Why am I warm blooded?

0:00This BBC podcast is supported by ads outside the UK.

0:30at Whole Foods Market.

0:31Caroline Steel:Has Prince Harry and Meghan's American dream failed? I'm Asma Khaled, and I host the Global Story podcast from the BBC. The Sussexes are intending to return to the UK, six years after moving to California and striking multi-million dollar brand partnerships. Just last year, Harry told the BBC he couldn't see a world in which his family would return. So what's changed? Find out on the Global Story on BBC.com or wherever you get your podcasts.

1:03Caroline Steel:Oh my gosh, I can't believe you've already got a hedgehog Wow, oh my gosh, hello He's a boy, can you see? Oh yeah, I can see Lovely Oh my gosh, so cute Little face and the little ears Hi little one You're listening to CrowdScience from the BBC World Service I'm Caroline Steele and I'm joining some scientists on a disused golf course We're in search of its small, round and extremely spiky residents, hedgehogs. You know how to look for them? Yeah, I guess they kind of stand out a little bit. But in this long grass, it is pretty difficult. He looks so sweet and his eyes are open. He's staring at me like, um, hi.

1:48Caroline Steel:Importantly, hedgehogs are warm-blooded. In fact, they're a particularly curious type of warm-blooded, which is relevant to this week's question. Hello, I'm Matt from West Yorkshire in the United Kingdom. And Matt, what is your question for CrowdScience? My question is, why am I warm-blooded? Do you mean sort of in contrast to like a snake that's cold-blooded? Yeah, that's right. So why do I have warm blood? Why do most mammals have warm blood? why do insects have cold blood or the temperature regulating blood how does my blood get to the right temperature and how does it maintain that temperature yes because it's always about 37 degrees right which is interesting how do we get it there right so what made you think of this question it stemmed from the heat wave actually the recent heat wave in the uk i was watching some insects on the patio, some ants, and I wondered why, if they were cold-blooded and their blood temperature kind of regulated with the ambient temperature, why they didn't boil, why their blood didn't get to a temperature that wasn't sustainable.

3:00Caroline Steel:Yes, interesting question. How do insects or other cold-blooded animals cope when it gets incredibly hot? So what do you hope that we'll find out about being warm-blooded if you had to break it down into sort of a few things that you want to know so i'd like to know why that particular temperature range i'd like to know why we're not cold-blooded and what would be the advantages or disadvantages of being cold-blooded as opposed to being warm-blooded i haven't really thought about warm versus cold-blooded since being at school and learning that so it's going to be a steep learning curve and it's going to be fun.

3:41Caroline Steel:So thanks so much for asking your question. Thank you. Thanks for getting in touch, Matt. Regardless of the environment, be it a cold rainy day here in the UK or an uncomfortably hot day in Uganda, we all keep our body temperature at a comfortable 37-ish degrees Celsius, which is really quite impressive when you stop and think about it. So how do we manage such consistency? And is it the same story for all warm-blooded animals? And what about cold-blooded animals? Lizards bask in the sunshine, snakes warm themselves on hot rocks, which sounds quite enjoyable, but why is it a good evolutionary strategy?

4:22Caroline Steel:Plus, lying in the sun actually sounds quite warm to me. So what do we mean by cold-blooded and warm-blooded anyway? Cold-blooded really means the body temperature depends on the temperature of the environment. We call that ecto term. Ecto means from the outside. Term means temperature. So your body temperatures come from the temperature of the environment. You regulate your body temperature based on the temperature of the environment. This is Manuel Leal. He's an evolutionary biologist at the University of Missouri, Columbia in the US. He researches lizards in the Caribbean. A warm-blooded is an endotherm.

5:02Endo means inside and term against from temperature, which means that the heat you generate is used to maintain body temperature because you're able to trap that heat.

5:13Caroline Steel:So warm-blooded animals or endotherms, like us humans, generate their own heat, while animals that rely on the world around them to warm up are described as ectotherms or cold-blooded. Coal is relative, right? When you say coal, it means I feel that it's colder than I am. But it turns out if you were to grab a lizard in the desert, you will say this lizard is harder than I am. So in that sense, you will be the cold-blooded. OK, so if I picked up a lizard in the desert, it wouldn't feel cold, it would feel warm. It will feel warmer than you, some of them, yeah. Some of them might have a temperature that is above your body temperature.

