One of the most little known species of ants in North America is the leaf-cutter ant. This is mainly because it lives in tropical environments and it is not aggressive to animals or humans if not disturbed. The leaf cutting ant is a social insect. Alone the ant is virtually helpless but with the colony it can be a thing feared by animal and human alike. The leaf-cutting ants have a very important role in the tropical forest. They create and manipulate the environment around them. They also can do major damage.
The leaf-cutting or fungus-growing ants are distributed from northern Texas to central Argentina. These ants are injurious since they cut the green vegetation from trees, shrubs and crops, and carry it into the nest, where they cultivate fungi on it. They have been known to denude a tree or ornamental plants in one night. It has been estimated they do $1 billion damage per year in North and South America today, these ants still cause millions of dollars in crop losses in many South American countries. Although primarily an agricultural pest, this insect on occasion may invade the home for cereals. In the United States, the Texas leaf-cutting ant, iAtta texana
I, occurs in Texas and Louisiana. This ant is believed to cause a total yearly loss of $5 million in the United States (unison services. 1998).
There are about 9,500 named species of ants. These ants are divided into 16 sub families and 300 genera, all which belong to the family called Formicidae, the family of ants (Hoyt. 1996). The leaf cutting ant belongs to the genus called iAtta.
The leaf-cutter ant looks pretty much like a regular ant in North America except that it is a little bigger than most ants. Looking at the ant in the untrained eye a person usually perceives that it is a primitive organism. Looks can be deceiving. The leaf-cutter ant is a complex superorganism unique social, environmental, and food gathering behavior.
The anatomy of the leaf-cutter ant is pretty simple.
It has a one segment «waist» (pedicel) between thorax and abdomen. Sharp spines on waist and backward from head. Antennae 11-segmented very long and elbowed without distinct club. The legs are very long. This ant can be light to dark reddish brown (Smith. 1997).
The leaf-cutter at is just like other ants in many areas in appearance. The ant has a hard exoskeleton to which the muscles are attached. It also protects the internal organs. The main feature of the ant is its head. The large solders have huge serrated blades which they can have up to 0.5 cm space between the mandibles. The size range can be from 1
16 to ½ an inch depending on the type of species. The ants are divided up into three different types according to size. Minima ( 5mm long) and soldiers (a distinct class with oversized head and mandibles and a total body length of more then 13mm) (Whitehouse & Jaffe 1996). The average worker leaf-cutter ant lives from 4-6 months (Howard, Henneman, Cronin, Fox, Hormig. 1996). The worker ants commonly perform superhuman feats.
If we magnify the operation to human scale, so that an ant’s 6-millimeter length grows into a meter and a half, the forager runs along the trail for a distance of about 15 kilometers at a velocity of 26 kilometers an hour. Each successive mile (to convert to familiar Anglo-American sports distances) is covered in 3 minutes and 45 seconds, about the current human world record. The forager picks up a burden of 300 kilograms or more and speeds back to the nest at 24 kilometers an hour – hence 4 minute miles (Holldobler & Wilson, 1994).
The queen is the largest of the ants and can reach length larger than ½ an inch and her life span can be 10 years or more (Smith. 1997). The queen has wings and when she decides to leave the nest she carries a small pellet of mycelium (the fungus) in a special pocket on her head. When a new nest is found, the mycelium is used to start to grow food again. The colony growth is slow at first but soon proceeds rapidly and reaches maturity in about 5 years.
The Nests – Fungus garden
The nests of the leaf cutting ant can get up to 200 square meters in length and 6 meters deep. Up to 5 –8 million ants can live in one nest (Holldobler & Wilson, 1994). The ants nests have many entrances and exits which facilitate travel and air supply into and from the nest. Some underground pathways can reach up to 200 meters in length. The nest consist of many chambers—2000 in an average sized colony. Each chamber is 8 to 12 inches (200-300 mm) in diameter. Nest building raises a huge mound of soil. Ants constructing an average-sized mound carry 88 tons (80,000 kg) of soil to the surface (Fogel R. 1997). Under the ground there are many paths which lead to multiple cavities in which fungus gardens grow. The fungus garden is a spongy mass like a sponge with lots of spaces. The leaf-cutting ant has a mutualistic relationship with the fungus, which they cultivate on fresh plant material. The species of fungus used is from the basidiomycte family called iLepiotaceae.
