Israeli Scientists Solve the Mystery of the Fruit Bat’s Missing Neurons

Studying fruit bats in a tunnel, Nachum Ulanovsky and the Weizmann team hone the Nobel-winning theory of how we (and bats) navigate in the vast real world, not a box in the lab

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An Egyptian fruit bat in motion
An Egyptian fruit bat in motionCredit: Weizmann Institute of Science
Ruth Schuster
Ruth Schuster

Fruit bats, aviating wonder of nature. Their beauty aside, we can all, with the exception of farmers perhaps, appreciate their propensity to pollinate. Without them we wouldn’t have durian fruit, for instance.

Yet oddly enough, fruit bats don’t necessarily stick to the neighborhood. They may flap vast distances in search of their meals, even dozens of kilometers.

On this note, a team at the Weizmann Institute of Science in Rehovot has resolved a mystery, and modified a Nobel Prize-winning theory while about it: exactly how it is that fruit bats manage to navigate such long distances without having enough “place cell” neurons in their fruit bat brains to do so, theoretically.

Their discovery, following years of work, modifies the theory of place cells postulated half a century ago, which won their discoverer John O’Keefe a Nobel in 2014. It has to do with how we create maps in our minds.

Let us explain. We, fruit bats and other animals have nerve cells called “place cells” in our brains.

The place cells are in the hippocampus, an area in the brain that is important to memory in general, and to navigation. When the hippocampus is damaged we can suffer memory loss and can’t navigate, explains Prof. Nachum Ulanovsky of the Weizmann Neurobiology Department, who led the study.

When Alzheimer’s hits the hippocampus and place cells become damaged – that explains why people suffering that condition get lost, he adds.

How do place cells work? Each represents one singular space in the environment, it was thought. It’s responsible for remembering that one space, it was thought. When these cells work together, we get a cognitive map of the environment.

In the case of the bat, since they fly in 3-D unlike the earthbound among us, the place cells need to provide a 3-D map, and the bats also need a 3D compass – up, down and around.

“When taken together, I know where I am,” Ulanovsky observes – and so do they. Simultaneous activation of multiple place cells thus enables orientation within a space and enables navigation in the environment, he explains.

The million-bat question is, what environment.

Like a rat in a cage

Thing is, Nobel Prize-winning neuroscientist John O’Keefe formed his place cells theory 50 years ago while working on our friend the rat, in tiny environments such as one-by-one meter boxes. It’s like studying the entirety of human navigation skills on people living in jail cells.

Say the place cells get activated when we enter a particular space in our environment. One place cell would get triggered when the rat enters the southern corner. Another would get triggered when it moves on to the space next to the corner. Another would fire when said rat goes to the northern corner, etc. Okay. Based on his rats-in-small-spaces study, O’Keefe concluded that each place cell was “responsible” for an area about 10 by 10 centimeters in size.

But if indeed each place cell in the batty brain represented an area of about 10 centimeters, then the fruit bats would need at least ten trillion neurons to compute their long-distance flights, Ulanovsky and the team explain. Probably more.

But their entire hippocampus consists of only about 100,000 neurons. Yet bats may flap as much as 20 kilometers to their favorite fruit trees (using skyscrapers and other city icons as navigation markers, separate research has shown). And then they navigate back home.

Clearly there was something missing from O’Keefe’s theory. In his defense, Ulanovsky points out that the original research was done with rats in small boxes, because that’s what fit in the lab.

The Weizmann team, on the other hand, ran their experiment on fruit bats in a 200-meter long tunnel. The bats were equipped with tiny “neural logger” devices on their little heads: These wireless devices, which the team has developed, allowed them to record the activity of the bats’ hippocampal neurons in flight.

The fruit bat brain cell elucidation tunnel at Weizmann - it's the long white thing.Credit: Weizmann Institute of Science

Don’t those neural loggers on their heads discombobulate the bats? “No, they can fly 20 kilometers in one session with these things, without a problem,” Ulanovsky reassures Haaretz. The scientists spent years miniaturizing them, he adds.

True, a 200-meter dark tunnel isn’t a 20-kilometer marathon to a luscious loquat but it’s more realistic than a one-square-meter box.

And it became clear that O’Keefe’s results only pertained to laboratory conditions, not the real world, Ulanovsky explains to Haaretz.

“At a much bigger scale – we find suddenly that everything is very different,” he says. The area “covered” by each place cell in nature isn’t 10 centimeters or even 10 times that. One given place cell can represent multiple “place fields,” not just one, as had been thought. And the size of the place fields of the same neuron can vary widely, up to 20-fold.

This is a radically different neuronal code than has been found to date in the mammalian brain, Ulanovsky says, adding: “The new neuronal code is 100 times more efficient than the classic one.”

Which begged the question of what “neuronal code” means. Ulanovsky explains: “That refers to how neurons respond to the world. The classic view of place cells was that each reacts to one place, in one space. We found a very different thing: One neuron may operate in many places, and each neuron has different resolution in different places – a type of neuronal code which we showed is much more efficient for representing extra-large spaces.”

And that is why fruit bats can navigate vast distances, up, down and sideways, go time and again to favorite fruit trees, and return to their cave or Dizengoff Center or an underground parking lot.

Apropos their favorite fruit, separate research done at Tel Aviv University found why male Egyptian fruit bats let lady fruit bats eat food right out of their masculine maws. It’s exactly the same reason why human males have been known to spring for dinner and a movie and it isn’t altruism. That charming practice has become known as the producer-scrounger feeding interaction.

The new study was led by Ph.D students Tamir Eliav and Shir Maimon, together with Dr. Liora Las; the theoretical part of the work was done in collaboration with theoreticians Prof. Misha Tsodyks from Weizmann, and Dr. Johnatan Aljadeff, now at the University of California in San Diego.

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