5:51I think the idea of cold-blooded came from an easy way out to explain how an animal maintains its body temperature. And in the case of humans, it happens because we produce heat and we are able to maintain that heat. And therefore the idea of warm-blooded.

6:11Caroline Steel:So the terms warm-blooded and cold-blooded are a bit misleading. If you find a snake first thing in the morning, its blood may be pretty cold. But if it has a chance to bask in the sun, its blood could reach a higher temperature than many warm-blooded animals or endotherms. We all have ectotherms as ancestors, right? We all come from a fish. Let's just not go too far. We know fish are where our ancestors and all fish are ectotherms, right? The division was that animals develop either feathers or hair and that allowed them to start maintaining body temperature. and then that's when you get the endotherms versus the ectotherms, what you call the cold-blooded, which again is the most common in terms of animals.

6:54Most animals are ectotherms.

6:57Caroline Steel:Ectotherms make up the majority of animal life on Earth. And in many ways, endotherms got lucky. Over evolutionary time, our ancestors developed ways of generating more heat. And insulation like fur and feathers helped them hold on to it. Now millions of years of evolution later, it's a lot more complicated than that. In fact, we humans are nearly furless. So how do we keep ourselves warm? As mammals, what we do is we generate a lot of internal heat just by staying alive with metabolic activity. This is Simon Hodder, Director of the Environmental Ergonomics Research Centre at Loughborough University in the UK.

7:38You could think about it as a smart central heating system. So it will help us when we're too cold to keep us warm, but it will also help us to lose heat when we get too hot.

7:52Caroline Steel:And rather than warm water running through pipes, in our central heating system we have blood running through veins. The blood circulates around the body, taking heat to the extremities and then bringing cool blood back. Even if we don't do any external work and we're just lying sleeping at night, we're still generating heat. Yes, we've got less internal heat that we're building up, but we have to maintain this 37 degrees. OK, we need to stay at 37 degrees and our blood helps us with that. So you mentioned that if we're too hot, more blood flows to the surface and I guess that's to cool the blood down?

8:37So you have your main arteries and veins and then you have lots of blood vessels off those so when we get hot what we need to do is to actually lose some of that heat to the environment and so we push as much as we can to the surface of the skin and that's something called vasodilation. typically we would say you have a core temperature of 37 degrees your skin is probably around about 33 oh okay that's why when we think of comfortable temperatures we probably think about 25 degrees and so we've got this temperature gradient of 33 to 25 and we're easily losing heat to that

9:20Caroline Steel:A difference in temperature between our skin and the air around us means we lose heat from our blood. Now, during the heat wave in Europe this summer, I found it very hard to cool down as the temperature was above 33 degrees, our skin temperature, for days on end. A feeling which many listeners will be all too familiar with. So that's where sweating comes in and that active element of the sweat coming onto the surface of the skin, the moisture there evaporates off and then that recalls the skin. How do we cope with being too cold? Do we redirect our blood again? So when we get too cold or the environment's too cold, skin temperature is one of our first indicators about our environment and it's telling us what to do.

10:09so it will note down that it's now getting cooler. But that then triggers a warning. So now the body wants to conserve its heat. So now it actually shuts off this blood flow to the periphery and that's something called vasoconstriction. So the idea there is about trying to maintain the temperature of your internal components and that's your heart, lungs, liver, brain.

10:42Caroline Steel:How do we control all of this? What's going on? Is it inside our brains? Yes, there's part of the brain called the hypothalamus which acts as almost like our central heating control system. So that's looking to provide signals to the body to maintain that 37 degrees that we want. So it will be telling us either to sweat, shiver, vasoconstrict, vasodilate, and that's the area that does that. And it's picking up information from the periphery of the body. We've got skin temperature sensors, which are actually sending signals back, and it's integrating that and responding to how it's feeling. So it's getting information from your skin saying how hot the environment is.

11:33Caroline Steel:Yeah. Then making decisions about how we should respond, how constricted or dilated our blood vessels should be, how much we should be sweating. Yes. And all of this is happening outside of our consciousness. Yes. It's what we could probably think of if you go back to your central heating. It's your thermostat. So we humans keep ourselves at a comfortable 37 degrees, thanks to the hypothalamus, our brain's thermostat, and a clever system of sweating and widening and narrowing blood vessels. But not all endotherms work quite like us. In fact, I had a close encounter with a particularly unusual example.