I Some researchers say that all gardening ants cultivate a single form of the fungus species, iLeucocoprinus gongylophorus
I(Hoyt 1996). Leaves are brought in by workers and then reduced to a pulp and incorporated with their saliva and fed to the fungus. These fungi produce swollen hyphae (gongylidia), bunches of which are known as staphylae. These provide food for the ants and their larvae (Weber 1972). For the best productivity, the fungus garden must have a structure that combines a large area for the production of staphylae (the ant’s food) with the smallest chamber volume which can be maintained and with accessibility to the workers (Bass & Cherrett 1996).. Most of the fungus garden can be only accessed by minima workers because the small passageways in the fungus limit the size of ant that can pass through. In the fungus garden there is more surface area on the inside than the outside and there is more fungus (staphylae) on the inside than the outside. Thus there is a much greater need for small ants in the fungus garden than large ants. Since the outer surface can be accessed by all ants, there is a much greater harvesting pressure on it than the interior surface of the fungus garden. The outside of the garden appears to be heterogeneous. Another important feature of the fungus garden is how long the substrate material will support the growth of the fungus. It is found that fungal gardens have a life cycle of 7 weeks to four months. The length of time the fungus can grow on the substrate is related to the season the type of substrate the ants use. The outside of the fungus garden is heterogeneous. The upper part is a green-gray color with large cells, and this is a young garden where most of the fresh substrate is added. Fungus gardens get older from the top down. The further down you go on the garden, the grayer it will look and the cells will become smaller because of the pressure from the fungus on top. Eventually the holes become so small that minima ants can not have access to the interior of the garden. Old gardens have thinner walls then new gardens. Since the walls become thinner and the holes become smaller the surface are per unite of volume becomes greater in older gardens. On an average, 74% of the total garden surface area is internal. The interior cells of a fungus garden have an average area of 2.17 mm square and a circumference of 5.71 mm square. About 90% of the ants found in the interior of the fungus garden were minima. When the ants start to put down new substrate, the first staphylae is produced in about 4-5 days. The best production comes in about 20-30 days after substrate is added. Smaller cavities have more staphylae per unit are than larger ones. Therefore the older the garden becomes, the more food the ants get until about 7 weeks later when the fungus runs out of substrate. As you can begin to see, there is two antagonistic things working for and against the ants. The ants want to maximize production (keep cells small so more staphylae grow per unite of area and to maintain a high humidity) but also they want to keep the garden accessible for all workers (Bass and Cherrett 1996).
After reading the above paragraph you are left with some questions. Do the leaf-cutting ants have any mechanical action on the fungus to improve its productivity? Does the fungus garden need to me tended after substrate is given to the fungus? I will answer these questions using (Bass & Cherrett 1996). The presence of workers on the fungus stimulates staphylae production in a number of ways. Workers frequently defecate onto substrate that they have just added to the garden (Quinlan & Cherrett 1977). It turns out that the fungus lacks enzymes to degrade protein so it can’t receive any nitrogen compounds from the substrate. The ant feces contain ammonia and a mixture of amino acids (Boyd & Martin 1975) which supplies the need of the fungus. The ants also prune the fungus when they eat the staphylae. Studies have shown that about four days after pruning, the fungus grows more staphylae then if it hadn’t of been pruned. If a fungus garden is not pruned, then large white growths appear on the fungus surface. This coincides with low worker populations and a small number of staphylae. The ants eat staphylae when they are of medium size. Old staphylae are large. It is interesting that if the fungus is not pruned, staphylae is not produced. This show the implication that the leaf-cutting ant and the fungus have evolved in such a way that both can not live without the other. How does the fungus keep from being totally consumed by the ants? It turns out that most of the hyphae of the fungus is mixed up with the inedible fragments of vegetation. The hyphae becomes available to the ants only when it grows out of the vegetation to produce the edible staphylae (Bass & Cherrett 1996). As we can see, the fungus ant relationship is a unique and complex system of controls and dependencies. So even though there is greater harvesting pressure on the outside of the fungus garden, the more the ants prune and eat the fungus, the more the fungus responds and produces nutrient staphylae.
