A mysterious map: how researchers discovered a unique brain organization in bats


New research from Dr. Andrew Halley and the UC Davis Krubitzer Laboratory details how bat brains are highly specialized for echolocation and flight

By MARGO ROSENBAUM — [email protected]

People often wonder how the mammals we see swinging through trees, swimming in the ocean, or flying overhead relate to us. We reflect on how millions of years of evolution have resulted in so many mammals of varying intelligences and abilities.

If only we could look straight into their brains.

Dr. Andrew Halley, a postdoctoral researcher at UC Davis’ Krubitzer Lab, did just that. With the help of fellow researchers at UC Davis, Simon Fraser University and UC Berkeley, Halley performed brain surgeries on anesthetized bats to better understand the motor cortex – the region of the brain controlling voluntary movements through the body.

Publication of the results on May 25 in the journal Current biology, Halley and the other researchers found that bat brains are highly specialized for two unique aspects of their biology: echolocation and self-propelled flight.

To make this discovery, Halley, the paper’s lead author, and her colleagues mapped the regions of the brain controlling the movements of these fruit bats, focusing on areas dedicated to echolocation and flight.

The bats represent a quarter of all living mammal species, but until recently much of their brains and evolution remained a mystery. Halley and her fellow researchers sought to change that.

Prior to this study, a species of bat motor cortex had never been mapped. This achievement now allows researchers to understand the part of the brain involved in planning, controlling and executing voluntary movements.

Ways to study evolution

Fascinated by evolutionary questions, Halley studies evolutionary neurobiology and comparative neuroscience. A native of Philadelphia, he majored in psychology and worked in a genetics lab as an undergraduate at Pennsylvania State University. Halley said he grew up more interested in the humanities, but always had a fascination with psychology.

Biology piqued his interest when he started taking biology classes in college, especially after learning the theory of evolution. With the questions he started asking, he realized he needed to know more about neuroscience to answer them and wanted to study brain evolution.

Halley has completed her doctorate. at UC Berkeley in 2016, after studying biological anthropology and working on a project tangentially related to neuroscience, in which he studied differences in embryonic development between species.

This fascination with evolution and neuroscience brought him to the Krubitzer Lab at the UC Davis Center for Neuroscience as a postdoctoral researcher.

“The Krubitzer lab was sort of a natural fit; [Dr. Krubitzer] is one of the foremost researchers of brain evolution,” Halley said.

Dr. Mackenzie Englund, a former graduate student in the lab and co-author of the paper, shares Halley’s appreciation for evolution and sensory systems, which he says “are this means by which we interact with the world. “. Englund came to UC Davis for his Ph.D. to search for similar questions.

“Evolution has always been one of those things that made me feel really connected to the world,” Englund said.

Moving away from the study of traditional model organisms

Led by Dr. Leah Krubitzer, the lab largely focuses on studying the evolution of the neocortex, which is “the part of the brain that most people think of when they think of a brain,” according to Halley. The lab is interested in multiple aspects of the neocortex: its function, the interconnectivity within the structure, and how it connects to other parts of the brain.

By studying a range of mammals, researchers in the lab seek to understand how evolution results in varied brain organization from one species to another. Halley said the lab takes a comparative approach and studies animals that deviate from traditional model organisms, such as mice and zebrafish.

The lab is working to understand if parts of the brain have evolved to match the uniqueness of the body of mammals such as opossums, platypus, primates, tree shrews and, more recently, with the help of Halley, bats.

“You can learn a lot just by looking at the extreme adaptations you find in the natural world,” Halley said. “Comparative research, on the one hand, is inherently interesting because we want to understand how evolution works, and more specifically how brain evolution works.”

Studying animals other than model organisms is important, according to Halley, because studying only these animals tells researchers little about the role of evolution in modifying the brains of many different species.

“There are a handful of biological models that are typically used to do a kind of ‘bread and butter’ neuroscience, and they’re also very widely used for translational research to try to develop drugs,” Halley said. “There are limits to the extent to which a laboratory mouse is a good model for a human.”

Brain surgery in bats

Halley’s recent work is part of a larger Krubitzer Lab project to illustrate how species’ brain regions are organized based on differences in their bodies and behaviors.

This study focused on understanding the motor cortex in bats: its variation, what it represents, and whether flight and echolocation have resulted in unique morphologies, such as the extra elongated toes of bats. , with membranes connecting the fingers, forelimbs and hindlimbs to form their giant wings.

“It varies from individual to individual…the motor cortex is so much more variable than other sensory areas because the cortex can be built up by things we do, our behaviors,” Englund said.

All mammals have a motor cortex, so understanding this important part of the brain in bats could suggest understanding brain function and evolution in humans.

“What’s really important is determining common motor cortex themes across species and what may vary,” Englund said.

Using bats from a breeding colony at UC Berkeley, Halley, Englund and the other researchers performed brain surgery to investigate their questions.

After anesthetizing the bat being studied, Halley and the scientists cut open the bat’s skull, exposed the neocortex, and used electrodes to stimulate different areas of the motor cortex. By applying small amounts of current, they sought to determine what muscle and limb movements were created by stimulating various parts of the motor cortex.

“Applying small bits of current to different parts of the brain was essentially an artificial way of mimicking what happened in a naturally behaving bat,” Halley said.

Halley and Englund worked together and took turns in the experiments, often resulting in workdays lasting 12–15 hours. Because “each animal’s life is so precious,” they wanted to get as much data as possible from each experiment, Englund said.

“We would shut down in the experiment room, giving ourselves breaks so we could go get a coffee and maybe a granola bar,” Englund said.

In the end, their new discoveries were worth the grueling days.

In particular, the researchers discovered that in Egyptian fruit bats, large regions of the motor cortex are devoted to their tongue, which emits sounds for echolocation, and to the muscles propelling their limbs for flight.

Map a motor cortex

After the experiments, the researchers were able to create a map of the spatially separated areas of the brain that regulate body movement. The “map” is topographic in relation to the body, that is to say that certain parts of the body are larger or smaller depending on the species. Larger areas on the map mean a body part is overrepresented in the brain, Halley said.

“The key findings from our study were that…different parts of the brain are enlarged in different species depending on their behaviors or body types,” Halley said.

Areas of interest in the motor cortex can likely be explained by their unique biology and adaptations. Egyptian fruit bats have unusual methods of echolocation – instead of using their larynx like most bats, these animals use their tongues. In the study, more than 40% of the stimulated sensory and motor cortex controlled tongue movements. Moreover, the vast majority of the motor cortex was responsible for the coordinated movements of the shoulders and hind limbs, explaining a possible reason for the particular morphology of bat wings.

Despite all the work by Halley, Englund and others in the Krubitzer lab, more studies are needed to understand the full extent of the motor cortex and other parts of the brain in bats.

These animals are becoming increasingly common as model species for study, but many of their bases in neurobiology remain poorly understood. Creating and maintaining colonies is complex, and their unique body morphology makes them more difficult to use in neuroscience research, Halley said.

Future research in evolutionary neurology may involve more studies in bats based on Halley’s findings: Mapping the motor cortex is just the first step.

Written by Margo Rosenbaum — [email protected]


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