12:14We are working with hedgehogs and this is the end of the active season for hedgehogs. They will probably start hibernating in September and we are trying to find hedgehogs and equip them with thermosensitive radio transmitters to monitor the hibernation behaviour.

12:30Caroline Steel:This is Yulia Novak. She's an ecophysiologist at Liverpool John Moores University here in the UK. She studies how animals respond to changes in the environment, particularly when it comes to regulating body temperature. and at the moment she's focusing on hedgehogs which brings us to a disused golf course in Liverpool here in the UK. And is this a particular hedgehog hot spot? Do hedgehogs really like golf courses? That's hard to tell. We have found them on several golf courses so depending on the golf course yes I think they are a good location for hedgehogs and there is a really large population on this golf course.

13:05Caroline Steel:Despite having never seen one in the wild hedgehogs are one of my favourite animals. They're about the size of a small melon covered in brown spiky spines and when they feel threatened they simply curl up into a ball. An inspired strategy. So how do they go about keeping warm? So hedgehogs are actually warm-blooded animals in a way so they're endothermic but they are special in the regard that they are heterothermic so they're heterothermic endotherms which means that their body temperature is not stable during all of the year. So hedgehogs are are heterothermic endotherms, which means they generate their own heat, but they don't keep their body temperature constant.

13:46Caroline Steel:They are seasonal hibernators, and that means during the winter, their body temperature will decrease. So they will change the set point for the body temperature to a lower level, which saves them lots of energy and allows them to survive during conditions when it's colder and less food is available. And why is it that you're fitting transmitters to hedgehogs? The reason why we work with hedgehogs is that they are a seasonal hibernator, but we actually do not know much about the hibernation behaviour in the wild. We do not know whether they can use the hibernation behaviour flexibly. We do not know whether they can change their hibernation behaviour in response to urbanisation or climate change, so any environmental changing conditions.

14:29Caroline Steel:So you don't know how hedgehogs respond to changing conditions? That's what you're trying to find by tracking them? So by understanding when heterothermic endotherms like a hedgehog choose to either maintain the high metabolic rate or to reduce the metabolic rate, we understand more about the costs and benefits that have shaped the evolution of endothermy. Your metabolic rate is essentially the rate at which your body uses energy. A hibernating hedgehog has a much lower metabolic rate than an active one. Yulia is researching the hibernation patterns of hedgehogs in response to changes in the environment like climate change, which might give us some clues about the trade-offs that come with being warm-blooded.

15:09Caroline Steel:Trade-offs that have played a role in our evolutionary history. That's what we're going to be looking at next. Prep for a busy week with Whole Foods Market. Start your day with fully cooked breakfast sausages from Amy Liu, 365 brand frozen waffles with no bleached flours, and of course Whole Foods Market eggs, which are all cage-free or better. In the evening, bring home a build-your-own family meal that feeds four for just$35. Choose one entree and two sides. Shop smarter, not harder, at Whole Foods Market. Has Prince Harry and Meghan's American dream failed? I'm Asma Khaled, and I host the Global Story podcast from the BBC.

15:52Caroline Steel:The Sussexes are intending to return to the UK, six years after moving to California and striking multi-million dollar brand partnerships. Just last year, Harry told the BBC he couldn't see a world in which his family would return. So what's changed? Find out on The Global Story on BBC.com or wherever you get your podcasts.

16:21Caroline Steel:You're listening to CrowdScience from the BBC World Service, the show that's been answering your science questions for nearly a decade. Later this year, it's our 10th anniversary. And as the show is driven entirely by you listeners, we also want you to help us celebrate. So what's your favourite ever episode of CrowdScience? And why? Have you had your question answered? And what did that mean to you? Has an episode of CrowdScience changed the way you thought about something? Or maybe it's inspired you in some way. We want to hear about all of your CrowdScience stories. Please email them to crowdscience at bbc.co.uk.

16:56Caroline Steel:or you can WhatsApp us a message or voice note to plus 44 8000 314 773. I'm Caroline Steele and this week we're answering a question from our listener Matt who wants to know why he's warm-blooded. Although it turns out that warm-blooded and cold-blooded are pretty misleading terms. Ectotherms or cold-blooded animals can get very hot while some endotherms or warm-blooded animals can drop their body temperature to hibernate. Back on the golf course, after tramping through long grass at night with head torches, Yulia's team have found a hedgehog. Mission accomplished. You've got a lovely hedgehog in your hands.

