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  • NATURE PODCAST

The brain cells that help animals navigate in 3D

Hear the latest science news, with Benjamin Thompson and Nick Petrić Howe.

In this episode:

00:47 Mapping a bat’s navigation neurons in 3D

Grid cells are neurons that regularly fire as an animal moves through space, creating a pattern of activity that aids navigation. But much of our understanding of how grid cells work has involved rats moving in a 2D plane. To figure out how the system works in a 3D space, researchers have mapped the brain activity of bats flying freely around a room.

Research Article: Ginosar et al.

07:44 Research Highlights

How a ‘toxin sponge’ may protect poison dart frogs from themselves, and the world’s oldest known coin foundry has been located.

Research Highlight: An absorbing tale: poison dart frogs might have a ‘toxin sponge’

Research Highlight: Found: the world’s oldest known mint and its jumbo product

09:59 A flexible fabric that transforms from soft to rigid (and back again)

Researchers have created a ‘tunable’ fabric, inspired by medieval chain mail, that when compressed changes from flexible to rigid. The stiffened structure can hold 30 times its own weight, and the team behind it suggest this material could be used to build temporary shelters or have medical applications.

Research article: Wang et al.

16:33 Stark warning from the IPCC’s latest report

This week the UN’s Intergovernmental Panel on Climate Change released its long awaited report compiling the latest climate science data. Nature’s Jeff Tollefson joins us to discuss the report and the warnings it contains for our warming world.

News: IPCC climate report: Earth is warmer than it’s been in 125,000 years

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doi: https://doi.org/10.1038/d41586-021-02204-3

Transcript

Hear the latest science news, with Benjamin Thompson and Nick Petrić Howe.

Host: Benjamin Thompson

Welcome back to the Nature Podcast. This week, mapping how navigation neurons work in 3D…

Host: Nick Petrić Howe

And the fabrics that switch between being stiff and flexible. I’m Nick Petrić Howe.

Host: Benjamin Thompson

And I’m Benjamin Thompson.

[Jingle]

Interviewer: Benjamin Thompson

Getting from A to B is a complex process. Knowing where you are and figuring out where you’re going requires many different types of brain cell. Much of what we know about navigation at a neurological level has come from studies in rats, watching which neurons light up as an animal moves about a space. But there is a drawback to this, as many of these studies have been done in 2D, looking at a rat scurrying over a flat surface. But, as we know, the world is a 3D place and life has its ups and downs.

Interviewee: Nachum Ulanovsky

Of course, different animals have different degrees of three-dimensionality in their movement. There are animals that fly or swim, like bats, fish, dolphins, whales et cetera, and they really move strictly in three-dimensional space.

Interviewer: Benjamin Thompson

This is Nachum Ulanovsky from the Weizmann Institute of Science in Israel. This week in Nature, he and his colleagues have a paper out looking at how one particular group of neurons fire in 3D space to help an animal work out where it is. Specifically, Nachum has been looking at grid cells, which are pretty well understood in the 2D world.

Interviewee: Nachum Ulanovsky

Grid cells are neurons which are activated whenever the animal, rats usually, traverse one of multiple locations in the room. A neuron could be activated in one location or a second location or a third location or a fourth location, and if you look at how are these locations arranged in space, it turns out that they form a hexagonal lattice, much like a honeycomb.

Interviewer: Benjamin Thompson

So, specific grid cells fire in multiple specific places in space, forming this regular repeating hexagonal pattern or lattice, which is thought to help an animal judge distances and know where it is. But what happens when you go from 2D to 3D? What would the pattern of grid cell firing look like? Would it be lots of hexagonal layers, one on top of the other, making an overall 3D hexagonal structure? Well, this has been a longstanding question in neuroscience and Nachum wanted to find out.

Interviewee: Nachum Ulanovsky

And in fact, there were theoretical predictions of what one might expect to get in this situation because this hexagonal lattice on a two-dimensional surface, it’s the best packing of circles on a plane. And what we were after was to see whether this beautiful geometry exists also in three-dimensional space. And to answer this question, we took sort of the most extreme three-dimensional navigator among mammals, which is the bat.

Interviewer: Benjamin Thompson

Nachum has been using bats – Egyptian fruit bats specifically – in his research for some time now, and in this work encouraged them to fly around a large room as he recorded where and when the grid cells fired.

Interviewee: Nachum Ulanovsky

What we’ve done is we’ve placed between 6 and 11 little spheres on which they can land and get little pieces of bananas, and they were at different heights, all around the perimeter of the room. This would encourage them to fly through three-dimensional space, and we recorded the neural activity by using this wireless electrophysiology system that we’ve developed, this neural logger, that allows to record neurons from the brain and store the data onboard the animals.

