Leaf Cutter Ant

Characteristics

Leaf-Cutter Ants are dark red in color. In addition to the standard ant anatomy, the back of the thorax has three pairs of teeth, or spines which help them maneuver material, such as leaf fragments, on their backs. Size varies by caste within each colony: the smallest are merely 0.7 mm wide (head size) and the largest up to 5 mm wide, with the foragers typically about 2.0-2.2 mm wide.

Species Specifics

Atta ants are unique because they culture fungus from cuttings of live plant material.

Physical Characteristics

Leaf Cutter Ants have heads of various sizes, allowing them to perform specialized jobs. They also have the ability to excrete antibacterial liquids, to protect the fungus from infection.

Ecology

Habitat

They are found in rainforest and deciduous forests as well as in open woodlands and scrub forests. Atta ants will also invade agricultural areas that supplant their forest habitats.

Distribution

Diet

Leaf cutter ants of Atta cephalotes exclusively feed on one type of fungus, Leucocoprineae.

Ecological Web

Atta ants are often considered agricultural pests. However, they help maintain ecosystem health by aerating soil. They provide crucial nutrient redistribution in the forest through their discarding of spent fungus and their removal of their own waste material. The ants are also a valued source of protein for other forest animals.

Activity and Behavior

Activity Pattern

During the day, leaf cutter ants leave the nest to forage. When a suitable food source is found, the ants will secrete chemicals from their poison gland sacs. These chemicals have two functions. Firstly they serve as a recruitment signal, attracting other ants from their colony to join them. Secondly these chemicals function as long-lasting orientation cues for the foraging trail. Many chemical and behavioral details of the ants' poison gland are not well understood. However, we do know that the deposition and potency of these chemicals depends on both the quality of food and the need of the colony's fungus for new vegetation. When an ant finds a high quality food source, they summon other nearby ants to join them by stridulating, or rubbing their body parts together to make a vibrating sound. Tender leaves, higher in sugar, are usually deemed the most desirable, as opposed to tough, thick leaves. The more desirable the food source, the more ants tend to stridulate. When a leaf carrying ant is ready to take a leaf fragment back to the nest, she will stridulate, attracting a minim worker to join her on the journey home. These minim workers ride on the leaf fragments, and defend the leaf carriers from attack by parasitic phorid flies, that try to lay eggs on the leaf carriers' bodies. Minim ants also perform another job while riding on the leaf fragment. It is thought that minim workers inspect and clean leaf fragments, so they are free of any harmful bacteria or fungi.

Behavior

Social Behavior

Leaf Cutter Ants subscribe to a caste system, where ants carry out specific jobs based on the size of their head. Ants with a head size of 1.6-5.0 mm work as foragers, cutting vegetation, and carrying it back to the nest site. This is usually done in an assembly line leading back to the fungus garden. Smaller ants work as gardeners, with head widths of 0.7-1.0 mm. The larger 'gardeners' receive cuttings and clip them into fragments 1.0-2.0 mm across. Next, smaller gardener ants come along and crush and mold the fragments into moist pellets, add fecal droplets, and insert them into a mass of similar material. Then even smaller workers pluck loose strands of fungus from places of dense growth and plant them on the newly constructed surfaces. Finally, the smallest ants patrol the garden. They pluck out spores and hyphae of foreign species of mold.

Reproductive Behavior

Atta ants typically only have one fertile queen, and hundreds of thousands or even millions of sterile workers. Each year, mature colonies produce young reproductive males and females, referred to as alates. These winged alates then depart from their mother colonies and go on mating flights. The time of day at which these flights take place is usually synchronized with other nearby colonies. Prior to departing on this flight, females will store a small wad of the mother fungus in a cavity located beneath the opening of her esophagus, called the infrabuccal pocket. The new females will mate with 2-10 males, storing their sperm inside her body for the remainder of her life. Mating with multiple males, or polyandry, allows for an increase in genetic diversity within the colony, which is thought to improve the colony's vitality and resistance to disease. Another explanation for polyandry in leaf cutter ants is that multiple males are needed to supply the queen with enough sperm to last her for over a decade. In this time, she will produce 150-200 million offspring. After mating, the males die. The queen then sheds her wings and attempts to start a new colony. Only a small percentage of new colonies will survive more than a few months. To form a new colony, the queen must dig out a new nest chamber in the soil. She then spits out the fungus wad she has been carrying, and feeds the fungus with her first eggs. By the third day, fresh fungus has started to grow, and the queen has laid 3-6 eggs. By the end of the first month, she will have a brood consisting of eggs, larvae, and pupae embedded in the center of the mat of fungus. In this beginner phase of colony construction, the queen maintains the fungus garden by herself, mainly fertilizing it with fecal liquid. She does not eat the fragile fungus, but instead keeps her energy up by consuming 90% of the eggs that she lays. When the larvae first hatch, they are also fed eggs. When these first workers become adults, they begin to feed on the fungus and take over the fungus culture activities. The egg laying rate of the queen now increases, but not all of her eggs are viable. Some of her eggs are referred to as trophic eggs, formed by the fusion of two or more malformed eggs. These trophic eggs are fed to developing larvae by the worker ants. After about a week or so, the young workers begin to start foraging for leaves to feed to the fungus culture. The queen now assumes her role as an 'egg laying machine', a role which she will have until the end of her life. During this period, she is constantly surrounded by workers who groom her and feed her trophic eggs laid by workers. Workers can lay eggs, but will only lay deformed trophic eggs in the presence of a fertile queen. If workers were to lay fertile eggs, it would negatively affect colony efficiency. Because of this behavior, we know that worker ants are constantly informed about the fertility status of the queen. Although the method for this widespread communication between the queen and workers is not completely understood. One theory suggests that the queen's eggs are coated in a pheromone that signals the queen's fertility status. As she lays the eggs, workers distribute them to different areas of the fungus colony, spreading the signal.

Offspring

A queen can produce around 150- 200 million daughters throughout her lifetime. Ants undergo a complete metamorphosis, meaning they hatch out of their eggs as larvae. During this time, they are fed and cared for by worker ants. After the larval stage, they then enter a pupa stage, and finally emerge as adult ants after 40-50 days.

Conservation

Status

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