Scientist Stories: David Haig, The Huddler's Dilemma

OK, so let's get going. Here are 3 pictures that have something in common. So I'm thinking pink here. These are some blue— OK, these are some blue tit chicks sitting in a nest begging for food. These are a nest of mouse pups. These are a set of young bats all huddling together. The three of them have a number of things in common. One of them is that they're pink and naked. They've also got their eyes closed. These are highly autretial, relatively helpless offspring. The other thing that they're doing is they're engaged in a very simple cooperative behaviour. They're sitting there helping to keep each other warm. And you see all sorts of particularly small warm-blooded creatures do this. When it gets cool, relatively speaking, they get close together. The advantage of doing that is twofold. One of them is if you're sitting next to a warm body, you're not losing any heat across the surface where you're in contact. If you're at the same temperature, there's no net heat transfer between the two of you, and so you're only losing heat to the environment on the exposed— oops— on the exposed outer surface. If this were an enclosed nest, there's an additional advantage that your individual furnaces that are generating heat heating the surrounding air, so you're raising the ambient temperature which is also reducing the rate at which heat is lost. These examples are somewhat different. These are probably the chicks of a single mother who has laid a clutch of eggs. They may have multiple fathers in that clutch, and the same could be said with this litter of mice. They would have the same mother, but there might be some degree of multiple paternity. Whereas the bats that we're looking at here, bat mothers only give rise to a single offspring at a time. This is a harem-forming bat, so they might be related through their fathers and not through their mothers, but these are different sorts of kin groups that we're looking at in these huddles. This is just a picture of slightly older bats. They're starting to develop fur and develop pigmentation, but once again getting really, really close together and not doing much, but there's substantial energy savings there. So let's move to a famous example of huddling. So these are— this is a huddle of emperor penguins. So these are male emperor penguins. They've got an egg sitting on their feet which they are incubating and transferring heat to that egg, and particularly in cold temperatures they get together in these massive huddles. It can be below freezing outside and you can get up to 70 degrees Fahrenheit sitting in the centre of these huddles. Some interesting things here. Notice that they've all got their backs to the outside and their fronts are directed towards the inside of the huddle. I'm not quite sure why that is. They're Eggs are sitting on their feet here, so it may be something about maximising insulation for the eggs, but I'll show you some chicks doing similar behaviours later. Okay, so a little bit about emperor penguins. The adult males, while they're incubating their eggs, fast for a period of 4 months in Antarctic winter conditions. And during that period of time they've got to maintain their bodies and keep their bodies warm, working off basically fat reserves that they establish before the start of incubation. When the chick hatches out, they've got to regurgitate food to the chick to keep it fed until mum comes back. She's been out at ocean collecting more fish and then there'll be a transfer of the chick from father to mother. It's been calculated that without huddling, a male emperor penguin cannot survive the winter. An isolated emperor penguin male at those temperatures by his own would run out of fat reserves and he would be unable to maintain high body temperatures. So they can get together in huddles of about 5,000 males huddled together in bad weather Huddling reduces weight loss by 25 to 50%. So the weight loss is the measure of how much organic molecules are being burnt. So this is substantial energetic savings from huddling behaviour. When the wind is blowing, the windward males, so the males who are exposed to the strong winds, move towards the flanks of the huddle. Exposing the next rank of males behind, and so the huddle tends to sail before the wind, slowly moving in the direction of the wind as there's this constant churning of individuals in the huddle. It's been described as everybody taking their turn on the outside. There are other explanations of the phenomenon. So this is sort of diagrammatically. Here are the winds blowing. The exposed coldest individuals here. Move to the outside exposing the next rank who then move to the outside and so you get this constant churning behaviour during strong winds within the huddle. The churning of the huddle has been described as cooperative behaviour in which each male takes his turn on the outside but I think a model of self-interest in which each male moves to where he is warmer getting away from the strong winds is adequate to explain taking turns on the outside. So it's sort of an emergent property of each individual maximising their own body temperature results in everybody taking a turn on the outside. Now the huddling behaviour of adult male emperor penguins, in a sense you can think of it as being neotenic. It's similar to the formation of what are called creches by chicks in other penguin species. So most adult penguins have a well-defined personal space. They don't like another penguin coming close to them. Adult emperor penguins get up close to other adult emperor penguins because they're benefiting from their body heat. But you see the same behaviour in chicks of other penguin species. This is an emperor penguin picture. So here we've got one