17:37Caroline Steel:What are your plans for him? So Katie has just done some behavioural tests to see how bold or how shy this individual is. Katie Crawford is Yulia's PhD student. So we place them on their back and then they're curling up and we are seeing how long it takes for them to actually stick their nose out or their feet out. And was he bold or was he shy? He took like just under five minutes. So about half of them take under, about half of them take over. Cute. Okay, so you've established that he's average in terms of temperament. We're going to weigh him now and then give him some heat shrink markers. So we have a little heat shrink that we put on the spines with a number on so we can identify them again.

18:20And then we will put a radio transmitter on him.

18:23Caroline Steel:I'm amazed that he sort of sits still and isn't trying to run off. He's surrendered to his weighing fate. 883 grams. You could hibernate it that way, very comfortably. Yeah. You'd have a nice winter. Yulia is attaching a small tracker to the hedgehog's back, and I'm having a quiet chat with Katie. We don't want to disturb Yulia. So it's a radio tag. So it sends out a VHF signal, a very high-frequency signal, and then that gets picked up by the receiver that we have. But these ones are quite special because they have a thermal attachment. So the beats on the radio tag, so the frequency of the beats, like the number of beats, correlates to the temperature of the hedgehog.

19:06Caroline Steel:So you can tell if they're hibernating or not. So the tag is giving out a radio frequency. It's giving out like a signal, like a pulse of signal. And if it's hotter, there's more pulses? Yeah. So that's how it contains the information about the temperature of the hedgehog. So not only can you find where the hedgehog is, but you can also find out the temperature, the skin temperature of the hedgehog. So that's how we can tell when they're hibernating. Once the hedgehog starts hibernating, Yulia will check on him daily so she can build a detailed picture of what's going on. Hibernating means he can get through winter without constantly searching for food.

19:40Caroline Steel:But he'll also slowly burn through his precious fat reserves until he warms up again. It has its pros and cons. Now, our listener Matt wants to know the pros and cons of endothermy and ectothermy. Our furry ancestors evolved the ability to generate and hold on to their own body heat. But what advantage did that give them? And if most animal species are still ectotherms, relying on the environment for warmth must be a pretty successful strategy too. Here's our lizard expert Manuel again. From a lizard perspective, they can eat less because they can just use the environment to provide that temperature.

20:16So their metabolic rate, which correlate usually with the amount of energy you need, is significantly lower than the metabolic rate of a human or of any mammal. So in terms of energy consumption, it's pretty advantageous not to be completely dependent on your own to produce the heat. And there are also two types of ectotherms. terms. Well, there are many types, but if you want to think about the extremes, there's those that always live in shaded areas, and therefore they have adapted to be able to have a functional body, their muscles, everything is adapted to work perfectly fine in cooler temperatures.

20:57And then there's those that go back to the desert, and it's just so hot then that they wait until the night to come out. But the sun trapped enough heat that when they come out at night, the sun then is liberating heat and they can sit on top of the sun, warm up their bodies and go about their lives.

21:19Caroline Steel:This makes me think of a question listener Matt had. He sort of begun thinking about this because he was watching ants during a heatwave in the UK here. And he was wondering how their blood didn't boil, how they were okay, sort of getting so hot in the sun. What's going on there? How are they managing? Ants can actually ventilate very nicely. So all insects, they don't have lungs like us. They have another way for ventilation and they can perspire very quickly, exchange body temperature. That helps. The other thing that is interesting with ants, and this is true with a lot of small animals, is that they can go into the sun and be sort of warm, but very quickly move into the shaded and become equally quickly cool.

22:07Let's imagine now something much bigger like a dinosaur. On a cold day, yes, they will be very sluggish. However, once they warm up, then it takes a long time for them to lose all that heat that they have game.

22:20Caroline Steel:This comes down to something called the surface area to volume ratio. Smaller animals, like ants, have lots of surface area compared to the volume of their body. So they can gain heat quickly in the sun, but lose it just as quickly in the shade. Bigger animals, like a large dinosaur, have proportionally less surface area, so they take longer to warm up. But once they're warm, they hold on to that heat for much longer. If you could pick, would you pick to be an endotherm or an ectotherm? If I can pick, I will be an ectotherm. Really? Interesting. Why is that? A lot of the energy we consume just go to maintain our body temperature.