Nature Podcast

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Adam Rutherford: This week, the formation of rare but powerful massive stars.

Mark R. Krumholz: When you add up the contributions from the small stars and the big stars, the big stars wind up providing most of the energy in the universe. So, massive stars are sort of the central players in galactic ecology.

Kerri Smith: A special report on a deadly disease from Zambia.

Michael Hopkin: Tracking the spread of malaria usually requires blood samples taken by trained professionals, but the researchers here have devised a way to detect the pathogen in saliva or urine meaning that malaria monitoring could be done by ordinary people in their home.

Adam Rutherford: And why all the world's current climate change policies will fail?

George Monbiot: While there are endless government policies for reducing the demand for carbon, there are currently none as far as I can discover anywhere on earth for reducing the supply.

Adam Rutherford: That's the view of journalist, George Monbiot on our podium later in the show. This is the Nature podcast, I'm Adam Rutherford.

Kerri Smith: And I'm Kerri Smith. First this week, we plunge into the oceans where we find Charlotte Stoddart pursuing some predators as they forage for their next meal.

Charlotte Stoddart: If you are a shark, then finding food in a vast featureless ocean is pretty tricky, especially when your target is too far away to see or smell through the surrounding seawater. So how do you do it? A new study shows that sharks and many other marine predators follow a search pattern that matches a mathematical relationship called a Lévy distribution. This pattern or Lévy walk consists of small clustered movements interspersed with longer range jumps into new foraging areas and it's the best strategy when looking for patchily distributed prey. Lead author, David Sims from the Marine Biological Association in Plymouth, UK told me how he and his team managed to track these movements in not only sharks but also tuna, cod, turtles, penguins, and seals. Nature 451, 1098–1102 (28 February 2008)

David W. Sims: We tracked them using sophisticated electronic tags, which recorded not only their sort of position, but also their diving behaviour and it was the diving behaviour that we used to look at the movements that the animal makes through the water column and by analyzing that in a particular way, we could get a window on the decisions that the animals were making.

Charlotte Stoddart: And what did you find then?

David W. Sims: Well, we were struck by the pattern of the movements, the dives that these animals took. There were often quite short dives that the animal undertook and these would be interspersed or its, sort of, intermittent with these long deeper dives and this struck us as being quite an interesting pattern.

Charlotte Stoddart: And the pattern that you are describing is called a Lévy walk. I wonder how this puts a search pattern relates to the distribution of prey. Did you look into that relationship too?

David W. Sims: We did yes, I mean that was one of the interesting things of this study, was that no one really had ever looked at Lévy walk-like movements of animals in relation to prey before. So for example, we looked at krill, which are our staple, sort of, food source, for lots of, sort of, predators in the ocean. And we looked at those patterns and amazingly we found that they were very similar in their, sort of overall form in terms of their density changes - so the amounts of krill or zooplankton per unit volume. So we hypothesized that these two patterns may be linked in some way.

Charlotte Stoddart: And what do you think the evolutionary relationship is between this link?

David W. Sims: In the study, we attempted to, sort of, try and reveal what the nature of this linkage might be and so we produced a computer simulation that would give us an idea of what the benefit and consequently the costs of foraging in a, sort of, patchy environment according to different sorts of prey distributions. So for example, we would have a Lévy-like world that the animals, the predators could forage through and we would have a random, sort of, world, a random distribution of prey patches and then we would get model animals that were Lévy-like movers and then we would have random movers. So we looked at the different combinations and these simulations supported the idea that Lévy-like patterns of movement would be best in a Lévy-like world.

Charlotte Stoddart: It's really striking that you found this Lévy-like behaviour in so many different species. What does that tell you?

David W. Sims: We think and we suggested in the paper that the animals are adopting similar sorts of decision processes. The decision process is what underlies the observed behaviour and so we think that because the animals are faced with similar problems, they have evolved the same rule in response to, sort of, patchy resource distributions.

Charlotte Stoddart: And how about humans; did our hunter-gatherer ancestors use Lévy walks?