of these crèches of emperor penguin chicks.. So they're all getting together in the creche. Notice the same pattern, backs towards the outside, beaks inside. Here we've got an adult who's just getting down with the chicks here, probably to keep warm with all these nice fluffy bodies. The sister species of the Emperor Penguin is the King Penguin. So there the adults keep their distance from other adults, but here we have a crèche of emperor penguin— sorry, of king penguin chicks. Crèching behaviour was traditionally described as a defence against predators so that they get together and they can defend against skuas and things like that. So that's probably one of the advantages, but I think the thermal benefits are also very important, probably most important. Some of the evidence for that— so these are king penguin chicks. If this was primarily about defence against predators, rather than the beaks pointing inwards, the beaks should be pointing outwards. So I think the predominant thing that's happening here is a sharing of body heat. Now here is— this was a datum that I— at first I found a little mysterious. So this is taking cloacal temperatures in male emperor penguins.. So they put a probe up their external orifice and they're getting an internal body temperature. And the body temperature of isolated males was about 38 degrees Celsius, whereas the males in the huddle actually had a cooler body temperature. And my first assumption was these ones would be warmer than those males. But what this is reflecting, this is actually the energetic saving they're making. These males in the huddle So this is a core body temperature and there's a temperature gradient from the core to the outside and if they're sitting in a huddle to stop their outside from freezing, they can turn down the internal furnace because they're needing to expend less energy to keep their body warm. So this is actually a measure, this difference, of the thermal advantages of huddling. Now from this study, there was a French group that put probes into 4 male emperor penguins during the breeding season to look at their body temperatures. So what these are showing here, this is at the, when the males and females get together on the breeding ground. This is looking at the core temperature of the male. There's interestingly a decline in the male's body temperature during the courting period. He starts off warm and gets cooler, and then the vertical line is the point at which the female lays the egg which is immediately transferred onto the male's feet. And at that time, the male immediately raises his body temperature and maintains a high body temperature. And you can see that happening here. The egg is transferred You're maintaining a stable body temperature. This one, the egg was transferred somewhat later in the season, but after the transfer, maintaining a stable body temperature in the male. Now this is like Sesame Street. One of these things is not like the other, and which one is it? It's clearly this male here who transfers the egg. He starts off maintaining a high body temperature and then suddenly his core body temperature drops and also becomes very much more variable. And this male, at that point, he lost his egg. So he's no longer needing to provide heat to incubate the egg, and so the thing that he does is he immediately drops his body temperature. And I suspect that what he was doing here is that he's allowing himself to be passively warmed within the huddle. So he's sitting with all these warm bodies around him. He reduces his own heat production, which he doesn't need to be keeping his egg warm. And for no energetic cost he allows himself to be warmed by the bodies around him. And I would love to know what's happening in the variance here. Is these really low core body temperatures when he's sitting in the middle of the huddle and he's got a lot of warm bodies around him and he's maximizing his heat? Unfortunately, we don't have that information from this dataset. Okay. So that was the one male who lost his egg. Now for our biblical reading. Our biblical reading of today comes from Ecclesiastes. And this is a wonderful explanation of cooperation. If two lie together, then they have heat. But how can one be warm alone? So this is the getting together to sleep, to keep warm, is a simple cooperative behaviour. That is partly what these pups are doing here. Interestingly, these huddles of mouse pups are doing a similar churning behaviour that you see in the emperor penguins. The best place to be is in the centre of that huddle with a lot of warm bodies around you, so mouse pups tend to try to burrow into the huddle and the ones that are sitting on the top are cooler and so they're constantly trying to get into a favoured location. So here's a study from Bruce Alberts, 2012, just looking at huddling behaviour in laboratory mice. Now what this is looking at here, this is ambient temperature, so we're starting off at a high temperature, 36 degrees Celsius, coming down to cooler temperatures at about 22 degrees Celsius here, and then the temperatures are rising again. And these pup units is just a measure of what is the circumference of the huddle measured relative to the circumference of a single mouse pup. So this is a measure of whether they're, they're, the individual pups are far apart or they're getting closer together and reducing their surface area to volume ratio. So one way to think of a huddle is what it's doing is reducing the exposed surface area for the total volume of pups. Pups. So this is at postnatal day 2. At that time, the pups have very little ability to move, and as the temperature gets colder, at cold temperatures there's a little bit of a reduction in this pup unit measure. That's largely that the pups are just pulling their