Interviewer: Benjamin Thompson

So, by logging where in 3D space the bats’ grid cells fired, Nachum and his colleagues could look to see what sort of pattern the activity took in the room. Did they find the hexagonal lattice pattern, so well defined in 2D?

Interviewee: Nachum Ulanovsky

The short answer is that we didn’t find it. We didn’t find even a single neuron that significantly and convincingly showed a hexagonal lattice.

Interviewer: Benjamin Thompson

So, they didn’t find the pattern that had been theorised.

Interviewee: Nachum Ulanovsky

It’s what we expected to get and what we looked for, for more than two years, and this research took many years in part because it took several years to realise that we are sort of looking and looking and looking and not finding this and okay, now we need to recompute our bearings, so to speak, and rethink what we’re looking for.

Interviewer: Benjamin Thompson

But after a lot of head scratching and some complex mathematical modelling, the team found that while there may not be this regular repeating pattern that was expected, the firing of the grid cells wasn’t random either.

Interviewee: Nachum Ulanovsky

What we looked at is the local distances between nearby firing fields, nearby grid fields. So, each one of them, I can ask who are the three nearest neighbouring spheres and I can look at those three distances and then locate the next field and those three nearest distances et cetera, and then when we found that in many of those cells there was a fixed distance or a characteristic local distance between nearby fields.

Interviewer: Benjamin Thompson

Instead of being a very ordered overall state with a perfect hexagonal lattice, the team found a semi-ordered organisation. There were pockets of local order where the location that neurons fired was close to others, and these locations were always separated by a fixed distance. But of course, this begs the question: why is there one pattern for grid cells firing in 2D but another in 3D? Well, Nachum suggests that they’re actually part of the same system, and it’s the fixed distances between where the grid cells fire that’s the key.

Interviewee: Nachum Ulanovsky

What our results argue is that the hexagonal lattice structure is the secondary property, but what’s more fundamental is actually that nearby fields have fixed distances from each other, and if you have that, then in two dimensions you would automatically get a hexagonal lattice, and in three dimensions you get this semi-ordered thing. So, it sort of puts the emphasis, so to speak, not on the hexagonal lattice in two dimensions but on the fixed distances in two dimensions.

Interviewer: Benjamin Thompson

So, after a long time wondering, it seems that researchers now have a better insight into how grid cells fire in 3D space. But of course, this is just one animal, the bat, and one particular experimental setup. Another paper out today in Nature Neuroscience looked at rats able to climb in a 3D environment and showed a different pattern of activity, although also not a hexagonal pattern, so there’s still lots to learn about how all this works. Regardless, Nachum says that his finding might mean that researchers will have to have a bit of a rethink when it comes to working out the mechanisms of how navigation works. A lot of weight has been placed on a hexagonal system for grid cell activity that may differ from what’s really happening.

Interviewee: Nachum Ulanovsky

It will require quite some work to produce a model that on the one hand is consistent with the perfect, beautiful hexagonal lattices in 2D, but on the other hand can produce these semi-organised or locally organised fields in 3D. So this is a major challenge for the field, and it challenges some of the existing mechanistic models of how grid cells come about.

Interviewer: Benjamin Thompson

That was Nachum Ulanovsky from the Weizmann Institute of Science in Israel. We’ll put a link to his new paper in the show notes.

Host: Nick Petrić Howe

Coming up, we’ll be hearing about so-called ‘tunable’ fabrics that can change properties on command. But before that, it’s time for the Research Highlights, with Noah Baker.

[Jingle]

Noah Baker

Poison dart frogs lace their body with some of the most deadly toxins in the animal kingdom, but what stops them poisoning themselves? The answer might lie with ‘toxin sponges’. The poison batrachotoxin found in poison dart frogs kills by disabling proteins embedded in nerve-cell membranes. Scientists have long theorised that batrachotoxin-laden animals, like poison dart frogs or some birds, have evolved mutant membrane proteins that batrachotoxin can’t bind to. But a new analysis has found no such mutations in the golden poison frog. In fact, researchers showed that captive captive-raised golden poison frogs do have batrachotoxin-sensitive membrane proteins but don’t succumb to the toxin’s effects. The scientists propose that the animals sequester the compound with ‘toxin sponge’ – a type of protein that sucks up the toxin before it can damage the membranes. No such protein has been identified for batrachotoxin, but the team did show that a sponge protein found in the American bullfrog can soak up a similar toxin and provide protection in the process. Read more in the Journal of General Physiology.