David W. Sims: That's a really interesting question and there have been some suggestions that hunter-gatherers used this sort of movement pattern. There's been a study in Namibia for example on hunter-gatherer societies there and they seem to show these, sort of, short steps, much like the short shallow dives our animals showed, but then they're interspersed with these much longer distant movements to try and locate resources outside of their immediate environment and so there are parallels in the system. So it does look that the, sort of, scaling laws of marine predators that we've shown here operating in a complex world to optimally acquire resources, probably does relate to more universal patterns in very diverse animals, perhaps from microbes right up to humans.

Charlotte Stoddart: So, now that we know that so many different species of animal adopt this Lévy behaviour, what use is this knowledge?

David W. Sims: Yes, I mean that's a very good question, isn't it? There are two, sort of, immediate ways in which this could be applied. In fisheries for example, we don't know much about the movement patterns of fish and in fact in spatial models for fisheries at the moment, the movements are assumed to be random. Well, of course, if what we're finding is that they adopt these Levy-like movements, that's a perfect tool in which to parameterize some of these spatial models. Another example might be in robotics for example. Optimally sampling hostile environments such as active volcanoes or the deep sea for example or even other planets and in fact we have begun to look at the movements of the Mars Rover and in the initial exploratory phase, they do show these Levy-like patterns which again are resonant of the marine predators that we've been studying.

Kerri Smith: David Sims of the Marine Biological Association in Plymouth, UK, we quietly explore here in the Nature pod and whilst Charlotte was 20,000 leagues under the sea, Adam has been scouring distant galaxies. Adam.

Adam Rutherford: Yes, I've been looking up to the stars with Mark Krumholz at Princeton and Christopher McKee at UC Santa Cruz. They've worked out exactly the right cosmological conditions for the formation of massive stars. These giants are the dominant source of energy in the universe. Despite being rather rare, I asked Mark what defines a star as being massive and why they occur so infrequently? Nature 451, 1082–1084 (28 February 2008)

Mark R. Krumholz: Massive stars – there isn't a hard and fast dividing line, but roughly speaking they are stars more than about 10 times the mass of the sun and they go up to a 100 times the mass of the sun perhaps even a little more. As for why they are so rare, that's an excellent question and part of the reason for doing this research. We know that the typical star, the most common star, is a little smaller than the sun; its perhaps two-tenths the mass of the sun, half the mass of the sun, and they become increasingly rare as you go upward in mass and why the typical mass is what it is and why they become increasingly rare as you go up in mass is not completely well understood.

Adam Rutherford: Okay and you say, as well that massive stars are the dominant source of energy in the universe. Can you just explain that a bit?

Mark R. Krumholz: So, massive stars are very rare, but they are also extremely bright. The brightness of the star that's about 10 times the mass of the sun is several thousand times that of the sun. So, even though they are very rare, when one forms it's so incredibly bright that when you add up the contributions from the small stars and the big stars, the big stars wind up providing most of the energy in the universe. So when you look at a galaxy like the Milky Way, if you were to look at it from outside the Milky Way, most of the light you would see would be coming from newly formed young massive stars.

Adam Rutherford: Massive stars at the end of their life cycle, they turn into supernovas, is that right?

Mark R. Krumholz: That's correct. They are also responsible for producing most of the heavy elements in the universe. They are responsible for controlling the energy balance of the interstellar medium, the gas in between the stars and the galaxy. So massive stars even though they are very rare are, sort of, the central players in, you could think of it as galactic ecology.

Adam Rutherford: So, in your new paper, you have addressed the question of how and where these massive stars form and you've used a concept called column density, can you just explain what column density is?

Mark R. Krumholz: So, you can think of column density as telling you how thick something is along your line of sight. And an analogy might be, imagine taking a book and I'm shooting a bullet through it, all right and you ask if I look at the hole the bullet leaves, how much mass was in that hole before I shot my hole at it, if it was one gram and say the cross-section of the bullet was 1 square centimetre, well then the column density of that book was 1 gram per square centimetre, so that's what column density is. Its says if I punch a hole in something that's 1 square centimetre in area, how much mass will be in that hole.

Adam Rutherford: And your calculations in this paper, you've given a minimum threshold for column density for massive star formation.

Mark R. Krumholz: That's correct. We estimate that massive stars only form in clouds where the column density of the cloud is bigger than about 1 gram per square centimetre, which is incidentally about the column density of a 1 cm deep layer of water.

Adam Rutherford: Seems like a very high density for the universe.