legs, you know, they're trying to crunch up to keep warmer. But at postnatal day 4 and then postnatal day 8, you see this regulatory behaviour. That as the temperature gets colder, everybody's moving closer and closer together and reducing the perimeter of the huddle, and they're getting energetic savings from that. And then as the temperatures get warmer again, they're moving apart. And so this is sort of like a homeostatic mechanism of regulating communal body temperatures. As things are warm, you're further apart. As things get colder, they move closer together. This is just some more data from that paper. This is postnatal day 4 of groups. So we're looking at the huddling groups here. Not much going on. So the green is the perimeter. Not a lot going on there. But at postnatal day 8, you've got this getting closer together when it gets cold and then moving apart as it gets warm. The black line is the difference in temperature between the interscapular region of a mouse and the rump of the same mouse. So the heat-generating tissue in a mouse pup is a deposit of so-called interscapular brown fat, which is just a furnace which burns fat to generate heat. That's what's keeping the pup warm. And so the difference between the temperature there and on the rump of the mouse is a measure of how much heat they're generating. And so you can see as things get colder, they switch on their furnace to generate heat, and then as the temperature's rising, they switch off their heat production. Now, heat generation by a pup, they're drinking milk from mother. The milk that they get from mother, some of it can be used to help keep themselves warm, can be burnt as fuel,, and other milk can be put into growing bigger bodies or competing for milk or other sorts of resources. So heat generation by a pup reduces the resources it has available for growth and competition with its sibs. So there's an individual cost of generating heat. They get some sort of personal benefit from that heat production, but the other individuals sitting around them are also sharing in that heat. Pups benefit from the generation of heat by their littermates, so we're looking at a classic public goods problem with the danger of free-riding. So the maximal growth would be achieved by free-riding on the heat generation of your siblings, being a cool pup in a warm puddle. You're putting— in a warm, not a warm puddle, a warm huddle. You're putting all your resources into growing larger, and you're being kept warm by your siblings. The problem is that this relation is symmetrical, and you expect that public goods, in this case heat, tend to be undersupplied at equilibrium. And I want to ask the question, how much should a pup contribute to the communal heating bill in this public good situation? And so I'm going to introduce you to a family of mice. Here is mum. Sorry, I used the Australian spelling of mum rather than mom. Oops, come back. Oops. And what I'm doing here, I'm just taking a hypothetical locus. Here is the two alleles at a locus in mum represented by a blue and a red dot. And I'm assuming she mates with two males, so she's producing a mixed paternity ladder. Litter. This is dad 1 and dad 2, and I'm representing the 2 alleles at just an arbitrary locus in those 2 males by different colored dots. And so we're wanting to look at the distribution of maternal and paternal alleles in the resulting litter. So she produces a litter of pups. All of them either get, say, the red allele is the allele she got from her father, so 50% of the pups get a red allele. 50% get the allele that mum got from her mother, the blue allele, distributed across them, and then they all get one of the 4 different alleles coming from the 2 fathers. So here they are sitting in a huddle, and I'm just going to concentrate on the 2 alleles sitting in this central individual here. So the red allele is the one that this pup got from its mother, and the purple allele the allele that this pup got from its father. So pull the others around and you can immediately see there's an interesting asymmetry here. So suppose we have a mutation that increases heat production and suppose that mutation is a maternally inherited mutation in the red allele. In that case, 50% of the other members of the litter who are gaining the benefit of that increased heat production are carrying a copy of the same allele. And moreover, if this pup increases heat production because of its maternal allele, it's going to be getting heat contributed to it by roughly speaking 50% of the other members of the litter. However, for the paternally derived gene, because of multiple paternity, the other members of the litter that are carrying a copy of the same gene are less than 50%, so the benefit of donating heat to other individuals genetically is diminished, and also this individual can only count on getting added heat from one other member of the litter in this case. So the outcome of this is that if you have multiple paternity litters, you expect that genes of maternal origin solve the public goods problem better than genes of paternal origin, and so maternally derived genes will be running the furnace at a higher temperature than paternal genes within the same mouse pup. So the prediction is that paternal genes should set the thermostat on their furnace at a lower temperature. Now as I mentioned earlier, the principal heat-generating tissue in mammals is a tissue called brown adipose tissue. It has uncoupled mitochondria, so the mitochondria are just burning fuel. And rather than pumping protons, the protons are pumped out, they immediately leak back across the membrane, and by the, what is it, the first law of thermodynamics, that's because that's just being generated ultimately as heat being produced. Now, here is an