Social Changes Change Behavior and Appearance, Too

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

Biological Strategy

Desert locust

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Adapt Phenotype

Living systems evolve over time in response to selective pressures. Selective pressures are environmental factors that reduce the reproductive success of some individuals in a population. Genetic makeup (genotype) is one way in which living systems mitigate selective pressures, but another is adaptable phenotypes. A phenotype is an observable characteristic that can be thought of as the expression of the genotype, combined with modifications caused by the environment or developmental conditions. Individuals, populations, or ecosystems that have phenotypes capable of reducing the effects of selective pressures can survive. Some plants, for example, modify their leaf shape in response to changing environmental conditions. A single olive tree has variable leaf shapes on sunny compared to shady areas of the tree, yet the next year, those same buds may develop differently shaped leaves.

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

Introduction

Swarming locusts bouncing into each other might seem like a strange comparison for humans. The locusts’ response to such contact though presents an intriguing model for us to consider.

The key points are that locusts use changes in population density to trigger major changes in their behavior and activity, and they announce loudly and clearly that those changes have occurred.

Image: David Nunn / Some rights reserved

A locust stands on a tree branch.

Swarm of desert locusts, Schistocerca gregaria, in Sudan

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

Individuals of a swarm of desert locusts, Schistocerca gregaria, settle near Aeterba, Red Sea coast, Sudan

The Strategy

The physical touch from other locusts detected by the hind legs, or simply the sight and smell of other locusts nearby can trigger a release of serotonin that makes a normally loner locust drawn to living closely with many others of the same species.

But this is not the only change the locusts undergo when swarming. They also slow down their ability to learn to dislike certain foods, and they “forget” their earlier aversion to them. What this means is they no longer avoid toxic plants, and instead begin to consume them. The locusts themselves still suffer from a toxic malaise, but more importantly it makes them toxic to consume.

This toxicity is related to a change in appearance as well. When living as individuals, desert locusts are green or brown and blend in with their surroundings, avoiding being eaten by avoiding being seen. Once they travel conspicuously in giant swarms, they change the production of red pigment, causing their existing colors and patterns to appear much more vibrant and striking. This signals to predators that eating this abundant new food source will actually be harmful––especially if eaten in high quantities.

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The Potential

We can learn a lot from this example in nature when considering potential interactions between humans, machines, or humans and the machines we build. Sensory inputs could trigger changes to behavior that allow robots to be more positively received by humans when around them, but more optimized for performance when alone. Following the locusts’ lead, this change of behavior and intent can also be communicated outwardly through physical appearance, so that the humans know that the robot is now operating under different instructions.

Serotonin Increases Sociability, Triggers Swarming

Desert locust

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

Biological Strategy

Adaptive Camouflage Helps Blend In or Stand Out

Cuttlefish

The skin of cuttlefish changes color rapidly by expanding or contracting elastic pigment sacs.

photograph of green beetle with black spots standing on a rock

Biological Strategy

Humidity Changes Exoskeleton Color

Hercules beetle

The exoskeleton of the Hercules beetle changes from green to black with increasing humidity using thin film interference by reversible modification of layer thickness.

green, red, blue and brown chameleon on brown tree branch

Biological Strategy

Shifting Crystals Change Chameleons’ Color

Short-horned chameleon

Pigments and nanoscale structures collaborate to help chameleon change its look.

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Last Updated March 1, 2024

References

“At high population densities, billions of locusts form immense swarms, which naturally expose them to predators. Consequently, the migratory locust, Locusta migratoria (Orthoptera: Acrididae), within the swarms develop a sharp contrast pattern of a black tergum and a brown sternum. In contrast, conspecific solitary locusts living in low density tend to have cryptic green coloration that aids them in blending with their surroundings and avoiding predators (6–8). The body coloration of gregarious locusts serves in collective behavior, thermal regulation, and alerting against natural enemies (7, 9, 10).”

Journal article

Spatially differential regulation of ATF2 phosphorylation contributes to warning coloration of gregarious locusts

Science Advances | August 23, 2023 | Xinle Kang, Meiling Yang, Xiaoshuang Cui, Huimin Wang, Le Kang

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Reference

“These behavioral modifications include changes in feeding behavior that support a shift in the locusts’ antipredator strategy from crypsis to conspicuousness: whereas solitarious locusts reject toxic food, transiens and gregarious locusts readily feed on toxic plants to acquire and maintain unpalatability to vertebrate predators.”

Journal article

Phenotypic Transformation Affects Associative Learning in the Desert Locust

Current Biology | December 2, 2013 | Patrício M.V. Simões, Jeremy E. Niven, Swidbert R. Ott

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