Mark R. Krumholz: Well it is an extremely rare and high density for interstellar space and I should make clear the volume density we're talking about here is very high for interstellar space, but still more or less vacuumed by earth standards. The volume density in a region like this might be as much as a million to a billion atoms per cubic centimetre which is more or less better than the best vacuum chamber on earth.

Adam Rutherford: So now you can predict, actually where massive stars will form?

Mark R. Krumholz: That's the main idea of the paper that massive stars can only form in clouds where the column density is greater than about 1 gram per square centimetre and the underlying physical reason for this is that when an interstellar gas cloud is starting to collapse and to make stars, normally these clouds break up into pieces that are about the mass of the sun or a little smaller, and the reason they do that is because the size of a piece is controlled by the competing forces of pressure and gravity. Pressure wants to smooth things out; gravity wants to clump them together. Normally, the balance between pressure and gravity in an interstellar gas cloud makes things better about the mass of the sun or a little smaller, but in very high column density regions, once you start making little suns, they can heat the gas up a lot and so the pressure becomes greater and that balance between pressure and gravity begins to change instead of making things that are about the size of the sun or a little smaller, you can all of a sudden start making much, much larger things. So that's the underlying physics behind of this result.

Kerri Smith: Mark Krumholz from the Department of Astrophysical Sciences at Princeton, sharing some news of how a star is born and that paper along with all the rest covered in the podcast this week is available at

Adam Rutherford: Now its time for the Podium, this week environmental journalist, George Monbiot.

George Monbiot: I can make the following statement with confidence. All the world's climate change policies are currently destined to fail. They will fail despite new technologies, the better deployment of renewables or even a massive improvement in energy efficiency. How can I make such a sweeping statement? How can I write off the efforts of all the brilliant people seeking to reduce our emissions of greenhouse gases? Because, while there are endless government policies for reducing the demand for carbon, there are currently none, as far as I can discover, anywhere on earth for reducing the supply. The global policy for preventing runaway climate change seems to me to work like this; encourage companies to extract as much coal, gas, and oil from the ground as they can and pray to Almighty God that they won't be used. Take the government of the United Kingdom. In an effort to prevent dangerous climate change, it has imposed a legal obligation on itself to cut carbon emissions by 60% by 2050. It's too little too late, but still a radical piece of legislation. Yet, the same government intends and I quote "to maximize economic recovery from remaining coal reserves." It has subsidized the coal industry over the past 8 years to the tune of 220 million pounds. It also has a policy I quote again of "maximizing the UK's existing oil and gas reserves". In December the treasury minister explained the purpose of a new tax break for oil and gas companies working in the North Sea. It was she said "to make sure, we are not leaving any oil in the ground that could be recovered." The same policies are pursued by all governments. If they have reserves of fossil fuels they'll encourage companies to exploit them. There are, I think, three reasons. One is that drilling and mining generate lots of tax revenues another is that they keep both the corporate and labour lobbies happy as they produce profits and jobs, the third is that the idea of leaving a valuable resource unexploited conflicts with everything we've been brought up to believe about what progress means. Recently Jan-Peter Onstwedder, who until December was BP's head of Risk Management, calculated that proven oil gas and coal reserves, those ready to be extracted will produce 703 billion tons of carbon dioxide when they are burnt. In 2006, the British government published a report entitled 'Avoiding Dangerous Climate Change' and in it Malte Meinshausen, an emission scientist, shows that we must produce no more than 470 billion tons of carbon dioxide by burning fossil fuels if we are to have a chance of preventing more than 2 degrees of global warming. This assumes that we can radically reduce the amount of CO2 produced by agriculture and deforestation. If not the total comes down to just 370 or even 320 billion tons, roughly half the amount waiting to be released from proven reserves. So why are we still prospecting for fossil fuels when we already have more than we can safely burn? The reason is that governments are pursuing two completely different policies. One is to encourage the production of fossil fuels; the other is to discourage their consumption. Until this conflict has resolved our carbon cutting programs will fail. No company extracts fossil fuels as a hobby. Once removed from the ground, they will be burnt whatever demand side policies say. May I propose a new kind of carbon capture and storage, which is geologically stable and guaranteed to work? Leave the damn stuff in the ground.