[Jingle]

Noah Baker

Palaeontologists have found the oldest known mint – no, not the breath-freshening kind, the money kind. The site, which was found in what’s now China’s Henan province, formed part of a bronze foundry that was built around 780 BC. At first, the facility produced mainly weapons and objects used in rituals, but moulds found at the site revealed that at some point it also started processing a type of large, spade-shaped coin that was used as currency from the seventh to the third century BC in China. Radiocarbon dating of charred millet seeds in one of the pits showed that minting began between 640 and 550 BC, predating all other known active mints. Some of the moulds found were also unused, which suggests that the entire minting process, from mould-making to casting coins, occurred on the site. Read more in Antiquity.

[Jingle]

Interviewer: Nick Petrić Howe

Next up on the show, I’ve been looking into a material that can be stiff or flexible. Clothes are great, but what if, as well as delivering style, comfort and a certain je ne sais quois, they could give you a leg up? Imagine, for example, you’re trying to lift a heavy object. Wouldn’t it be great to get support from your slacks?

Interviewee: Chiara Daraio

If you think about warehouse workers, they have to lift heavy weight for extended periods of time, you may think of creating wearable fabrics that could be stiffened or that could become more rigid as well to aid in the lifting weight process while working.

Interviewer: Nick Petrić Howe

This is Chiara Daraio, an engineer who’s been working on so called ‘tunable’ fabrics – fabrics that can change properties. In this case, a flexible cloth that can stiffen on command. Such fabrics could offer more than giving heavy lifting a boost. For example, imagine a new type of medical cast. Their flexible form could be draped over a broken limb and then stiffened once in place. This would allow the cast to be adjusted as the injury healed and even reused for the next broken bone. But how do you swap between flexibility and stiffness at will? Well, in Nature this week, Chiara has a paper about a material that seems to achieve this.

Interviewee: Chiara Daraio

What we created is a new fabric that is very much inspired by the medieval chain mail armours, which can be tuned from being soft and foldable to being stiff and rigid like a solid.

Interviewer: Nick Petrić Howe

Like medieval chain mail, the fabric is made from lots of tiny links. In this case, they are 3D printed and made of plastic. Chiara refers to them as particles. When relaxed, her special 3D printed chainmail is flexible, but if you compress the particles together – and we’ll get on to how they do that a bit later on – they lock and become stiff, as long as the particles are the right shape.

Interviewee: Chiara Daraio

You need to increase the average number of contacts that each particle perceives from neighbouring particles. So, the higher these contacts, the more these particles are tightly, snugly fitting together, the stiffer the fabric will get.

Interviewer: Nick Petrić Howe

It turns out that the little rings that make up bog-standard medieval chain mail do this pretty well, but they have a downside.

Interviewee: Chiara Daraio

If you take these two-dimensional particles, like a ring, you can stack them like a tower, and those stacks provide a lot of opportunities for particles to be in contact. However, this comes at a cost and the cost is weight because if you jam-pack these rings or this square very snugly together, the fabric becomes very, very dense, almost like having a full, solid sheet.

Interviewer: Nick Petrić Howe

And with density comes weight. That may be fine for Sir Lancelot, but it isn’t ideal by today’s standards. So, instead, Chiara wanted to find a shape for the particles that could make a material that perhaps isn’t quite so stiff and strong, but is light enough to be worn.

Interviewee: Chiara Daraio

We focused on these octahedrons. Those have more hollow spaces. They are mostly empty, and even when you stack multiple layers of these kinds of fabrics, they are, by and large, 80% hollow.

Interviewer: Nick Petrić Howe

Chiara’s fabric is made from lots of tiny interlinked octahedrons — think two hollow pyramids stuck together. And despite the trade-off, when stiffened, the material was able to hold weight 30 times that of the fabric itself. But that’s a good point. The fabric only stiffens when it is compressed and the octahedrons lock together. So, how do you pressurise a pant? Chiara’s solution sucked.

Interviewee: Chiara Daraio

For the purpose of this proof-of-principle realisation that you see described in this paper, we use a membrane, think a bit similar to a zip lock bag that we use in our kitchen, and we use vacuum pressure. Effectively, we connect these membranes to pump and suction out the air so that the membrane conforms to the sheets, to these fabrics, and compresses them in all directions.

Interviewer: Nick Petrić Howe

Think of a vacuum sealed bag of coffee beans or rice. Chiara’s solution worked well for the paper, but it was only intended as a demonstration. A vacuum isn’t the most practical solution. Unless, like in Back in the Future 2, it could also dry your clothes at the same time. But enough of my musings, back here in reality, Chiara is working on potential alternatives to make the switch between flexible and stiff.

