Dengue, decaying dead bodies, and a stone age deer trap

Researchers in the United States have identified what appears to be a network of around twenty types of microbe that universally drive the decomposition of animal flesh. It’s thought the discovery could have huge implications for forensic medicine. And it was by studying the decomposition of human bodies in specialised facilities built for this purpose in America that led Jessica Metcalf, from Colorado State University, to identify these key microbes. She told Chris Smith about the work…

Jessica - In the United States we have these forensic anthropological facilities. These are places that definitely have really big fences around them, they're outdoors, and where people can choose to donate their bodies to be part of a forensic science; a sort of anthropology bone collection. We coordinated across three of these facilities to decompose 36 human donors over four seasons. We sampled these donors, who were placed outdoors to decompose daily for 21 days. These were swabs that we either rubbed a little bit on the skin or we also sampled the soil right near them. We were capturing on those samples the microbial communities that were active. Then we were able to get the DNA out of those and we used the DNA then to profile what microbes were there at that time. We built that up and we ended up with a time series of snapshots over 21 days for each of those 36 cadavers. That's a really powerful data set. It allowed us to see what microbes were there and what they were doing. Also, we looked at some of the small molecules they were making as they broke down things like fats and proteins that were coming from the cadavers.

Chris - Are you testing different environments and also different times of year? Because I would think that might make a difference to these outcomes, wouldn't it?

Jessica - We sampled bodies that were placed at three different locations that covered two climates, and then across all four seasons. What we found is that actually there's this core set of microbial decomposers that show up regardless of all of those variables. When we started looking into, well, what is the reservoir? Where are they hanging out? Where are they persisting when there's not something decomposing? We found no evidence for them being either gut or soil, which were our two big environments that we thought might be responsible. Maybe people were carrying them around. We also looked at these big data sets that are available on the human gut, human body and mini soils, and we really find very little evidence. There's one or two of these microbes that appear that you can find in the soils or associated with humans but, for the most part, the only place we found them were on insects. What we think, and this makes a lot of sense, if there's an animal, it could be a deer, it could be a bunny, whatever, decomposing in the forest, that particular location may not have had an animal decomposing on it for a thousand years. So how do these decomposers microbes persist? Well, they're hanging on to the blowflies and the carrion beetles that can move to visit bodies. To me, ecologically, it really made a lot of sense. Previously, we'd known that a couple of microbes that are involved in decomposition came in on blowflies, and now we know it's a lot more than a couple. The main, key decomposers, pretty much all of them are associated with insects.

Chris - That's fascinating because we always used to assume that your own bugs just declared war on you as soon as your immune system stopped when you died, that you were consumed by the army from within. What you're saying is, actually, that isn't the case and it's all an invading army that comes on the feet of flies.

Jessica - We have a study going on right now of indoor bodies and so we'll see, because it doesn't mean that these core decomposers are doing all the decomposition, but they're certainly a network that's doing the bulk of it and they're specialists at it, so they're really good. Some of your microbes could still be betraying you after death and going after your nutrients but, for the most part, no.

Chris - How does this change our understanding, forensically, in that case, given that you've now found this and you've found these species come in, you know the timeline, how does this enable us to have a better understanding of what goes on if someone is killed or dumped or whatever out in the wilderness?

Jessica - The cool thing is that because there's these microbes that are everywhere, we can build this fairly general model from which we can predict how long a person's been dead based on the microbes that you sample when the body's found. We can do that fairly accurately. This provides another tool in a death scene investigator's tool set which, depending on how long a body has been decomposing, there may not be many tools available to you. In some cases this could be the only one.

We've all had fun teasing our friends and family before. Such jokes will often be verbal - but scientists have also observed this behaviour in children as young as eight months old, which suggests that language is not an essential element of teasing: for example, a playful tap on the shoulder. In light of this, cognitive biologists have been seeking an origin for our sense of humour in our closest animal ancestors, and have observed teasing behaviours in all four great ape species, meaning it's likely to have been in our biology for at least 13 million years, as Erica Cartmill, from Indiana University, explains to Chris Smith...

