Motherless gorillas and how hummingbirds hum

Plus, why elephants don't get cancer, and why deforestation causes peaks and troughs in malaria cases
27 May 2021
Presented by Chris Smith
Production by Chris Smith, Eva Higginbotham.


A hummingbird in flight about to feed.


This month: how hummingbirds hum, how elephants evolved anti-cancer genes so they can sustain big bodies, gorillas that grow up without their mothers, and why deforestation causes peaks and then troughs in malaria cases...

In this episode

Gorilla with babies

00:35 - What happens when gorillas lose their mothers?

Studying the social dynamics of gorillas who lose their mothers while at a young age

What happens when gorillas lose their mothers?
Robyn Morrison, University of Exeter

Losing a parent, and particularly your mother while you’re still a youngster, has well documented, profound lifelong impacts on a person’s life course. So the obvious question is, what happens to other social species that are closely related to us, like gorillas, for instance. As Chris Smith heard, Robyn Morrison has been combing through data collected across more than half a century by the Dian Fossey Gorilla Fund…

Robyn - It's one of the difficulties of studying wild populations, right? Especially endangered wild populations. You can't do any sort of experimenting yourself. You have to wait for these things to happen. And it is, of course, really rare that young gorillas lose their mothers. So the data that we use has been collected over 50 years. So it's taken all of this time to have kind of a big enough sample for us to really answer these questions and say, you know, what does happen to these gorillas when this rare thing does happen.

Chris - Was your starting point that we know from epidemiology and psychology and sociology, that when this happens to a human, it does have really very profound consequences on some individuals? Were you coming at it from that angle saying, well, are gorillas as another social species similar?

Robyn - Yes, exactly. That and, in humans, it's interesting because in some populations we see really, really detrimental effects of losing one's mother. And in other populations, we see much less dramatic effects. And part of this study we wanted to do is to kind of understand why that might happen looking at one of our closest relatives, the gorilla.

Chris - How did you get enough data to come up with with statistically robust findings then?

Robyn - I work with the Dian Fossey Gorilla Fund and in 1967 they started monitoring mountain gorillas in Rwanda, and they've been there ever since. So there's this really incredible dataset contributed to by hundreds of different researchers and fieldwork people. And so it was kind of all of that really long-term data, that was the only way we can really answer these sorts of questions.

Chris - What were the outcome measures then?

Robyn - So what we found is that in terms of fitness, we couldn't see any detrimental effects of young gorillas losing their mothers. So we looked at gorillas that were over the age of two and not quite eight, so eight is when they are kind of first classified as adults. And what we found is that they were just as likely to survive, they were just as likely to reproduce. So we looked at females and we found that they were actually reproducing ever so slightly earlier, so about six months earlier they had their first offspring. So they seem to be kind of succeeding in life all around. Not only are they surviving but they're also kind of reproducing and kind of contributing to the next generation of gorillas.

Chris - Well, that flies in the face of where I thought you were going to say it was going to go then, that you'd say it was the same as with other primates where there's a big catastrophe if you lose your mother. How do you think that they're doing so well when this happens to them then?

Robyn - Yeah, it's a really good question because we do see that this is really problematic, especially in chimpanzees as well, which are super closely related to gorillas. And I think part of it is to do with this really cohesive social structure that gorillas have, right? They live in these groups, they're usually about 12 individuals, but they move together. They feed together, they build nests at night together, and they're always in this really cohesive social group. And so part of our study was looking at "how did their social relationships change when they lose their mothers?" And what we found is that they kind of actually became better integrated in the group. They had stronger relationships with everyone else. They were really central. They had really strong relationships with the dominant male. And when we compare this with other primates, the problems that the other young primates face is that they actually kind of struggle to be part of this social group when they lose their mothers. So they struggled to get kind of access to food, access to mates, all of those sorts of things.

Chris - Is it that the group recognises that the individual is now vulnerable because it's lost its mother, or is it that the individual knows it's vulnerable because it's lost his mother and behaves differently to the group? Or is it both?

