Transcription
Look at that wicked countenance. The mere sight of it gives you goosebumps. I'm a little bit scared of them. I don't want to be in like a small room with a wild rat. They're survivors and will protect themselves and can be a bit aggressive. But I have a grudging respect for them. They're so intelligent and versatile and can use all sorts of resources.
We tend to see them and think of they're disgusting or odd or funny or just something to be concerned about, but you know, they have their own biological story. Initially, I just wanted to know what is a New York City rat? Where did they come from? But then we started digging into that question and realized we didn't really know in general how this species moved around. Answering that question means looking beyond New York and diving into genetic samples of rats around the world to determine their origins.
The main evidence we have for where brown rats originally come from is actually kind of circumstantial. So, it's from things like the presence of current free-living, wild-living populations that aren't associated with human settlements. But this is something we see in Northeast Asia that we don't typically see elsewhere in the world. It's also pointed to by the genetic evidence. So, the greatest diversity genetically seems to be again in Eastern Asia. But we have a problem there that that's also where most of the genetic research has been done in the past.
But genetics alone can't solve the mystery of how brown rats spread. We're trying to bring pretty much every form of evidence we can to bear on this question. We do of course have written records pointing to the presence of brown rats in certain times and places. It's very difficult to push anything back before about kind of 1730 cuz increasingly hard to associate with the particular species if we move any earlier. One traditional problem is that it's hard to distinguish the bones of brown rats from those of black rats. We pretty much solved that problem now. We have a method called zooms which allows us to get a very quick, cheap identification.
But the huge problem we have is dating. Because rats are burrowing animals, you don't always know if they were at the site at that time or if they burrowed in there and then died. We identify some interesting ways you can get around that problem like rats on shipwrecks. We're pretty confident that they went down with the ship and you can date their existence to the downing of the ship. The earliest evidence of brown rats arriving in North America traced back to a merchant ship that sank in 1760. They likely arrived in Europe centuries earlier.
But how did they reach these destinations so far from their native range in East Asia? Based on genetic analysis of modern rats, it's been estimated that brown rats dispersed out of Northeast Asia and down to Southern China and Southeast Asia maybe 800, maybe 1,000 years ago. So, one possibility is that they dispersed along the South Asian coastline and then perhaps through the Caucasus across the Iranian plateau. Another possibility that's been suggested in the past is kind of a dispersal through Central Asia along the Silk Road. The one that I'm perhaps inclined to favor at the moment would be kind of a direct maritime spread. So, by the 18th century, you have quite a high volume of European colonial shipping between South and Southeast Asia and Western Europe and also the Americas for that matter.
What is interesting about North America is that we think there are at least two separate dispersal processes by which brown rats got there. For the Atlantic coast, I think we really need to see that in a joined-up way together with dispersal to Europe. But at the same time, it's also seems fairly clear that brown rats were also dispersing to the Pacific coast of North America by an entirely independent route. Most populations were established of essentially one lineage of rats. Which would indicate that rats were introduced, established themselves, built a large population, and then there weren't a lot of additional migrants coming in.
Rats do seem to have kind of like a bias against outsiders. They're very social and familiar with their colony mates and they will allow some dispersers to come in from nearby, but if you just bring a bunch of unfamiliar rats, they don't like it. There may be movement of rats still, but they maybe don't have a large impact on populations when they get there. To thrive and outcompete other animals in these new environments, rats quickly took advantage of their new food sources. For a long time, they were almost certainly eating other things, right? Like wild seeds and insects. Over time, it was just such a beneficial strategy to stay with humans.
We have some archaeological evidence that really quite a long time ago, >> [music] >> maybe as early as perhaps 6,000 BC, it's already turning up in human settlements. As human settlements got denser, they would have larger trash piles with favored things like bones and fat and meat and so forth. Eventually, humans were living in larger, stable societies where it was a stable strategy for rats to just stay there. After spreading across urban and rural landscapes and settling in with humans, researchers are starting to see signs of how brown rats have adapted to their new environments.
They occur in a much broader array of climates than they originally evolved in. Given how many different environments they occupy now, it's likely there's a lot of selection pressures on different populations. Rats are are quite broad in their dietary preferences. When you eat different food sources, you incorporate isotopes from those food sources and they differ depending on what you're eating. Rats in urban areas tend to eat more meat and protein than rats from rural areas. And so, that may be driving these dietary adaptations [music] that we suspect are common around the world.
There's a lot of metabolic adaptation that happens in rodents. It also shows up in morphology, which is something we found in in rats in New York City is their skull shape has changed over the years. It's very common in rodents that the size of their teeth and their jaw and so forth changes from diets that are harder to process to ones that are easier. While researchers can delve into their bones to see evidence of change, there's still so much to learn about how rats leverage the built world around them.
We've put a lot of effort into managing this species and trying to remove it from our living spaces. But on the other hand, we haven't spent a lot of time understanding its biology when it's outside of the laboratory, when it's living freely. You're up against the brutal math of rat reproduction. Rats can become sexually mature in a matter of weeks. They can have several litters throughout the year. In terms of the urban landscape and how we build cities, are there specific types of urban construction or infrastructure that either promote or inhibit the movement and success of rats? Do subway systems in general tend to allow rats to move freely and and have this [music] safe subterranean habitat? Are there particular types of sewer systems that are rat resistant or rat prone?
The so-called war on rats would have benefited from some careful study before embarking on traps and poisons that can be cruel to the animals we're trying to exclude and also have secondary effects on other species including humans. My prediction is rats are going to become an even larger presence and more important in cities moving forward because of climate change. They probably are likely continuing to evolve to urban conditions because they haven't been there for that long.