interesting paper. So this was a paper looking at the differentiation of a preadipocyte here into brown adipocytes. So this is the different— this is the recruitment of heating units within brown fat tissue. And this is one of those classic spaghetti diagrams. There are all sorts of genes sitting here that are interacting in complex ways determining whether you get more brown fat cells. And I looked at this figure and there's something really interesting in this figure. And that is these two genes here, PREF1 and NECTIN. So they're both inhibiting, they're having a negative effect on the differentiation of brown fat cells. So they're inhibiting the production of heating units. And these two genes are known to be imprinted genes. These genes are only expressed when paternally derived and not expressed when maternally derived. When maternally derived. So this is saying genes of paternal origin in this brown adipocyte are actually inhibiting the production of the energy-producing heating tissue, consistent with the expectation that genes of paternal origin are running furnace at a lower temperature. So PREF1 and Nectin are paternally expressed genes that inhibit differentiation of preadipocytes into brown adipocytes. Once the cell has become a brown fat cell, it needs to receive sympathetic innervation to switch on the heat production. And so this is done through beta adrenergic receptors. So you have norepinephrine coming from the sympathetic innervation, binds to that receptor. It leads to the release of GS-alpha. This is a G protein and then by a downstream effect, GS-alpha switches on non-shivering thermogenesis to switch on heat production. GS-alpha— oh, we've got a— that's an alpha there— is preferentially translated from its maternally derived allele in brown adipose tissue. So now we have a maternally expressed gene which is responsible for switching on heat production in those cells. So that allele, when it's paternally derived, is not switching on heat production. When it comes from the mother, it is switching on it. And so once again we're having this as a third gene, but now it's a maternally expressed one. The same consistent pattern: genes of maternal origin favouring more heat production, genes of paternal origin less heat production. Okay, so a little bit of behavioral ecology now about rat huts and rat pups in huddles. I'm having a lot of spoonerisms today. So this is classical behaviorists now looking at rat behavior. So the intervention they're using here is that if you give a rat pup a beta blocker, you can switch off the signal to switch on heat production. So you can force a rat pup not to be generating heat. So control huddles are stable at 15 degrees Celsius. So it's cold temperature, these are control rat pups, they get together and they form this nice stable huddle being close together. However, if you block thermogenesis with a beta blocker, in some of the pups within that litter, the huddles break up at that temperature. If you block thermogenesis in some of them. So this is a situation where some pups are warm and some pups are cool and they don't form stable huddles. However, if you give all of the pups beta-blockers, so none of them are generating heat, you get a nice stable huddle. They get together to reduce their heat loss, reduce their exposed surface area. Area. And so this instability in the huddle is actually an outcome of being close to an individual who's cooler than you. So having temperature differences within the huddle is not conducive to forming this stable interaction. So what I think's happening here is that rat pups are very sensitive if they're next to a body that's cooler than themselves. And so in this situation, The nice warm brown color, everybody's generating heat, you've got this nice stable aggregation. If nobody's generating heat, you've got a stable interaction. But when some individuals are warmer than others, you can't form that stable huddling interaction. I'm really conscious actually when I'm lying in bed next to my wife. I'm incredibly sensitive who's warmer than who. We really are sensitive of who has got the warmer body temperature than others. I suspect I'm more sensitive to relative body temperature than I am to the temperature of the blanket or the temperature of the pillow that I'm sitting next to. I now want to work— look at some adult huddling behaviour in group hibernators. So I'm going to now look at group hibernation. Now during hibernation, The major costs of hibernation— so there's energy savings at cold temperature by just shutting down metabolism so you're not having to burn a lot of fuel to stay warm. However, during hibernation, individuals have to periodically rewarm for reasons we don't quite understand. So they don't go through the whole winter at a low body temperature. They periodically have to rewarm, and they're the major energy costs of hibernation during the winter. It's the time when you're warming up your muscles to come up to high body temperatures. Some people have suggested that they need to sleep, and you need to sleep when you're warm. And so what happens in rewarming? First of all, you switch on your brown fat, which is generating heat that warms up the muscle. Once the muscles are warmed, the muscles can start to shiver, which is an alternative way of generating heat and so the whole body temperature is raised in that way. Now if you're huddling in a group, if you all do this in synchrony, you're sharing the costs of rewarming. But you can imagine an advantage to a laggard who doesn't switch on heat production. It's sitting next to a warm body that's burning fat to warm itself up and some of that heat is helping to warm up your muscles and so you've got an energetic saving by reheating yourself, starting reheating after some other