Interviewee: Chiara Daraio

We are, for example, exploring external fields, like magnetic attraction between adjacent particles or tension created by fibres and pulleys that could be interwoven in the hollow space within the particles. Those actuated either by a small motor or, for example, in response to external stimuli like temperature, pH or light, I would expect would become a lot more practical in applications.

Interviewer: Nick Petrić Howe

And Chiara sees a lot of possible applications. We mentioned medical casts and lifting supports but she also points to things like defensive shields or emergency shelters that could be transported easily in their flexible state and then popped up as necessary. Now, there are still plenty of kinks to work out before these tunable fabrics become a practical reality – not least how stylish they look – but Chiara has high hopes that this technology will be able to tackle real-world problems.

Interviewee: Chiara Daraio

If we could make something that truly provides advances in the medical field or in aiding or augmenting performance of workers or in defence, I think that would certainly be exciting to me and to us and it would be a useful next step.

Interviewer: Nick Petrić Howe

That was Chiara Daraio from the California Institute of Technology in the US. For more on stiff or flexible fabrics, we’ve also got a video where you can see them in action. We’ll put a link to that, along with Chiara’s new paper, in the show notes.

Interviewer: Benjamin Thompson

This week, the United Nations’ Intergovernmental Panel on Climate Change (the IPCC) has released its newest report that collates all of the latest climate research data to assess the current state of our warming world and what might be to come. To find out more about what it says, I called up Jeff Tollefson, senior reporter here at Nature, who’s been covering the story.

[Jingle]

Interviewer: Benjamin Thompson

We’re recording this on Monday, Jeff, and today is the day that the UN’s IPCC has released a long awaited report and, my goodness, it makes for sobering reading. But before we get into what’s in it, maybe you can tell us, what is this report?

Interviewee: Jeff Tollefson

So, this report is the product of about eight years of work since the last one by more than 200 scientists, but the thing to remember is that this isn’t a report by scientists, this is a report by governments that’s compiled by scientists. So, the scientists went over something like 14,000 papers, the full scientific literature since the last one was released in 2013. They compiled a report, a compendium, of all of that science, what it says, what it means, and they hand that over to the Intergovernmental Panel on Climate Change, which is, as it sounds, a panel made up of government representatives, representing 195 countries across the world.

Interviewer: Benjamin Thompson

And so, a huge amount of data that’s been collated and been pulled together. Let’s get straight to it then, I suppose. What are some of the headlines then in this report?

Interviewee: Jeff Tollefson

Well, the main message is that global warming is real, it’s happening now, and we can feel it today, and it’s going to get worse if we don’t take action. That’s been the bottom line for a very long time, but this report makes it clearer than ever. Perhaps the most shocking number is that scientists can now say with considerable confidence that the planet is warming at a rate that is faster than any time in the past 2,000 years, and that it is now warmer than it’s been in 125,000 years. That dates back to before the last ice age, so these are some very stark numbers.

Interviewer: Benjamin Thompson

Well, one of the things that stood out to me in this report is that scientists are trying to update and reduce the uncertainty in what future warming might be. How has that come about, Jeff, and what are they thinking might happen?

Interviewee: Jeff Tollefson

So, one of the things that scientists have done is to try to constrain their models, and one of the metrics that they use is something called climate sensitivity, which refers to how much long-term warming you would expect from a doubling of CO2. The new report basically says that climate sensitivity is somewhere between 2.5 and 4 °C, with a central estimate of 3 °C. So, that constrains climate sensitivity considerably on both the lower end and the higher end. So, what that means for the models is that when climate scientists run their mission scenarios, we have more confidence in what to expect in the year 2100 under any given atmospheric CO2 concentration.

Interviewer: Benjamin Thompson

And what might we expect then? What do researchers think might happen?

Interviewee: Jeff Tollefson

So the IPCC doesn’t try to predict the future. What it does is it runs various simulations looking at different scenarios over the coming century. In some of those scenarios, humanity undertakes aggressive efforts to reduce emissions. In other ones, the emissions skyrocket. So, if you look at kind of a central estimate in which the world doesn’t really change much from what’s happening today, you basically get an estimate that temperatures are likely to rise between 2.1 °C and 3.5 °C by 2100. That’s substantially above the Paris goals of limiting warming to 1.5 °C to 2 °C.

Interviewer: Benjamin Thompson

Yeah, and of course, that was 2015 that was agreed. Have researchers given you a sense of whether this is still achievable?