Reproduction and the colony
The reproduction and initiation of a colony is very similar in all species of the iAtta
I tribe. Mating season is usually around May or June but the environment at the time has a major influence on mating times. The reproduction process always begins with the nuptial flights (Holldobler & Wilson, 1994). “Some species such as iAtta sexdens
I, hold the flights in the afternoon, while others such as iAtta texdens
I of the southwest United States, conduct them in the darkness of night” (Holldobler & Wilson, 1994). While beating their wings, the heavy females meet and mate with as many as 5 or more male in succession. While in the air, each queen will receive 200 million or more sperm from her mates, who will die within a day or two. She stores the sperm in her spermatheca and it is there the sperm will die for up to 14 years which is the known maximum lifetime of a queen. Each sperm will be used individually to fertilize a single egg. A single queen can produce as many as 150 million daughters which are the majority of the workers. Some will turn into queens capable of founding a colony on their own. Others will arise from unfertilized eggs to become short-lived males. After a queen has successfully mated, she will descend to the ground and rakes off her wings at the base. Then she will dig a cylindrical hole about 30 cm deep where at the bottom she widens a room about 6 cm across. She then spits out the packet of fungus she stored in her pouch at the bottom of her mouth onto the floor so her garden can start. The she lays 3 to 6 eggs. The fungus grows and when she has laid more then 20 eggs she will put the eggs and the fungus together. All this time the queen is cultivating the fungus by herself. At regular intervals about an hour or two she takes a piece of fungus and defecates on it with a brown drop of fecal liquid. In about 40 to 60 days the first adult workers emerge (Holldobler & Wilson, 1994).
All during this time the queen survives on the energy obtained from the metabolic breakdown of the wing muscles and fat from her own body. Also at the beginning she eats up to 90% of her eggs. This puts a lot of pressure on the queen. Day by day she grows thinner while caught in a race between starvation and creation of enough ants that can sustain her life. With the queen’s resources rapidly running down she must create a perfectly balanced work force on her first try. In the first crop of adult workers, there are no soldiers or larger-sized workers. Only the smallest foragers and still smaller workers who raise the fungi are present When the workers emerge they immediately begin to feed on the fungus and then dig their way up to the surface and bring leaves to sustain the fungus garden. The queen now turns into an egg laying machine. All she does is eat and produce eggs while the colony does the rest of the work. As the colony grows and prospers and size range of workers expands to include larger and larger forms. When the colony reaches about 100,000 members, the first full-sized soldiers are present. The colony’s beginning is slow but during the 2nd and 3rd years its growth is accelerated. When the colony begins to produce winged males it is a sign that the queen is old and soon the colony dies (Holldobler & Wilson, 1994).
When the queen first starts to lay eggs, Holldobler & Wilson (1994) found that the heads of the ants produced were in the range of 0.8 to 1.6 millimeters (heads of ants are proportional to body length, the larger the head, the longer the body). If the heads are greater than 1.6 millimeters, the queen will have lost too much energy in producing them and she will die along with the colony. If the heads are less than 0.8 millimeters, the ant is useless because it is too small to carry leaves and too small to care for the fungus garden(s). Now you must be asking yourself, how does the queen know what distribution of offspring to produce first? Holldobler & Wilson (1994) found that it is the size of the colony, not its age that determines that caste distribution. Example, if you took a large colony of leaf-cutting ants and killed all of the ants except 10,000. The queen would not produce huge soldier ants but would concentrate on producing small and medium sized ants until the colony reached a certain size. In essence, by killing the ants you made the colony young in its size and caste configuration (my example).