Robyn - That is a brilliant question, and we actually don't know. So from our data, we can see that these relationships are really strengthening, but what we haven't yet got into is who is causing it to change. Is it the young gorilla that's kind of seeking out much more social interaction with other individuals or is it all the group members that are noticing these changes and responding to it? And I think that's a really interesting question going forward. Particularly we found that the strongest relationship was with the dominant male. So it'll be really interesting to understand who is pushing that, who's kind of strengthening that relationship. And I think that's one of the next steps moving forward from this research.

Chris - And given that you're seeing this in these gorillas, does this argue then that this is some kind of evolutionary trait here because we see, you know, humans are also successful because we're pretty good at reconfiguring groups and opening up a gap to slot someone in when they're vulnerable? Sounds like the gorillas are doing the same sort of thing, which would suggest that this is something that's innate to our behaviours

Robyn - Potentially, yes. I think it seems to be a very successful strategy, right? If there is this risk of losing certain individuals, having this cohesive social group can be really beneficial, right? Because you've got other social relationships to turn to it kind of buffers you in these extreme cases where, you know, something really bad happens and you lose a really important social contact. I don't know whether it means it's kind of the best strategy. I suspect, you know, there are different strategies in different ecosystems, different environments, different species that are beneficial, right? So each species is finding its optimal way through this evolutionary landscape, but we don't seem to see it in chimpanzees as well. And that's kind of another of our really close relatives. So it's kind of complicated. It's kind of uncertain when these sorts of behaviours might have evolved. Maybe it's evolved kind of separately in humans and gorillas. Maybe it evolved kind of before we split from each other. There's some really interesting research in Bonobos as well. There's quite a lot of evidence of adoption and even adoption of individuals outside their own social group.


06:54 - How do hummingbirds hum?

Using sensitive force detectors to unpick the physical mechanism of a hummingbird's hum

How do hummingbirds hum?
David Lentink, University of Groningen

Hummingbirds are so named owing to the sound they create as they hover by beating their wings extremely rapidly, up to 80 times a second. Now, with the help of high speed cameras, extremely sensitive force detectors, and a vast array of microphones, the physical mechanism that causes a hummingbird to hum has been unpicked. And Eva Higginbotham heard how from David Lentink...

David - We were curious about how hummingbirds hum. So whenever you see hummingbirds fly in front of a flower, you hear this characteristic humming sound, and we were curious what was causing it. And so what we did is we developed a special measurement set up with more than 2000 microphones so that we could record the acoustic field in 3D and really figure out where it is coming from, what the origin is.

Eva - How did you get a hold of 2000 microphones? Was this like a 3d stadium that you put a hummingbird in to try and make it make its noise?

David - I love that comparison. It is a little bit like a stadium. We developed a special flight chamber and we collaborated with a company called Sorama in the Netherlands, who develops these microphones. And so invited the CEO to come to California where I worked at the time where we did this research at Stanford University, for some coffee and fun experiments where we would just record the humming sound with all these microphones. And that's really where it took off. And then the key thing is that we didn't stop with just making those recordings, but we also ended up recording the forces that the hummingbirds generate while they're hovering. And by forces I mean the aerodynamic forces that enable them to stay aloft, so the lift force, but also to drag force that they have to overcome to generate this lift force that lifts their body weight up in the air, enables them to hover perfectly still in front of the flower. And that was also a first where we measured these forces. And then we were able to show that the fluctuations of these forces as the wings beat back and forth, that those really nicely predict the acoustic field, the humming sound that we hear.

Eva - Wow. So you found that you could, by measuring the forces, you could predict what kind of sound you were going to get out the other end?

David - Yes, and that's the key thing. And to predict this we actually used a mathematical model. Now the wonderful thing about acoustics is that it's basically just governed by Newton's law of motion, but then for fluids, right? So for air. And of course it's not super simple, but it is manageable. So we had this mathematical equation that predicts how the forces would generate sounds based on first principles and what that acoustic field would look like. And then we compared that with our recordings and they matched up.

Eva - One thing I'm wondering is how did you manage to get the hummingbird to stay in your experimental setup? You can't tell it to hum, I assume, it's going to do what it wants!

