Aphids have mouth parts than pierce plant veins and suck up the sugary sap that is being transported in the phloem vessels of the veins. Once they have digested the sap, the insects release a sweet liquid called honeydew from the ends of their abdomens. Honeydew is actually the feces or poop produce by the aphid's digestive system.
Honeydew is rich in sugars, as it name implies, but it also contains amino acids, vitamins, and minerals. Aphids often flick the expelled honeydew away from their body. Ants feed on these honeydew deposits when they find them on plants or on the ground. Some ants take a more active role in obtaining the secretion by "farming" the aphids, however.
The relationship between ants and the aphids that they farm is an example of mutualism. In mutualism, both animals benefit from their association. In this case, the ants receive a regular source of honeydew and the aphids receive protection from predators.
Ants that care for aphids often stimulate their charges to release a drop of honeydew by tapping or stroking them with either an antenna or a foreleg. The ants are sometimes referred to as aphid milkers. They protect their herd from potential predators and carry the aphids around to new plants when necessary. They also collect new aphids when necessary. Some ant species also collect the eggs produced by their charges and store them in their nest over winter. They take the eggs back to the plant leaves in the spring.
Researchers have found that some subterranean ants also feed on sweet secretions from other insects. In some areas, thief ants (Solenopsis molesta) obtain honeydew from ground pearls. Ground pearls are round scale insects that feed on root sap. Citronella ants (Lasius californicus) care for mealy bugs and feed on their honeydew. The bugs feed on liquid from plants. The ants get their name from the lemon verbena scent that they release when they are threatened or crushed.
Leafcutter ants are found in South and Central America and in the southern United States. They belong to one of two genera—Atta and Acromyrmex. Many species exist. The ants cut pieces from leaves or petals with their sharp jaws and then carry the plant pieces to their nest. Leafcutter ants are sometimes known as parasol ants, since as they travel they hold their piece of leaf or petal above their head. The leaves are used to produce a fungus "garden". The ant colony feeds on the fungus.
The ants may roam as far as two hundred and fifty metres to collect plant material for their nest. They find their way home by following a trail of chemical pheromones which they deposited as they moved away from the nest. In some areas, they can be a pest because they strip trees of their leaves.
A Colony and a Fungus Farm
Atta cephalotes is a leafcutter ant with a wide distribution in South and Central America. The nests of this species may be huge and the colony may consist of as many as five to eight million individuals. The organization of the colony is impressive.
The largest ant in the colony is the queen. Next in size are the males. They are followed in size by the soldiers, who protect the nest. Smaller worker ants (maxima ants) collect the plant pieces. In the nest, the leaf carriers pass the leaves to even smaller ants (media ants), who chew the leaves and turn it into a mulch. The mulch supports the growth of a specific type of fungus, which the ants eat.
The smallest ants of all are called minima ants. Their job us to take care of the fungal garden. The insects tend their fungus crop carefully, removing debris and parasites and even destroying invading fungi of a different species.
About 10,000 species of ants have been identified, but scientists think that around twice as many species actually exist. According to the AntWeb website run by the California Academy of Sciences, more than one thousand trillion individual ants are thought to exist on Earth.
It's true that some ants can be annoying and that some are harmful for plants, other animals, or humans. I think that they're interesting insects, though. There may be many more strange ant behaviours waiting to be discovered.
- Information about zombie ants from Pennsylvania State University (2014)
- Zombie ant brains are left intact from Penn State (2017)
- Information about Adetomyrma venatrix from Wildscreen Arkive
- New dracula ants have been discovered from the phys.org new service
- Relationships between lycaenid butterflies and ants from the Australian Museum
- Lycaenid butterfly larvae manipulate ants from Kobe University
- Facts about honeydew gatherers from antiwiki.org (a website run by ant biologists)
- Honeydew ranches exist underground from Scientific American
- Information about a leafcutter ant (Atta cephalotes) from Wildscreen Arkive
- Atta cephalotes facts from antweb.org
© 2011 Linda Crampton