Another reason for the unique distribution of the roles and sizes of ants is related to physics and chemistry. In each role of each size of ant, Holldobler & Wilson (1994) found that when they measured carbon dioxide given off, that the ants minimized the amount of carbon dioxide given off. This means that the ant who is suited for the best caste for the job, is the one who gathers the most vegetation with the least expenditure of energy. The most efficient group of leaf carriers proved out to be the ones whose heads were 2.0 to 2.2 millimeters in width. When soldiers were given leaf carrying jobs, they use much more energy.
The sex of a leaf-cutter ant is very easily determined. If the queen fertilizes an egg it produces a female and an unfertilized egg produces a male. Since the male progeny come from unfertilized eggs, they are haploid; that is, they inherit one of the mother’s two sets of chromosomes and nothing from the father (Hoyt 1996). The female on the other hand is diploid. They receive one of the mother’s two sets of chromosomes and the father’s one set (only one set of chromosomes exist in a gamete or sperm).
The key is that one set – the half of the genes that daughters get from their haploid father. It is identical to every other set every female gets, and thus the sisters must be at an absolute minimum of 50% related to each other. The genes from their mother, depending on which of the two sets of chromosomes is inherited, may or may not be the same. On average, based on chance, sisters share 75 % of their genes with each other – more than 50 % they share with either parent. Thus sister ants in a colony can best ensure the survival of their own genes by helping each other. Males, meanwhile, with only one of the mother’s two sets and nothing from a father, share only 25 % of their genes with their sisters, on average, and predictably contribute less labor than females to the colony welfare; their purpose is reproduction through a single mating, after which they die (Hoyt 1996).
This is one of the main differences that separates the ant species from other insects. For all hymenopterans are haplodiploid. Also a difference within hymenoptera is that the iAtta
IGroup is a social insect while most hymenopterans are solitary.
Now you must be asking yourself, what causes a female to grow into a fertile queen instead of a sterile worker? Holldobler & Wilson (1994) said that the deciding factors are environmental rather than genetic.
All the females of a colony possess the same genes with respect to caste – and any female after conception [as previously mentioned, most ants in the nest are female] can turn into either a queen or a worker. The genes merely provide the potential to develop into either a worker or a queen. The controlling environmental factors are several in kind, varying among species. On is the amount and quality of food received by the larva. Another is the temperature of the nest at the time the larva is growing up. Still another is he physical condition of the queen. If the mother ant is healthy, she produces secretions during most of the year that inhibit the larva from developing into queens [this is a reason why only older nests produce queens and males at the same time]. In this one category the mother deserves the name we have given her – queen, or ruler of the colony. She not only determines whether an offspring will be male or female, but also assigns caste to her daughters. Yet even here, the workers exercise a kind of ultimate, parliamentary control. They alone decide which of their growing brothers and sisters will live or die, and hence they determine the final size and composition of the colony (Holldobler & Wilson 1994).
The leaf-cutter ant has posed serious problems for evolutionists. For what is the advantage an individual ant gains in putting all her energy into the selfless tasks of a colony, from leaf-cutting to caring for the larva to fighting as a soldier – without ever having a chance to mate and produce offspring of her own? This question has led evolutionists to one of the secrets of the superorganism. “The ants’ success lies in the evolutionary concept of uKin selection
U” (Hoyt 1996). This concept occurs when members of a species work for the survival of relatives – genes by descent from a common ancestor rather than for one’s survival of its own offspring (which is called individual selection). This is a very complicated process in which many people have devoted their lives to its study.