Kerri Smith: That was George Monbiot and his book Heat: How to Stop the Planet Burning is out now. And the latest climate change news and research is available at Nature Reports Climate Change that's at


Adam Rutherford: This is the Nature Podcast. Coming up shortly, a new drug target for treating psychosis, but first a news feature in Nature this week highlights a new way to monitor and help wipe-out malaria. Mike Hopkin sent us this report from Zambia.

Michael Hopkin: Its night time in Zambia and like everywhere in Southern Africa, the darkness buzzes with insects, but here in Macha, four hours drive from the capital Lusaka, people are most concerned with one of the quietest yet deadliest insects, the mosquito, or more specifically the malaria parasite that it transmits. During peak season Macha's Hospital sees dozens of cases everyday and despite the remote location, researchers here are at the forefront of the battle to improve our understanding of this ever present enemy. Tracking the spread of malaria usually requires blood samples taken by trained professionals, but the researchers here have devised a way to detect the pathogen in saliva or urine, meaning that malaria monitoring could be done by ordinary people in their home. The Malaria Institute Scientific director, Sungano Mharakurwa explains. Published online 27 February 2008 Nature 451, 1047–1049 (2008)

Sungano Mharakurwa: The fact that you have to use needles and sharps and the risks of HIV infection - it presses constraints, the use of needles and sharps requires specially trained personnel and bio safety packages to make sure that the sharps are properly containerized once they are used and properly disposed off as well, but if you can use saliva or urine, you don't require any of those things; in fact the patients can collect those things themselves and the ultimate aim is to be able to diagnose malaria at household level and those sort of things can be done if you have urine and saliva.

Michael Hopkin: The work at Macha is just part of the countrywide campaign to cut levels of malaria in Zambia and monitoring the presence of the malaria parasite is one of the main challenges says, John Miller, who works for an internationally funded project called the Malaria Control and Evaluation Partnership.

John M. Miller: Malaria itself is a difficult disease to measure. Well, just because it's a rural disease, a disease that largely affects poor people and to get out into communities to be able to see whether interventions are available and whether disease is coming down, it requires an extra effort. Certainly, we do our best to work with the ministry and partners to measure what's going on in terms of interventions and progress in reducing the disease burden.

Michael Hopkin: Sungano Mharakurwa and his colleagues are aiming to put together an ambitious proposal to remove malaria completely from the countryside around Macha. They'll use all the existing methods at their disposal including bed nets, insecticide spraying and anti-malarial drugs to try and develop a winning strategy.

Sungano Mharakurwa: The way to go about this is to employ everything that we have at this moment to control malaria, that includes indoor that is your spraying and use of effective therapies, so the idea is to find the most cost-effective way of doing it and if we can show that it can be done in an affordable and sustainable manner, yeah we would then roll it to a district level and probably to provincial level. I think if it can be shown that it can be done, chances are, the entire nation will want to adopt it and not spend too much time in hanging around when you know you have a way of getting rid of malaria.

Michael Hopkin: So much expertise is being developed here. The Public Health Organizations are hoping the Zambian model could even be rolled out to other African countries. John Miller's colleague Judith Robb-McCord is focussed on sharing Zambia's experiences.

Judith Robb-McCord: Right now we're working with Ethiopia, and we are going to be talking with Tanzania and we're also hopeful for a relationship in Malawi and we're looking at some targeted support potentially in Zimbabwe. We will also obviously be working here in Zambia with the National Malaria Control Centre as well. Basically what we see from the world of malaria and the technical approach is that there is evidence behind these approaches and so we are confident that the strategies that have been described as part of the National Malaria Program here in Zambia are of the same strategies that will work in Tanzania or in Zimbabwe, for example. They may have a different spin in terms of the adaptation in particular countries, how they are delivered. We're relying on the theory of practices here will work in other countries.

Michael Hopkin: And that will help to save the hundreds of thousands of Africans, most of them children who die of malaria each year. Macha's District Chief Leonard Moono Munansangu told me what it would mean if all those young lives could be saved.

Leonard Moono Munansangu: The children are the next world, is the next government, so if we lose the children then we have lost the nation, the children, mostly are the people who are greatly affected, we will need to save the lives of children.