Interviewee: Jeff Tollefson

So, again, the IPCC doesn’t try to predict whether these things are achievable. What they do is lay out possible futures that government policymakers can use to think about the decisions that they make. But technically, if you talk to any one of the scientists who are involved in this, they will tell you, yes, it is technically achievable that we can limit global warming to 1.5 °C or 2 °C, and the numbers are right there in the report, what you need to do. If we just continue at current levels of greenhouse gas emissions, we will burn through basically the carbon budget we have to remain under 1.5 °C in as little as a decade. But with aggressive action, if we basically cut emissions down to net zero by mid-century, then it is technically possible to remain under 1.5 °C even.

Interviewer: Benjamin Thompson

One thing that we’ve seen a lot of recently, Jeff, around the world are very specific and very catastrophic regional impacts of climate change, and this is something that’s been highlighted in this report, looking at those and the likelihood of them happening and the impact they may have. What are some of the things that have been said?

Interviewee: Jeff Tollefson

So, one of the big advances in this report is that scientists have come a long way in being able to document extreme weather and attribute that extreme weather to human emissions when you can document it now at a regional level. One of the advances in this report is you can kind of click in on any region of the world and you can see various impacts of extreme weather, drought, so that’s going to be of considerable help to governments as they consider their options moving forward.

Interviewer: Benjamin Thompson

Some things that we’ve talked about on the podcast before, I guess, are maybe low likelihood but hugely catastrophic events, like sudden ice sheet collapse or something like that. Now, as this report is looking at the scientific data, what does it say about things like this?

Interviewee: Jeff Tollefson

This is another innovation of this report. The IPCC has been criticised in the past for being too conservative, for focusing too much on what scientists can say, and that leaves out the long tail of extremes that scientists have a harder time predicting. And, like you say, this would be things like ice sheet collapse, changes in ocean circulation. It could be things like forest dieback. These are things that are harder to predict, and one of the reasons that they’re harder to predict is that they’re more likely to occur with extreme warming later in the century under levels of extremely high carbon dioxide. And this report basically says, ‘Although we think these extreme events are unlikely, we cannot rule them out, and if you are a policymaker thinking about long-term risks, it should be on your radar that there are things that could happen that would be catastrophic. Even if we can’t tell you the likelihood, you should be thinking about them.’ And it is different than the previous reports in that sense.

Interviewer: Benjamin Thompson

Speaking of policymakers then, of course, this report has been signed off by 195 governments, which shows that there is worldwide backing for the numbers and the stark warnings within it. But all the people you have spoken to when you’ve been reporting this story for Nature, do you get a sense of frustration that nothing will be done, that it will be just lip service?

Interviewee: Jeff Tollefson

Well, I think there’s long been a sense of frustration throughout the scientific community. The IPCC has been raising these alarm bells for three decades now and, like you say, this is a report that’s been signed off on by the very same governments that have thus far failed to take the kind of action that would be needed to seriously curb greenhouse gas emissions. But one of the messages that comes through loud and clear in this report is that every degree, every tenth of a degree, of warming matters. So, at some point, we have to kind of rein in emissions and halt this process. This is a message that scientists tend to focus on. It really is up to us. We have the power to stop emissions and we should have the motivation, based on the information in this report, to do so as soon as possible.

Interviewer: Benjamin Thompson

Well, we’re only a few short months away from the UN Climate Change Conference (COP26) when obviously governments will get together to discuss what can be done, what they will do, to try and tackle this problem. Do you think this report will move the needle at all?

Interviewee: Jeff Tollefson

It’s hard to say. Clearly, governments have kind of woken up to this issue, at least politically. Many countries, including the UK, the United States, even the European Union, have committed to net zero emissions by mid-century. What we have yet to see is the kind of action on the ground, the kind of policies at the local and national level, that will seriously begin to reduce emissions in a rapid way. So, everybody is looking forward to Glasgow to see whether governments will step up with additional commitments. But in some senses, the bigger question is, what happens after Glasgow? Will governments come home and really kind of take their own commitments seriously and begin to implement the policies that we need if we want to prevent warming in the future?

[Jingle]

Host: Benjamin Thompson

Nature’s Jeff Tollefson there. We’ll put a link to Jeff’s news story about the IPCC’s report in the show notes.

Host: Nick Petrić Howe

That’s all for this week. If you want to keep in touch with us then follow us on Twitter –we’re @NaturePodcast. Or if you want to send us an email then we can be reached at podcast@nature.com. I’m Nick Petrić Howe.

Host: Benjamin Thompson

And I’m Benjamin Thompson. Thanks for listening.

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