The gathering of food by the leaf cutting ants is a complex series of interactions and communication which I will discuss. The foraging for food first is started from the central nest. Special ants called scouts range off of ant trails in search of food for the nest and if they find food, they start to recruit workers to collect the resource. This sounds simple but the process is actually very complicated. Using Howard, Henneman, Cronin, Fox, and Hormig’s research (1996) I will explain the process. When a scout find a food source, the decision of whether or not to recruit workers is based on the location, quantity, and quality of the source. Also the scouts examine resource consistency such as protein, lipid or simple sugar content. If a scout finds a familiar resource, it will recruit workers faster then if it found an unfamiliar resource. How much an ant is familiar with a resource was found to be in direct proportion to the number of times the ant physically touched the resource or food. The same goes with the workers. If a scout presence a worker with an unfamiliar resource, it is more likely to reject it. If the resource is accepted, the worker ant will follow the special scent trail laid down by the scout to the location of the resource. The time delay for the ants to follow a new type of food can be a negative and a positive. This can carry costs by the delay in acceptance of a new resource such as food delay, overexploitation, and starvation. This mechanism can also protect the ants in that the ants know that the food they are feeding the fungus works but a new type of food can poison the fungus.
Wirth, Beyschlag, Ryel, and Holldobler (1997) studied the cycle of foraging in the leaf-cutter ant and they found some interesting results. They found that on an average, the total yearly input of collected green leaf material corresponded to1706 to 3,855 meters square of foliage area depending on the size of the nest. The number of leaf fragments collected per day ranged from 9,770 to 374,200. During the dry season the ants harvested more leaves than the wet season due to the loss of availability of green leaves during the dry season.. Also the type of material collected during the two seasons differed. During the wet season, greater quantities of green leaves were collected while during the dry season, non green material was collected (fruit parts, stipules, and flower parts). The ants collect resources according to their availability during that time. The amount of material collected also varies with the seasons. During the wet season, more material is collected. A possible reason for this is that since during the dry season the ants collect more flowers and fruits which contain more energy then leaves, the ants don’t need as much material to feed the fungus. The rain limits the amount of foraging that can go on during the wet season by physically stopping the ants from moving on their paths due to the abundance of water on top of the soil. This may be important in regulating colony size. The average area of a leaf fragment carried by individual ants was determined to be 0.79 cm square and with an average weight of 5.51 mg. The scientists discovered that fragment area and weight was determined by the type of plant harvested. Succulent leaves (which are normally thick) were cut into small pieces (
Special crops aren’t what make leafcutter ants unique
Acromyrmex octospinosus leafcutter ants. (Credit: Deadstar0 via Wikimedia Commons)
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Humans developed agriculture about 10,000 years ago, but leafcutter ants began cultivating massive subterranean fungus gardens more than 10 million years ago.
A complex genetic analysis has biologists re-evaluating some long-held beliefs about the way societies evolved following the invention of agriculture by these six-legged farmers.
Like humans, leafcutter ants grow crops, and like humans, farming allows the ants to produce enough food to support millions of individuals who work at specialized jobs.
A leafcutter ant of the native Texas species Atta texana tending a fungal garden. (Credit: Alexander Mikheyev via Rice)
“Our findings suggest that several of the things we thought we ‘knew’ about leafcutters are not true.”
In a study published this week in Molecular Ecology, biologists analyzed genetic data from samples collected at leafcutter nests throughout South, Central, and North America and concluded that the ants originated in South America and owe their success to something more than their choice of crops.
“The ability to grow domesticated crops was a major turning point in human history and evolution, and we thought, until recently, that a similar thing was true for leafcutters,” says study coauthor Scott Solomon, an evolutionary biologist at Rice University who collected many of the study’s samples as a graduate student and postdoctoral researcher at the University of Texas at Austin and the Smithsonian Institution in Washington, DC.
“Our findings suggest that several of the things we thought we ‘knew’ about leafcutters are not true.”
The research, which coauthor Ulrich Mueller, professor at UT Austin co-led, is available in both the newly published paper and a 2017 companion study, also available in Molecular Ecology.
What makes leafcutter ants special?
Leafcutter ants live only in the Americas. More than 40 species range from Argentina to the southern United States, and they are a dominant ecological player in any forest or grassland they inhabit.
The ability to consume plant matter they cannot directly digest allows a nest of leafcutters to consume about as much vegetation each year as a full-grown cow.
“They aren’t the only ants that grow fungi, but if you compare leafcutter ants with other ants that grow fungi, there are many differences,” Mueller says. “For starters, no other ants use freshly cut leaves to grow their fungi.”