David - Yes. We prefer the animals to do what they want! But the way this works is you provide a twig on which they can perch and you provide an artificial flower with unlimited sugar water that they know how to find, and then they'll just fly back and forth every 10 minutes. And as they fly over, we could already, through the acoustic microphones, see where it was flying. Because if you have about 2000 microphones, you basically have an acoustic camera. You can see where the sound is coming from. So we would see on the computer screen how the hummingbird was flying because of how its sound was moving in space. And then basically it hovers in front of the flower, and then we made our recordings. We have these over 2000 microphones, but also multiple high-speed cameras, tracking cameras. So we had a really good idea what the hummingbird was doing.

Eva - How do you go about measuring the force created by a hummingbird's wing? Because presumably it's not a very high level of force.

David - Yes, that's a wonderful question. What we did is we had a floor, but also ceiling and all sidewalls were instrumented with a very large panel made out of carbon fibre. And that was connected to four sensors that were recording the forces very fast. And by combining the pressure force exerted by the hummingbird below the hummingbird, above the hummingbird, on the sides and front and aft, by combining all of that, we have the net force it is generating within a wing beat, and wing beat resolved. And this was the first time we had a set up that could measure these forces in 3D. And the reason why we were really confident it worked is because we were able to point to the forces as being the source, and those are really easy to understand. So it wasn't the feathers rubbing that was the most important, it wasn't the turbulence generated that was the most important. It wasn't the feathers whistling or something. What we found is it's really just the wing moving back and forth and reorienting the aerodynamic force in space that generates acoustics waves, so pressure waves, and that is generating this humming sound that we hear. And so what you can do is, if you know the forces that the animal generates in flight, you can predict the sounds it's generating. And that's really the exciting part that we got out, we now have this really elegant model that can explain where wing sounds are coming from in flying animals.

Eva - What sorts of applications do you imagine this being used for?

David - What's so wonderful about this mathematical model, it's like we really developed it for a very complex wing. Bird wings are exquisite compared to, for example, the wings of aircraft or drones, but all of these wings generate noise. And what's really cool about the model is that it predicts the noise for a hummingbird wing that moves very complex, generates really complex forces, really well. So now we have a model that can be applied much more generally. If we want to make drones more quiet or fans more quiet, or actually anything that just rotates and generates forces, you can also think about things like wind turbines. So what you can now imagine is that we can design wings to have the right fluctuations in forces so that their sound, or humming sound, is more pleasing. And this is really, I think, the future for making our environment, our own environment, a little bit more pleasant. Where we design the sounds of the products and systems that we use instead of just accepting the sounds that they make so that we can enjoy more of the pleasing sounds like the hummingbird hum.


13:14 - How the elephant family is protected from cancer

How elephants evolved large bodies without increasing cancer risk

How the elephant family is protected from cancer
Vincent Lynch, University of Buffalo

A few years ago, we spoke here on the programme to Vincent Lynch. He’d solved “Peto’s Paradox” - the apparent contradiction that despite having a big body and therefore many more cells capable of turning malignant, elephants actually have very low levels of cancer. And they do it, he reported, by bulking up on copies of a protective gene, called p53, that kills off cancerous cells. Well now he’s widened the scope of that initial study both to look across the entire elephant genome for other anti-cancer genes and also to consider other members of the elephant family, as he told Chris Smith...

Vincent - So in a previous study we had shown that elephants have a bunch of extra copies of this gene called p53 - one of the master tumour suppressors. Its job is to go around all of your cells, and when it senses that the cells might give rise to cancer, causes those cells to kill itself. And we think that that's probably related to why elephants were able to evolve such large body sizes and such long lifespans. And what we did this time is say, well, instead of focusing in on the one gene p53, what happens when we look at all of the genes in the genome? So the genome includes 20 - 22,000 genes. So let's just look at all of them. It turns out that elephants have lots of extra copies of genes whose function is to surveil the cell for the kinds of damage that are associated with eventually turning into cancer, and either dealing with that damage or causing the cells to kill themselves. So it looks like one of the ways that elephants evolved their really large bodies and long lifespans is to have lots of extra copies of these anti-tumour genes.

Chris - Elephants of course, are part of a family of animals that are similar to them, but they're not all huge. So if we look at smaller animals in that same group of animals, do they also have this massive expansion or did the elephants alone as big animals have that?