Erica - I was studying gestural communication in orangutans, and I was watching this interaction between an infant orangutan and her mother. The infant orangutan was dangling over her mother. Mother was lying on her back in the straw, and the infant had a stick in her hand and she reached down the stick and sort of waved it over her mother. And as her mother reached up, her hand started to grab it, the infant pulled back the stick. That kind of, you know, here you go. Ha ha. Just kidding. Right? Give me five, too slow! And the infant kept doing it. She did it several times. The mother was playing along, didn't seem to get annoyed, kept sort of humouring the infant. I thought it was just absolutely charming. But what really fascinated me was when the infant stopped doing it, she dropped the stick. The mother then picked up the stick and started doing the same thing back to the infant. I really thought about it as having almost the structure of a joke. There's the setup, I'm offering you something and there's the punchline. I pull it back, right? There's the expected behaviour - take this thing and then the unexpected. Just kidding. And so that formed the centre of the kind of thing that I wanted to study in apes. I wanted to understand, was this something that they did a lot? Was this something that where they really, they understood what it meant for another to be surprised?

Chris - And obviously it fits in that group, in which we haven't shared an ancestor with some of the species you're mentioning for millions of years. This would argue that some of the behaviours we see in us, the reason we play with our children the way we do is exactly the same.

Erica - Absolutely. I wouldn't argue necessarily that the behaviours themselves are biologically inherited. You know, I'm not saying that we play peekaboo in the way that we do because of our biology, because of our genes, but I think we do things like play peekaboo because we do share underlying ideas about other individuals, what it means to interact with them. We understand that other individuals can have beliefs, they can have expectations, they can be surprised. You really need to, to understand something about their mind. And I think that those are shared across all of the great apes, those abilities.

Chris - How did you test that out though, to confirm for yourself that that was the case and that this is what's going on? This is a sort of primitive form of humour that has educational value.

Erica - Honestly, this is such a scientist's answer, but you know, it's something where more data needs to be collected. I mean, it's something that I think we're still exploring. I'm kind of going out on a bit of a limb here and saying, I think it's because of these kinds of understandings about other people, other individuals, that we see those behaviours. It's very hard to conclude that when you're just looking at behaviour. You can't interview an orangutan and say, what do you understand about this other individual? You need to draw on information from a wide range of sources. You need to look to see what they do spontaneously. You need to try to design clever experiments and games where we can see what they pay attention to? What information do they use? How do they make judgements in one situation versus another? In order to really fully answer that question. What we were doing in this study was kind of laying the groundwork for that. What we started with was one example, or you know, a handful of examples of what I had seen orangutans do. And I wanted to build a corpus of behaviours. I wanted to build a data set. Do they all look like this? I offer something and I pull it back at the last second. Or are there other kinds of behaviours that might have similar underlying structures, but appear quite different on the surface?

Chris - Do you think this goes beyond just higher animals like these primates that you are describing though? Because my dog turns up with a toy and will torment me because they'll wave it in front of me and then whip it away before I have the chance to take it off him. And it's clearly a game. They clearly are getting something out of it. Why would other animals be doing that?

Erica - I think that's one of the things that we hope will come out of this study. This isn't the first time anyone has seen these behaviours. You go to a zoo and you watch apes, you'll see some of them and you know, it'll probably be familiar to pet owners or people that work with or watch other animals. The thing that really we were trying to do is to argue that there isn't a sharp division between aggression and play. That there's an interesting grey area in between where what you do can be interpreted as playful or it can be interpreted as mean. And I think learning to navigate that space, learning what's appropriate with which individual, is something that social animals in particular have to figure out in order to thrive in their groups. And this is something that we know humans do. Now we know great apes do it. And so our hope is really that by sort of shining a light on this kind of behaviour and defining it and describing it as its own thing, we will encourage other people to look at it in a different way and to really start paying attention to it. And we'll start to collect examples, similarities, and differences across a wide range of species.