Ants that grow fungus on dead and decaying leaves have been around even longer than leafcutters, probably about 50 million years, Solomon says. But leafcutters’ ability to use living leaves was a quantum leap in evolutionary terms because it opened up the entire ecosystem. For example, Solomon says, the ability to consume plant matter they cannot directly digest allows a nest of leafcutters to consume about as much vegetation each year as a full-grown cow.
“Once you can use fresh leaves, it gives you access to so much more food,” Solomon says. “If you can grow and raise your crop on any leaf that’s growing out there, then the sky’s the limit.”
In comparison with other fungus-growing ants, leafcutter colonies are enormous, Solomon says. “They’re on the order of millions of individuals. Some leafcutter colonies are so large that they show up on photos taken by satellites in space.”
Leafcutters also have specialized tasks. Individual worker ants come in different sizes, and they have different jobs.
“Some are specialized on raising the young,” Solomon says. “Others are specialized on removing weeds and disease inside the nest. Others are specialized on going out and finding food, and yet others are specialized on defending the colony.
“All of the specialization is unique to the leafcutters,” he says. “With other fungus-growing ants, the workers are basically interchangeable. They don’t have these specialized tasks.
Mysterious ‘lazy’ ants have jobs after all
“One of the long-held truths of our field was that leafcutters grow a special and unique kind of fungus that no other ant could grow,” Solomon says. “It was thought that something about that unique crop allowed them to do these things that other fungus-growing ants couldn’t do.”
The new studies, which are the first to analyze the genes of fungi from hundreds of leafcutter colonies across the Americas, found instances where other ants grew the specialized “leafcutter-only” fungus, as well as instances where leafcutters grew more generic fungal crops.
“It’s not the crop that makes them special,” Mueller says. “We found that leafcutter ants and their fungi have co-evolved, and while that’s not a surprise, the evidence suggests that this co-evolution occurred in a more complex way than previously believed.
“For example, we found that the type of fungi that was long thought to be unique to leafcutters can be grown by other ants on dead plant material,” he says. “In one case, it’ll be grown on fresh vegetation, and in another case, it won’t.”
Ants have been trying to kill each other for 99 million years
Solomon says, “The question is what gives this fungus the ability to digest freshly cut leaves? It’s not something that is inherent in the fungus. There seems to be something about the way the leafcutter ants are cultivating the fungus that gives it that ability.”
Solomon began collecting leaf-cutting ants and their fungi in Central America in 2002 as a graduate student in Mueller’s lab. In 2007 Solomon expanded his work by spending a year working with study coauthor Mauricio Bacci Jr. at São Paulo State University in Rio Claro, Brazil. Solomon’s samples and dozens of others Mueller’s and Bacci’s teams gathered over the years allowed the researchers to pinpoint the origin of leafcutters to South America, probably in the grassland plains of what is now southern Brazil and Argentina, Solomon says.
“We sampled tons of different nests of leafcutter ant species throughout the entire range of all leafcutters, which goes from Texas in the extreme north down to Argentina,” Solomon says. “What’s novel about our approach is how much sampling there was, particularly in South America. In the past, there has been a lot of sampling, but it was focused in just a few different regions, particularly in Costa Rica and Panama.
“It turns out the leafcutters in those places don’t represent species that live elsewhere,” he says. “By going and sampling in other places, especially in the open grasslands of southern Brazil, Paraguay, and northern Argentina, we were able to show that the greatest genetic diversity of leafcutter fungi is in South America. Usually, wherever there’s the greatest genetic diversity is where a group originated. That is true for humans, and that’s just generally true of other species, and that leads us to believe the leafcutters originated in the grasslands of South America.”
Mueller says, “The study illustrates the importance in science of re-evaluating entrenched assumptions, amassing large data sets, and collaborating internationally before reaching conclusions.”
Agencies that supported the research include the NSF, the Brazilian Ministry of Education’s CAPES Foundation, and the São Paulo Research Foundation.