individuals. Okay, so laggards are warmed passively by neighbouring individuals. So let's look at a group hibernator. This is the Alpine marmot. So it lives in high altitudes in Europe. They get together in social groups to hibernate during the winter. So hibernating groups of up to 20 adults plus a litter of juveniles sometimes present. So this is a breeding pair, a set of subordinate adults, and then the juveniles who are believed to be the offspring of the breeding pair. Now it's been shown that the chance of marmots surviving the winter as a group goes up as the group size goes up, and there are some groups of marmots that just freeze during the winter and don't survive. Marmots need to maintain their body temperature above 2 degrees Celsius so that as a group they're sitting just above that sort of temperature. They're doing minimal heating within a burrow to keep their bodies from freezing. The juveniles, the young individuals, are found in the centre of the huddles. So they're in the privileged position. They've got these warm adult bodies is all around them, helping to keep them warm. As I've mentioned, winter mortality decreases with group size in absence of juveniles. So when you've just got a group of adults together, the larger the group of adults, the higher their chances of surviving the winter as a group. That's the benefit, the heat benefits of being in a larger group. Now when juveniles are present, the breeding pair lose more weight during the winter if juveniles are present. So they're burning off body fat reserves to help to keep the juveniles present. Older brothers also lose more weight if juveniles are present. So they're contributing to keeping the juveniles warm. It's just possible that they're not older brothers. So there's suggestion in marmots that sometimes these older individuals will mate with their, have been observed with mating with mothers, so it could be an incestuous system. So these might have some chance of being the fathers of the juveniles. But the kicker is that unrelated subordinates, so unrelated individuals who have joined the group, lose less weight if juveniles are present. So the increased heat production by the parents to keep their offspring warm is being exploited by the subordinates to reduce their heat production costs. Actually, let's forget about that one. Most solitary nocturnal primates, they were described as solitary because they forage alone. The time they get together as a social group is during the day when they want to sleep together and they're gaining thermal benefits at that time. I now want to move back to those pink offspring that we were seeing in the earlier pictures and ask the question, Why be naked? So, if, you know, at first thought, if you're trying to keep warm, why not have insulation, feathers or fur sitting around you? And here we've got a cooperative mammal, the naked mole rat. What's going on? You know, why have these animals lost their insulating cover of fur? Okay, so, and the reason why at first sight being naked appears a little paradoxical is that insulation reduces the body's heat loss if an individual is generating her own body heat. So if you've got an internal furnace working, you put on a fur coat or a feather, a down jacket around you to reduce the heat loss to the environment. And we have good examples of that. Here we've got a young emperor penguin chick with a big down coat to reduce its heat loss to the environment. A hooded seal pup, once again very thick fur coat there to keep heat inside. However, insulation— oops, sorry, back. Oh yeah. Insulation reduces heat transfer if an individual is donating heat to others. So if you're wanting to be an altruist and help to keep another individual warm, having an insulating surface between the two of you is reducing the rate at which you're transferring heat. And a simple example of that, this is an Adélie penguin. There are the eggs sitting there, and a lot of birds, when they're incubating, develop a brood patch. So here you have vascularized, heat-radiating tissue which is specialised for transferring heat from here to the eggs, and to maximise the efficiency of that is you get rid of the insulating layer of feathers. Insulation also reduces heat gain if an individual, rather than generating its own heat, is taking heat from other individuals, either from its siblings or its parents, and that's what I think's going on in these naked offspring that we're looking at. So here are some examples. This is a parrot chick. Once again, eyes closed, so this nakedness tends to be associated with relative helplessness of offspring, so-called autriciality. Does anybody know what this is? This is a giant panda. Okay, this is the baby of a giant panda. Once again, relatively thin covering of fur. Bears and pandas are fascinating for other reasons. I'd love to talk about that later. And then this curious neonate here. So humans are described as being secondarily altricial. So we've become this pretty helpless creature. Dick Alexander once described it as a convergent evolution with maggots. But we're relatively helpless and curiously, going with that secondary autriciality, we have evolved nakedness. But in this case, it's not only nakedness in the offspring, but it's also in the parent. And I'm just wondering whether there's something to do with cooperative heat transfer associated with the human evolutionary loss of— Fur. And with that, I normally don't give thank you slides, but in this case I want to give warm thanks to my close friends and ask you to think about why we use these particular terms. And with that, I'd be happy to answer questions. Sure. Yes? So from what you said, you think that penguins should have thinner coats, feathers on their stomachs than their backs. Is that true?

