Serotonin Increases Sociability, Triggers Swarming

Biological Strategy

Desert locust

Anika Hazra

Image: David Nunn / Flickr / Some rights reserved

Coordinate by Self-Organization

To create and maintain a healthy community of individuals and ecosystems requires that living systems coordinate their activities. Coordination doesn’t necessarily mean that there’s a leader orchestrating what happens. In nature, coordination is usually achieved through self-organization. In a flock of geese flying in a V-formation, for example, there’s no lead goose controlling where all of the others fly. The flock uses this formation because each goose gains energy from air vortices created by the goose in front of it. The lead goose doesn’t gain that benefit, so when it tires, it moves back and another goose takes the front position.

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Sense Body Awareness

Sometimes, it is important for organisms to be aware of their body motions, where they and their body parts are in relation to objects or organisms around them, and internal signals that indicate how they are physically feeling (such as hungry or sick). This is more complicated than simply sensing touch or pain; it requires being aware of positions and internal sensations by gathering and processing many cues simultaneously. For example, a vulture in flight must take in many internal and external cues to be aware of and maintain its position in the air. These cues can include its internal sense of balance, visual cues, and how air currents feel on its wings, face, and other feathers.

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Sense Chemicals (Odor, Taste, etc.) From the Environment

Chemicals are important for signaling and communication among living systems, either intentionally (such as when two living systems try to find one another) or unintentionally (such as when a plant emits a chemical signal that an herbivore can use to find a tasty bite). They are also important for other uses, such as navigating or finding sources for minerals. But chemical signals are often relatively weak and disperse when moving through water or gases. Therefore, detecting them requires specialized abilities, including a way to determine where they are coming from. A well known example of sensing chemicals can be seen in ants following a pheromone trail laid down by others in their colony to locate a quality and abundant food source.

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Sense Touch and Mechanical Forces in a Living System

Perceiving touch enables living systems to detect other living systems around them and environmental conditions, such as air movement, water currents, and temperature. This ability can help them sense danger as well as opportunity, as when a Venus flytrap’s hairs sense the presence of an insect to eat. Sometimes, a living system senses touch or mechanical forces at a coarse scale; other times, at a sensitive scale that detects very subtle differences. For example, a human elbow is not nearly as sensitive to textures as human fingertips. Fingertips have dermal ridges and many nerve endings that increase sensitivity, enabling them to explore the environment in detailed ways. Elbows don’t need to sense at that level of detail.

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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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The desert locust becomes social and forms swarms through an increase in serotonin triggered by the presence of other locusts.

Introduction

From ancient texts to contemporary news, across Africa, the Middle East, and South Asia, swarming desert locusts have had devastating impacts on agricultural areas. These locusts typically feed, move, and go about their lives independently––and even actively avoid each other in normal circumstances. However, if a locust is forced to be around other locusts, its behavior changes completely. This locust will become more attracted to others and will gather with other locusts that undergo the same change in behavior. Thus, a swarm is formed.

Image: AtelierMonpli / Wikimedia Commons / CC BY SA - Creative Commons Attribution + ShareAlike

Close-up view of desert locust, Schistocerca gregaria, subadult from front.

The Strategy

Swarming may be caused by an overabundance of locusts in general––a response to crowding to help locusts find food when there isn’t much left.

The sudden socialization of locusts is triggered either by physical touch from other locusts detected by the hind legs, or by simply the sight and smell of other locusts nearby.

These inputs increase the serotonin levels in a locust. In humans, the serotonin plays numerous roles throughout the brain and body, but is most well-known for contributing to feelings of happiness and well-being. Serotonin may play a similar role in desert locusts, causing them to gather with other locusts in order to maintain higher levels of serotonin.

The spike in serotonin lasts for less than 24 hours, but it only takes two hours of exposure to other locusts to flip the switch that makes a locust go from self-isolating to a social butterfly. The higher the spike in serotonin, the more social a locust becomes. Desert locusts can be prevented from becoming sociable by being injected with chemicals that keep their serotonin levels in check, or that prevent serotonin from being produced altogether. These chemicals are effective even when a locust is stimulated with physical touch.

Beyond the two-hour minimum, locusts become more social the longer they are near each other. This can result in a cycle where swarms grow bigger and remain intact for longer periods, which is bad news for the crops they feed on.

The Potential

More research is needed to understand the role of serotonin in influencing the behavior of the desert locust and its relatives. A better understanding could help inform our methods of pest control to protect agricultural areas from the damage of locust swarms. More abstractly, computer engineers and others could find applications that apply the principle of using the closeness of entities to trigger a change in their behavior or reaction toward others.

Related Strategies

Desert locusts shift from a camouflaging color when solitary to a contrasting pattern that makes them stand out when swarming.

Biological Strategy

Social Changes Change Behavior and Appearance, Too

Desert locust

Swarming locusts eat toxic plants and change color as a warning to predators.

Biological Strategy

Colonies Self-organize

Insects

Colonies of social insects self-organize and carry out complex group tasks through many simple individual interactions.

Last Updated March 8, 2021

References

Journal article

Serotonin mediates behavioral gregarization underlying swarm formation in desert locusts

Science | 30 Jan 2009 | Anstey ML; Rogers SM; Ott SR; Burrows M; Simpson SJ

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Reference

Journal article

Desert locust gregarization: a conceptual kinetic model

J. of Orthoptera Research | 1 December 2005 | Hassanali A; Bashir MO; Njagi PGN; Ely SO

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