How Did Rats Spread Across The World?

Rats often feel like an ever-present certainty in life. No matter where we move, visit, or explore, we can always be certain that a rat or mouse encounter is a distinct possibility. In fact, about the only place that we can ever truly escape from rats is by visiting the frozen reaches of Antarctica (or the Canadian province of Alberta) and even that's not always a certainty. 

But how did these rodents become such a common sight amongst human's across the globe? After all, most animals can only be found on one or two different continents. What makes the rat and mouse species so different from other animals? To get a firm answer to that question, we need to look at the evolution of human civilization and rodent adaptation. 

Every Story and Every Species Has a Beginning 

The human species has been around for a long time, but people often assume that human civilization is much older than it actually is.The start of human civilization is widely contributted to the creation of the city of Uruk in Mesopotamia around 5,000 B.C. 

This is the birth of agriculture, where humans began cultivating vast amounts of food through farming and domesticating animals as opposed to the previous hunter gathering societies. Consider just what a massive change this would be. Food was generally scarce and humans needed to migrate and continually search for new plants and animals to eat. This was true for most mammals. 

But for the first time, people were creating food stores that existed independently of the larger environment. This was a huge boon for humanity, but it would be equally beneficial to other species as well. Luckily for them, ancient Mesopotamia most likely lacked small omnivorous animals which could sneak into food stores. But as civilizations exploded across the globe, this would change. 

An Intersection of the Species 

We don't know exactly when rats and humans started sharing the same general space. However, we can generalize this period to have occurred thousands of years ago. Since the time of Uruk humanity had begun spreading agriculture and more modern ideas about food all over the globe. Agriculture provided nearly instantaneous benefits to any culture that adopted it. As such, it's little surprise that civilization really began to grow once that innovation occurred. People suddenly had more food than they even needed. Trade routes began to appear between different hubs of civilization. Traders would wander these routes with vast arrays of wares — including spices, food and seeds. 

During this period the traders would also attract the attention of the now familiar Norway rat and the black rat who were enticed by humans carrying and storing food. This began a period of what's known as commensalism. When species have such a relationship, there's no direct harm from one to the other. But one species benefits from the relationship while the other exists in a neutral state. In this case the rat was able to feed off of stored food. The impact wasn't severe enough at first to cause any real harm. But it was, of course, immensely beneficial to rodents. 

As traders moved through and expanded their routes the rat would follow along. The rodents would also begin to make their homes in the various towns and cities that trade caravans passed through. Both the Norway rat (Rattus norvegicus) and black rat (Rattus rattus) would take part in this expansion. 

Movement to Europe 

The black rat, and then the Norway rat, would eventually reach Europe. As is often the case, we don't know exactly when either species arrived, but know they were within Europe in the 1300s. The rodents would quickly make themselves at home in Europe's over populated and trash filled streets. It's thought that the silk road, a trading route between Europe and Asia, was one of the main paths by which rat migration occurred. 

The rapid spread of rat species at this time would also set the stage for the bubonic plague. This is known as one of the greatest pandemics to ever sweep humanity. The newly introduced rat population would act as a host to disease carrying fleas. As the rat populations spread, so did both fleas and the plague. 

However, Europe persevered and began the process of rebuilding and recovering from its losses. Part of this would also involve further expansion. People would head out from Europe for a wide variety of reasons, but we can always be sure of one thing. 

From this moment on, when humanity explores new areas they're almost certainly going to have a furry companion along for the ride. At this point rats had become quite familiar with the fact that humans could provide food, warmth and shelter. It's little wonder that they'd want to come along with us. This is especially true for ship-based travel. A ship was a virtual treasure trove for any rodent with an empty stomach. 

This was an era of naval exploration which reached every corner of the globe. Ships provided a wealth of resources due to the fact that they needed weeks or even months' worth of provisions. What's more, a ship usually provided a vast array of hiding places that could be used for nesting. Both mice and rat population groups would rise in almost any ship that was heading out for a long voyage. 

By 1776, rats had certainly reached the United States. Some even suggested the legend that this was a direct reaction to the American revolution. Historians speculate that Hessian troops hired by the British wound up bringing rodents along with extra supplies of grain. 

Another continent would see rat families for the first time just twelve years later. The European First Fleet reached Australia in 1788. It's believed that this was the point at which rat species first appeared on the continent. 

Some Particularly Impressive Feats 

So far we've seen some of the standard methods by which rat families reach new areas. Boats, and then trains, ensured that the rodents would reach as far as humanity itself. Our need to bring along large amounts of food ensured that a mouse or rat family would almost always have everything needed to come along for the trip. However, we also find some truly amazing feats associated with the rodents. 

For example, it's thought that the rat species reached Russia by means of the Volga River. Keep in mind that this didn't involve travel by boat. Rat colonies were actually seen swimming the huge river in order to reach Russia. This is hardly a unique event either. A rat can typically tread water for up to three days. What's more, they've been observed to swim for up to half a mile. 

But perhaps the most impressive and strange rat migration came from a pair named "no.18" and "no.12". Two descendants of those early swimmers in Russia would find themselves in the Korabl-Sputnik 2 space mission. The pair would launch into space on August 19th, 1960. They'd make their way back to earth on the following day. Before that point humanity had seen rodents in space. NASA in particular had sent mouse representatives of the rodent family, and some rat specimens had also made it into the upper atmosphere in high-altitude balloons, but this marked the first time that a rat had made it into orbit. 

We even find the occasional rat in Antarctica. However, these animals have typically existed in a similar state to what we see with outer space. Antarctica isn't a very hospitable place if humans aren't maintaining the environment, so when people depart the region a rat population typically won't last very long. However, in general, this leads to one amazing point. 

Rat and mouse colonies can spread at an amazing rate. They can leverage human transportation to help them spread, but even without us, the rodents can manage to traverse even large bodies of water. And once they arrive in an area they can become so common that most people wouldn't even know they aren't a native species. A species that originated in two distinct and distant parts of the world has now come to be everywhere on the face of the Earth. And in some instances, they've even managed to reach outer space!