They have, I don't know the front and the back side of that, why.

You're right. I hadn't thought, yes, you are right.

They're wanting to get, increase the efficiency.

They definitely do have, so in incubating the egg, they have a big flap that goes over the egg, and so there isn't much insulation between the male's circulatory system and the egg, but there is insulation towards the outside.

But yeah, I'll see if we've got data on that. Yeah.

Hey.

Hey. Really interesting talk. Are you familiar with Richard McFarland's recent work with vervet monkeys in South Africa, looking at how personality and more extroverted monkeys tend to have different positioning in sleep huddles? And they've done internal probing on that they lose less weight over cold periods of time than do less socially integrated extroverted individuals. Oh, that's cool. I'll send you that paper. It's really exciting stuff. —and they're also looking at how different patterns of grooming can change how hair lays for better thermoregulation.

Yeah, yeah. I really— so the last thing, warm thanks to my close friends.

It's remarkable how often we phrase how we feel towards other individuals in terms of thermal benefits. And I suspect that actually in the origin of sociality in mammals and birds, that but the thermal benefits have been a really important part of getting together as group members. Yeah? Do you have any idea how fever might be involved? Fever in response to pathogens is involved in any of this? So fever is generally thought to, in some sense, high body temperatures are helping to control the pathogen. I haven't thought of the social implications of that. Do you know of the malarial treatment of syphilis? No. So the Nobel Prize got from that, so before antibiotics, there was an effective treatment of syphilis so that you gave people a malarial infection to get their temperature above 104°F, which killed the the spirochete that causes syphilis and you control the malarial infection at the time with chloroquine. And so it was sort of an effective but sort of high-risk treatment to get rid of syphilis.

So, we just had a talk a couple— last week by Charlie on sleep as well and I blurted out in a moment there that I had spent 2,000 nights sleeping with hunter-gatherers. So I'm just going to blurt that out again and say that I'm really on board with a lot of what you're saying, the importance of thermal transfers. The first night I slept in the forest in southern Paraguay, it gets damn cold, -4 centigrade. Everybody's naked. I complained and complained. The next day about all the people who were on top of me all night.

After about 20 nights, I stopped complaining.

You were getting close. But I was wondering about hairlessness and cooperative breeding in mammals. And there's a huge interest right now in where cooperative breeding seriously emerges in the trajectory of human evolution. And could we look at genes that code for hairlessness and calculate when that mutation took place and use that as a guess of when cooperative breeding became really important in human evolution?

I think it would be a fascinating thing to know when hairlessness came in and, you know, if you could show a correlation between the two of them, but I don't know of any data of that. And of course, you know, there are other, you know, the question of human hairlessness, there are endless hypotheses about what that's about, but I don't think there's been much consideration of the thermal side of it. And just if you think about, you know, as babies and young children are more hairless than adults, but you know, once we go through puberty, we do get a bit more body hair and something. So for me, it seems that being really, really naked was possibly originally an adaptation of our neonates and then it's been transferred towards adults. You might know that a foetus has actually got a cover of so-called lanugo in utero, but it sheds that covering in the uterus before birth, and so you have these babies that are born, you know, with this completely naked skin. And, you know, there's now a lot of concern about, you know, skin-to-skin care of neonates, and in fact Adult, you know, putting a baby close to your skin is a remarkably effective way of raising its body temperature.