Kerri Smith: That report's from Mike Hopkin. Finally this week, a team who set out to study how LSD works might have hit upon a new drug target for schizophrenia. Stuart Sealfon of Mt. Sinai School of Medicine in New York and his team set out to study the effects of hallucinogenic drugs on the brain and found that they act on a pair of receptors called 2-AR and mGluR2 tangled into one molecular complex. Current drugs used to treat schizophrenia act on either one of these receptors alone; one of which responds to glutamate and the other to serotonin. Before now, no one knew that they form this complex, but the idea is that new drugs to combat the psychotic symptoms of schizophrenia might take advantage of this new result and acts on the complex as a whole rather than either receptor on its own. Here is Stuart Sealfon. Nature advance online publication 24 February 2008

Stuart C. Sealfon: This entire line of research did not start as schizophrenia research it started out trying to understand how hallucinogens like LSD work. What we found is that two receptors that were not previously known to be associated, a glutamate receptor and a serotonin receptor form a complex and this complex appears to be important in the effects of hallucinogenic drugs like LSD and in the effects of different classes of antipsychotic drugs is to treat schizophrenia.

Kerri Smith: So we knew about these two receptors and what activates them and we knew about their links to psychosis. So why hasn't it been obvious before that they interacted with each other in this way?

Stuart C. Sealfon: First of all they are actually related receptors in terms of their G-protein coupled receptors, they have that relationship, but they are in fact in different gene families, they have no sequence similarity with each other and previously receptors that form complexes across families like this have not been observed.

Kerri Smith: And what's the idea behind them having formed this complex then, what is that allow them to do?

Stuart C. Sealfon: In the function of the brain one would like more switches and more choices available and in forming a complex it allows them to form a very complicated switch, so that the effect of glutamate signalling at the complex can affect serotonin signalling and the effect of serotonin signalling can directly affect the glutamate signalling.

Kerri Smith: And so take us through briefly then how you managed to get your results? How you found these two receptors interacting?

Stuart C. Sealfon: We performed a variety of experiments for example using biophysical techniques where we labelled each of the components with fluorescent probes and looked for transfer of light between them that only occurs when they are very close to each other and we performed pharmacological studies in which the introduction of a compound that binds to one component of the complex affects the binding of compounds to the other component of the complex and these things can only occur if they are in fact physically in contact with each other.

Kerri Smith: After you had done these initial experiments with the complexes themselves to look at what actually happens in a disorder like schizophrenia, you looked to post-mortem brain samples from schizophrenic patients who had never been treated with drugs during their lifetimes and what did you find?

Stuart C. Sealfon: We found that the level of binding for the serotonin receptor was increased and the level of binding for the glutamate receptor was decreased and also the mRNA for the specific subtype of the glutamate receptor that we can't differentiate by binding the mRNA for that subtype was also decreased.

Kerri Smith: So, compared to normal brains, schizophrenic brains had different amount of these two receptors, so fairly sound evidence then that they are involved in these psychotic symptoms.

Stuart C. Sealfon: It's certainly a very striking observation that relates a number of different findings at the structural level and at the animal study level together.

Kerri Smith: Thinking of the implications then of this new result -- could we use this to develop new types of antipsychotic drugs, better drugs for schizophrenia perhaps?

Stuart C. Sealfon: I think that that's possible, we can first of all now use the effects on the complex that seemed to be associated with antipsychotic properties to screen for drugs that are, for example, specifically affecting hallucinogen signalling at this complex and in addition we have the possibility of identifying very novel classes of drugs that are in fact interacting with the complex but aren't specifically interacting with either of the components alone.

Kerri Smith: Stuart Sealfon of Mt. Sinai School of Medicine in New York.

Adam Rutherford: That's all for this week's show. Our Sound of Science comes courtesy of Stéphane Douady of the French National Research Agency CNRS. He has trekked all over the world recording the sounds of peculiar dunes that sing as sand avalanches down them. His CD Le Chant Des Dune is out next week, when the sand moves, the grains rub against each other at different frequencies and the resulting sound waves create these rumbles. I'm Adam Rutherford.

Kerri Smith: And I'm Kerri Smith, thanks for listening.

[Sound of Science]


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Kerri Smith: First you need to listen to the following three sounds of science taken from the podcast archives from autumn last year and then go to our web site that's and follow the link at the very bottom of that page. Here they come.

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