Vincent - The living elephants are big and the extinct ones like mammoths and mastodons, they're even bigger. So this lineage includes lots of big animals. But their closest living relatives are pretty small, so things like manatees and animals like a Hyrax, which is about the size of a Groundhog, and then a whole bunch of things, which are pretty small - about the size of medium sized dogs or mice. And when we look at their DNA, we see similar things, they all have extra copies of these anti-tumour genes. It's just that elephants have lots and lots of additional copies even compared to their close relatives.

Chris - So that tells you then, although all these animals share a common ancestor and that common ancestor had lots of copies of these genes, it was in turning into something with a very, very big body size, i.e. an elephant or bigger that there was this very strong selection for even more copies to enable that body size to be sustained?

Vincent - Yeah, that's right. So it looks like in this whole group of animals, there is a tendency to get extra copies of anti-cancer genes, but then as elephants evolved their large bodies, there was some additional selection that probably required them to have even more additional copies of those anti-cancer genes. So that's what we're seeing when we look at their genomes.

Chris - The interesting thing is the age of reproduction in these animals, because elephants really benefit from these extra copies of genes beyond the age of which they might be reproducing don't they? So is it because they live in big families that they've managed to concentrate these genes and select for the beneficial effects?

Vincent - Yeah. So that's actually a really, really challenging question to answer. So all we can do now is compare the DNA of all the living animals, so elephants, and there's a few different species of elephants, to all their close relatives and say, okay, how many of these anti-cancer genes do they have? And obviously elephants have more. But determining the direction of causation - is it that they got all these extra genes and that allowed them to get big and get large families and live a really long time? Or did it go the other way around? Was there some selection for elephants to get big and have large families and delay reproduction, and that required them to evolve some kinetic mechanism to do that? So we call that the direction of causation. We can't really know that, but all the things that go along with being big, like having long lifespans and delaying reproduction until you're older, and in elephants in that they live in these large families where the matriarch has a lot of knowledge about how to exploit the resources in the land. All these things probably together mean that elephants had this sort of increased selection to live longer, which means you need genetic mechanisms to allow that.

Chris - Do you think cancer would have been a major problem for these animals otherwise then? Because it just seems a bit surprising that only those genes, those anticancer genes, would have been enriched. Surely there would have been other problems that they would have run into as they got older. I mean, an old person claps out with heart disease and strokes, for example. Don't elephants succumb in the same way?

Vincent - Yeah, so I don't know about strokes. It's hard to sort of ask an elephant the questions that you're asked when you have a stroke. So we can't know much about cerebral cerebral vascular diseases. But they do end up getting a lot of coronary artery disease, so a lot of the kinds of diseases that we think are associated with old age in humans and other animals. So they suffer the same sort of old age diseases, the same diseases of old age as humans. But at least with regard to cancer, they get less of it.

Chris - So in other words, because they are so gigantic, they would have a substantial disproportionately big risk of cancer because they've got all those cells that can turn cancerous. So there would have been a very profound selection pressure to defend against that.

Vincent - Yeah, that's right. So if you can imagine that if you look at the sort of cells of all these different animals, and they all have the same probability of going from a normal, healthy cell to a diseased cancer cell, then big things which just have more cells than small things should have more cancer. So that means that, because they don't, they must have evolved really, really efficient ways of reducing their cancer. But that doesn't mean that they're not going to get other diseases of old age.


19:25 - Deforestation and malaria in Asia

How human activities can cause peaks and troughs in malaria cases

Deforestation and malaria in Asia
Francois Rerolle, University of California San Francisco

When we think about malaria, we often instantly picture mosquitoes, which are of course the culprit vector that spread the disease. What we probably don’t picture so often are what the people are doing in areas where malaria is rife, and how those human activities are changing the environment in ways that might impact on malaria transmission. And given the massive increase in urbanisation that’s happening, especially in the tropics, knowing how human behaviours like that are affecting disease rates is critical for effective public health strategies. As Chris Smith heard, UCSF’s Francois Rerolle has been looking at what deforestation does to malaria rates in Lao People’s Democratic Republic…

Francois - There is a connection because malaria is transmitted by mosquitoes and mosquitoes might enjoy the forest environment. And so what's happening in the forest might alter what's happening to those mosquitoes and therefore might alter what's going on with the disease, with malaria.