Yes, here, Athena, Athena, or this one first. Hi, actually on that subject matter that you just mentioned, I was wondering if you have any insight to the psychological development that could be associated with that, or if there's any kind of correlation with skin-to-skin contact and, you know, the psychology of a child's development in terms of like a parent with, you know, an infant.

It's not really my area of expertise. So there's the famous Harlow experiments of, you know, monkey, what they were, rhesus monkeys that were raised in cages without mothers and they were given choice of a wire monkey that was providing milk and a furry monkey to cling to.

These were models.

Now the one that they chose to cling to had a light bulb sitting in it and was actually warm, whereas the wire monkey model that was providing the milk was cool, and they preferred to move to the warm individual. You know, I know that getting up close to some individual, there's really something really, really nice about that. I've also, there was one really, really cold day, I tell my classes about these things, I've never been to the Yale-Harvard football match, but one year it was incredibly cold. And when the temperature's sort of warm, everybody maintains the personal space. And all those social inhibitions on a cold, on a really cold day, just disappear.

Everybody, just as Kim was saying, comes in really, you want to get as close as possible to the individual next to you.

I have two questions. One's actually very closely related to the point you just made, which is, so how are these systems going to get calibrated given that there may be a big range of the temperature that you're in? And especially if you have a developmental environment that is very hot, and then you're in an environment that's very cold, or vice versa, is that going to complicate how these systems might work. And then I'll just put the other question on the table too. You can answer one or the other or both. Which is, if this nakedness is an adaptation for heat transfer, you might also expect other specific adaptations for heat transfer, like if you're contacting skin in a particular place that you would want to actually direct blood flow there to both warm it and to receive the warmth on the other side.

I'm wondering if that's—

Yeah, I don't— so in the case of mouse and rat mothers, when they come to— so the pups are generating heat, and as soon as mum comes to sit on top of them, they drop their body. So they switch over from generating their own heat to being passively warmed.— and it makes sense then. So if you're wanting to conserve heat in the body, you don't have, um, you withdraw blood from the periphery to keep the warm blood central. If you're getting heat from the outside, you wanna put the blood to the periphery. It's one of the, sorry, it's a different sleeping claim. It's during the, um, During the Korean War, it was the coldest, the winters there were the coldest wars that the US Army has fought in, and there was a lot of frostbite, and the frostbite particularly affected the African American soldiers. So they had high rates of frostbite, and the claim is that both in, say, Australian Aborigines and a lot of Africans, what they do when sleeping at night is that they withdraw blood from the periphery to keep the core body temperature warm, and they reduce the energy demands by allowing the periphery to cool. So that's energetically efficient if you're in an environment where it doesn't get to freezing, freezing levels. Whereas people from northern climes, supposedly what we do during sleeping at night is we pump blood to the periphery while we're sleeping to keep the periphery warm, and that's a fairly good defence against frostbite. You know, the interaction between sleep and temperature, you know, getting to— putting warm socks on your feet suddenly makes you easier to sleep, you know, so subtle differences in temperature have big effects on sleep.

Sorry, I got distracted.

On the issue of temperature and early effects, I don't know anything on— so at one time people said brown fat was present in babies but not in adults, and we now know actually from the cancer literature that all adults have brown fat. But people do vary very much in how much brown fat they have and how active it is in generating in generating heat.

Now there's some old Japanese literature from before the Second World War.

This was looking at, this was on keeping cool in hot climates. So we all have the same number of sweat glands, but we can vary dramatically in how many of those sweat glands are functional. And this was a study of Japanese who were born in Okinawa, which is quite hot temperatures, versus Japanese born in Tokyo, and then they were transplanted between those regions. And the number of sweat glands that you had that were functional to help to keep you cool was determined by the environment you experienced as a child. So people born in Okinawa had lots and lots of functional sweat glands and sweated a lot, even when they were in Tokyo, whereas people born in Tokyo, when it would pay to keep cool, just had much less, in this case, sweat-generating capacity. So there could be interesting effects like that in brown fat generation, but I don't know about them.

