Some butterflies actually expand their brain by adopting new behaviors

A new study led by the University of Bristol has revealed that Heliconius butterflies’ brains grow significantly in response to adopting novel foraging behaviors. More specifically, a region of their brain known as “the mushroom body” (due to its shape) was found to be two to four times larger than in closely related butterfly species. 

These results suggest that the structure and function of an organism’s nervous system are closely related to its ecological niche and behavior.

“Heliconius are the only butterflies known to collect and digest pollen, which gives them an adult source of protein, when most other butterflies exclusively obtain protein as caterpillars,” said senior author Stephen Montgomery, a biologist at Bristol.

“This shift in diet allows Heliconius to live much longer lives, but they seemingly only collect pollen from specific plant species that occur at low densities. Learning the location of these plants is therefore a critical behavior for them, but to do so they must presumably invest more in the neural structures and cells that support spatial memory.”

How the research was conducted 

By using a unique synthesis of comparative data on large-scale brain structure, cellular composition and connectivity in the brain, and cross-species behavioral studies, the experts examined the relationship between mushroom body expansion, sensory specialization, and the evolution of pollen feeding.

The researchers built 3D models of the brains of 30 pollen-feeding species of Heliconius and 11 species from closely related genera, collected from Central and South America. Using this data, they measured the volume of different brain areas and mapped the locations in butterflies’ phylogenetic trees where major evolutionary changes in brain composition occurred. 

Next, the researchers investigated critical changes in neural circuitry by quantifying the number of neurons in the mushroom bodies and the density of their connections, while tracing the neural inputs from brain regions which process visual and olfactory information before sending it to the central brain.

Finally, in collaboration with the Smithsonian Tropical Research Institute in Panama, the scientists conducted a series of behavioral experiments in several species to assess whether the observed enlargement of the mushroom body corresponded to improved visual learning and memory.

What the researchers discovered 

The investigations revealed a remarkable range of variation in mushroom body size (up to 25-fold) among closely related species within a relatively short evolutionary timeframe, providing a compelling example of a phenomenon known as “mosaic evolution,” in which specific brain structures can vary independently during evolution when placed under strong selective constraints for behavioral adaptation.

“We identified that changes in mushroom body size are due to an increased number of ‘Kenyon cells,’ the neurons that form the majority of the mushroom body and whose interactions are thought to be the basis of memory storage, as well as increased inputs from the visual system,” Montgomery said.

“This expansion and visual specialization of the mushroom bodies were accompanied by enhanced visual learning and memory abilities. Through this synthesis of data types, we provide a clear example of a novel foraging behavior coinciding with adaptations in the brain and associated cognitive shifts.”

“This study provides a rare combination of neurobiological and behavioral data across closely related species, revealing a clear example of marked evolutionary changes in the brain over a relatively short time scale coinciding with improved visual learning and memory abilities. Identifying such relationships between brain adaptations and behavioral shifts are crucial to our understanding of cognitive evolution,” added co-lead author Fletcher Young, an expert in Evolutionary Biology at Bristol.

Better understanding the relationship between brain anatomy, sensory processing, and foraging behavior in these butterflies could provide new insights into the evolution of learning and memory not only in insects, but also in other animals, since the structure and function of mushroom bodies show significant similarities to aspects of vertebrate brains.

The study is published in the journal Nature Communications.

More about mosaic evolution 

Mosaic evolution refers to the evolutionary process where different parts of an organism evolve at different rates. In other words, various traits or characteristics of a species may not all change simultaneously or at the same speed during the course of evolution.

For example, in human evolution, our ancestors started walking on two legs (bipedalism) before they developed large brains. The anatomical changes required for bipedalism occurred at a different pace than the changes leading to increased brain size.

This concept is important because it challenges the notion that evolution is a uniform, linear process. Instead, it highlights the complex and multifaceted nature of evolution, where changes occur in a more patchwork or “mosaic” manner, influenced by a variety of factors like environmental pressures, genetic drift, and mutation rates.

Mosaic evolution is often discussed in the context of the evolution of hominids, or human ancestors, as well as in the context of other species. It’s an important aspect of understanding the full picture of how organisms have evolved over time.

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By Andrei Ionescu, Earth.com Staff Writer

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