Turrets Ventilate Nest

Small cones of dirt rise above the ground level

Biological Strategy

Leafcutter ants

Mary Hoff

Optimize Shape/Materials

Resources are limited and the simple act of retaining them requires resources, especially energy. Living systems must constantly balance the value of resources obtained with the costs of resources expended; failure to do so can result in death or prevent reproduction. Living systems therefore optimize, rather than maximize, resource use. Optimizing shape ultimately optimizes materials and energy. An example of such optimization can be seen in the dolphin’s body shape. It’s streamlined to reduce drag in the water due to an optimal ratio of length to diameter, as well as features on its surface that lie flat, reducing turbulence.

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Modify Material Characteristics

The materials found in living systems are variable, yet often made from the same basic building blocks. For example, all insect exoskeletons consist of a material called chitin. Because material resources are limited, each material within or used by a given living system must frequently serve multiple purposes. Therefore, living systems have strategies to modify materials’ softness, flexibility, and other characteristics. To ensure survival, the benefits of these modifications must outweigh the living system’s energy and material expenditure to generate them. For example, spiders store the liquid components of spider silk in a gland, converting them into silk thread when needed. Some threads have different characteristics, such as elasticity and UV reflectance, than others.

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Modify Pressure

For many living systems, modifying pressure provides extra strength. For others, it provides ways to move air. To use pressure effectively requires a reliable source of pressure, as well as mechanisms to create and release the pressure as needed. Often, modifications in pressure within a living system are created by water, although air can also be a source. An example of a water-facilitated pressure system is wilted leaves that use hydrostatic pressure to stiffen. They do so by bringing solutes (such as salts) into their cells, which causes them to draw in water.

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Protect From Temperature

Many living systems function best within specific temperature ranges. Temperatures higher or lower than that range can negatively impact a living system’s physiological or chemical processes, and damage its exterior or interior. Living systems must manage high or low temperatures using minimal energy, which often requires controlling responses along incremental temperature changes. To do so, living systems use a variety of strategies, such as avoiding high or low temperatures, removing excess heat, and holding heat in. Insulation is a well-known example of managing low temperatures by retaining heat using thick layers of hair, fur, or feathers to hold warm air next to the skin.

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Protect From Gases

Gases are part of most living systems’ environments, especially those exposed to the earth’s atmosphere. Two gases that can act as threats to living systems are oxygen and carbon dioxide; both, paradoxically, are also necessary to life. Thus, living systems must obtain a balance in gas concentrations, selectively protecting from too much gas in some situations and not enough in others. For example, mound-building termites protect themselves by building ventilation shafts to remove excess carbon dioxide from their fungal gardens.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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Turrets on nests of leaf-cutting ants enhance wind-driven ventilation

Introduction

Feeding young is a big job for many animals, but leaf-cutter ants take it to another level. They spend their days harvesting bits of leaves, which they haul into chambers deep within massive nests made of soil. There, specialized colleagues. mash the leaves, mix in liquids to help break them down, add fertilizer and pesticides in the form of specialized bacteria, and use the concoction to grow fungi, which they in turn feed to the young.

The Strategy

Underground farming brings some challenges though: the damp soil that makes up and surrounds the nest is too compact to allow air to flow naturally into the chambers, removing carbon dioxide and refreshing the oxygen supply. For some species, the solution turns out to be blowing in the wind––which ventilates the nest with the guidance of turretlike structures constructed at the surface. This approach to indoor agriculture allows the ants to carefully control the temperature, humidity, oxygen, and carbon dioxide levels their crops need to thrive.

Atta vollenweideri is one such species. Found on flat savanna habitats in Argentina and Paraguay, these leaf-cutters build massive mazes of tunnels and chambers that extend nearly 20 feet below ground as well as into a 3-foot-high (1-meter) mound they construct from the diggings. Atop the mound, the workers piece together bits of clay dug out from below, combined with bits of leaves and twigs, to create cone-shaped protuberances with one or more openings. When summer breezes blow, the air currents pass over these turrets, creating a low-pressure zone that causes air to flow outward from tunnels that open into the part of the nest swept by the wind. As it does, it sucks air in through other holes around the lower, peripheral part of the nest, increasing airflow through the nest three to 10 times compared to when the air is calm.

Scientists think two principles of physics brought into play by the shape of the structures are involved. One, Bernoulli’s law, describes the relationship between speed of flow and pressure of a liquid or gas; the rounded shape of the mound causes air to move faster across the top than across the base, creating a low-pressure zone at the top that draws air into the nest. The other, viscous entrainment, is related to the fact that air is calm inside the nest relative to the breeze outside. As the moving air contacts the still air, it pulls it along with it, much as flowing water in a brook carries silt from the bottom downstream.

This wind-driven circulation system, which is similar to that used by black-tailed prairie dogs (Cynomys luduvicianus) to keep fresh air moving through their burrows, rebalances the oxygen and humidity levels within. Experiments show that the ants use the concentration of carbon dioxide in the air exiting the tunnel system as a guide for shaping the turrets. If the carbon dioxide concentrations are high, they build shorter turrets with a greater total diameter of openings, apparently enhancing the ventilation. As a result, the conditions within the nest remain suitable for growing fungi, and the ants can maintain a stable crop to nourish the next generation throughout the warm summer season. (In the fall when the air cools the ants close up the nests, likely relying on the tendency of warm air to rise to provide the needed ventilation instead.)

The Potential

Leaf-cutter ants offer inspiration to humans in many ways. Their practice of tending a crop with fine-tuned inputs of nutrients and pesticides offers insights for optimizing inputs and outputs in human agriculture. Beyond that, their nest structures suggest ways to enhance the energy efficiency and sustainability of greenhouses, indoor farms, and gardens, and to improve the function of systems used to dry agricultural crops, paint, or other materials. Humans already have taken advantage of the principles demonstrated by leaf-cutter ants to enhance ventilation of homes and other buildings. This can save on costs of cooling buildings and reduce the threat of climate change by decreasing demand for fossil fuels. Organic farming. Wind-powered-HVAC engineering. Leaf-cutting is really just the start of the story of these impressive creatures.

Last Updated March 24, 2020

References

“To understand the significance of elaborate nest architecture for the control of nest climate, we investigated the mechanisms governing nest ventilation in a large field nest of Atta vollenweideri. Surface wind, drawing air from the central tunnels of the nest mound, was observed to be the main driving force for nest ventilation during summer. This mechanism of wind-induced ventilation has so far not been described for social insect colonies. Thermal convection, another possible force driving ventilation, contributed very little. According to their predominant airflow direction, two functionally distinct tunnel groups were identified: outflow tunnels in the upper, central region, and inflow tunnels in the lower, peripheral region of the nest mound. The function of the tunnels was independent of wind direction. Outflow of air through the central tunnels was followed by a delayed inflow through the peripheral tunnels. Leaf-cutting ants design the tunnel openings on the top of the nest with turrets which may reinforce wind-induced nest ventilation.” (Kleineidam et al. 2001:301)

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