There are those practices in many areas of northern Europe of having babies sleep outside when they're only just born.

Perhaps there's some calibration of the system there. That's it, yep.

What's one up here? Hi, I have a weird question. So I've heard that monks in Tibet meditate to ignite the fire within. Can you— do you have any ideas of like what part of the body should we meditate on to kind of have that same experience?

I'm sorry, I don't know the answer to that. There's a lot of, there's an immense amount of work going on to switching on brown fat to generate more heat because people think that this is going to be the solution to the obesity epidemic.

Because if you can just switch on the brown fat, you're just burning off calories, dumping them into the environment. Good thing. I think this is an example of, so what they wanted to do is convince the body it's cold to switch on brown fat, but I think this will be perverse.

What happens when winter's coming?

Yes, you burn wood on the fire to keep you warm, but the first thing you do is you lay on a wood pile in white adipose tissue, you know, so if the brown fat is being highly active to keep warm,, the body will put on more fat to have the fuel to run the brown fat.

Brown fat is also used in diet-induced thermogenesis.

So, you know, people who don't lose weight no matter how much they eat, and part of it is that they eat a meal and then the brown fat is just burning off some of those calories and dumping them as heat into the environment even when it's not cold.

So there are a lot of interesting questions about the regulation of brown fat when you switch it on. Another curious thing that I'd like to mention, people have assumed that brown fat is a variance of white fat because it has fat deposits.

The molecular signature of brown fat is it's much more like muscle.

And what I think, so the muscle did two things, you know, you have coordinated contractions of muscle for moving, but it was also a way of generating heat in shivering, which was uncoordinated contractions of fibres. And so I suspect brown fat is descended from a muscle that just became specialised in heat generation, you know, and shivering became more and more efficient and eventually got rid of the muscle fibres and just was a pure heat-generating tissue. But things have become complicated now because there's beige fat. So beige fat is white fat that gets converted to look like brown fat, and it's very fashionable at the moment.

So you mentioned that it's sufficient to explain the churning with kind of a Nash optimal IFD, where you're always kind of looking for the better spot. But just devil's advocate, so there's an ant that's relatively highly related that comes out on the surface in large sheets and appears to gather sunlight and then bring that heat back in. So I thought about, have you ever thought about especially maternally inherited traits doing the genetic accounting for other things that that churning could be, not actually moving to get more heat for yourself but maybe actually bringing heat to others?

So the emperor penguins are groups of non-relatives And so that's why I think, you know, the individual self-directed behaviour is critical. If you looked at the mammoths, you know, the, in that case, the babies are sitting in the centre of the huddle.

So now we're looking at a cooperative behaviour, at least for the parents that are keeping them warm. So I think it depends on context in whether you're explaining what's going on as a simple self-directed behaviour, or it's selfish behaviour.

And, you know, one of the places to test that, so sometimes one of the places to test that is if an individual is not doing well, if they're cooler, are they sitting in the centre of the huddle where the others are helping to keep them warm, or are they on the outside? So are the stronger individuals sacrificing themselves by being on the outside, or are they using their dominance to get into the favoured position in the centre?

So there are behavioural ways to sort of get at those sort of questions. Do you know if there are any studies on introversion and extroversion being associated with thermoregulation?

No.

Short answer.

It'd be, what Oh. So introverts get closer. Well, the prediction would be if you're, say, someone who doesn't keep warm well on your own, that the benefits from being social should be higher for you than if you're able to keep yourself warm on your own.

Yeah.

OK, that'd be interesting to look at.

Sleeping apart is—

and it was wonderful, Kim's thing, you know, this idea that we each sleep in our own beds. I know with my wife, on hot summer nights we move apart, on cold nights we get closer together. But in cold Europe, until recently, if you were poor, the whole family just lived together in the same bed. They were often actually, the beds were above the cows., so they were getting the heat generated from the cows, and in some cases you'd actually get and sleep with the— sleep next to the cows and the animals. That's a big heat-generating mass there. So yeah, we do strange things the way we sleep now, sleeping alone.

One more question perhaps.

Nobody wants to be the last question.

We have the warmest thanks.