Chris - Have we got evidence of what happens in other parts of the world when people have explored this sort of question?

Francois - Yeah, actually, and that was one of the motivations for this study. This is a research question that is pretty well studied in the Amazon. So there are some similarities between that region of the world and our region of interest around Laos.

Chris - And when people did look in the Amazon, what did they find? What was the relationship between malaria cases and what was happening with deforestation?

Francois - They led to the Frontier malaria story where deforestation is thought to increase malaria incidence in the short term, but in the long-term it's thought to decrease malaria incidence.

Chris - And why is that?

Francois - What's happening in the short term when deforestation starts, it requires humans to go in the forest, and those humans get exposed to the mosquitoes. But then in the long-term, deforestation might have such a dramatic effect on the mosquito population, you know, at one extreme you could imagine there are no mosquitoes left, and so there is no malaria left.

Chris - And is that what you found when you did the equivalent study in Asia?

Francois - So we did find some similar results in the sense that we also have a short-term increase and we also have a long term decrease in the malaria incidence following deforestation, but we find a different temporal relationship with the time at which this transition from short-term increases and long-term decreases - it was much shorter. It happened after two to three years in Asia, whereas the Amazon has this inflection point more around six to eight years.

Chris - How did you actually do this study then? Did you actually physically put boots on the ground or were you provided with all of the data that informed this?

Francois - A little bit of both. We were running some large studies and I was involved in the data collection of that study. So I went there and I had my boots on, but it's true that most of the data for that particular project was provided to us by the house ministries. Everyone that is tested for malaria is recorded in a registry and the data was given to us.

Chris - Therefore you can marry up who's getting malaria where, and when, and superimpose that on the local rates of deforestation and see what the patterns are between the two?

Francois - Yes, exactly. That's exactly how we answer that research question.

Chris - And how do you explain what you're seeing in Asia compared to what was originally documented in the Amazon with this different temporal relationship?

Francois - So human processes are for sure very different in Amazon. You have this colonisation of the forest deeper and deeper that started in the sixties where you have non-indigenous population getting deeper and deeper in the forest. Whereas in Southeast Asia, the populations that we are studying or indigenous populations that have been living in forest fringes area for not ever but for quite a long time.

Chris - But how does that explain why it takes longer in the Amazon than in Asia?

Francois - I'm not too sure. It may have to do with immunity that those people might have developed because, as in populations that have been hanging around mosquitoes and malaria for much longer, you might find some acquired immunity. And so this definitely will have a differential impact on malaria transmission in this area.

Chris - Is distance a factor as well, because if you've got a small group of people who live in one place and there's local deforestation, but they then travel farther afield to exploit that forest, you're going to see a very different dynamic than if you've got wide-scale deforestation, but wide-scale population at the same time. And that might be the difference between the two places?

Francois - Yes, that's right it might be. And actually, it's one of the points we're trying to make, because although the research question we answered was about the association between deforestation and malaria, what we were really interested in looking at was what is the impact of this population that like you said engage in forest going activities, engaging in maybe deforestation. And this type of population, this subgroup of the population, is believed to have a key impact on transmission in the region. And so we varied the spatial scale, we looked at deforestation happening further and further away from villages. And for instance, we found an impact 30 kilometres away from the villages, but not in the near vicinity of the villages, like within one or 10 kilometres. And this to us suggests that there is this subgroup in the population that is doing something in the forest, at least 30 kilometres away from the villages and that is sort of bringing back a malaria infection in the village.

Chris - Does this inform policy though, in the sense that if you've now got this data and it's much more fine grained, and you've got that geography to superimpose on the relationship, does this inform how we should approach where people are or are not allowed to explore forests? And also how we deploy resources anticipating where we might see big upticks in malaria rates because of people's activity?

Francois - Yes, so the results support the evidence around the importance of this population. And so they support any policies that will, you know, target resources and efforts to this population of forest goers. And this is really important because national control programs have set themselves a goal to eliminate malaria in the region by 2030, and focusing their resources and efforts on this key population might make the work a lot more efficient.


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