There's a good chance that you've seen some pretty big rats in your life and saying that they've made a strong impression would be an understatement. But just how big can a rat grow and what's the largest rat in history?
A Closer Look at the Biology Behind an Animal’s Size
To understand the size of a rat, we need to first consider the mechanisms governing their size. All mammals are thought to have descended from a now extinct species called Juramaia sinensis. 160 million years ago this tiny mammal ran around in a world ruled by dinosaurs. But when the dinosaurs became extinct, this warm blooded animal would find that thousands of ecological niches were now open. Over time this small animal, measuring about 2 1/2 inches long, would evolve into some of the largest creatures on earth. Even the blue whale, measuring in at 98 feet and 400,000 lbs., is a descendant of the tiny Juramai siensis.
All of this highlights the fact that evolution can produce some astonishing changes in an animal's size. This is an important point to consider when looking at the average size of a species. We can't simply assume that every rat will sit within a narrowly-defined size range. Sometimes all it takes is a single mutation to dramatically increase the size of an individual animal. It's often better to look at the biological order of a particular species to get a better idea of how large an individual animal might be able to grow. When we see the largest members of an order, we also get a rough idea of how much an animal's basic biology can scale up or down in size.
Looking at the Limits of a Rodent’s Size
The title of largest living rodent species goes to the coypu. The coypu, also known as capybara, is often mistaken for a rat. They have a furry hide and long tail which is easy to mistake for the more familiar rodent. The coypu also has a shockingly large frame.
A coypu usually weighs about 12 lbs., but has been seen to grow as large as 37 lbs. Their body can measure up to 24 inches and their tails can grow up to 18 inches. This means that a 37 lb. coypu measuring 42 inches long isn't out of the question. We can assume that rats are capable of existing in a healthy state at that size as well due to their biological similarity with coypu.
This emphasis on other species when trying to discover the largest rat might seem a little odd at first. But scientists are constantly stumbling on new subspecies of many different animals. And some of the more recent discoveries in the rat world are also the largest. It's important to consider what we know about the largest examples of rodent growth when thinking about potential limits of a rat's size.
Documentarians Stumble Onto a Shocking Discovery
Most people assume that scientists have discovered every large species on earth. But in reality, people are constantly finding new examples of life's diversity. When it comes to rat size, the perfect example of this phenomenon can be seen in a discovery back in 2009.
Documentarians and scientists were working on a story about an extinct volcano. They ventured out into the volcanic crater at Mount Bosavi in the hope of recording the region's unique scenery, but they wound up finding something even more remarkable — a new species of rat. This new rat, tentatively named the bosavi woolly rat, is remarkably large.
The rat's average length comes out to an impressive 32 inches, and their average weight comes in at 3.3 lbs. It's also important to note how recent this discovery is. Research on new species in a remote area typically takes a considerable amount of time. Scientists now have an average height and weight for the Bosavi wooly rat, but it remains to be seen just how large the biggest members of the species might become. However, even this average makes it the largest known rat species.
It's quite possible that a larger rat species might be out in the world awaiting discovery. And the current species of rat are subject to evolutionary pressure in the same way that the earliest mammals were. In fact, researchers speculate that we're seeing just such an evolutionary leap within a rat species in Sweden.
The Strange Case of the Mutated Swedish Rat
Think about how you'd feel if you set up a rat trap after seeing a particularly large rodent in your home. Now imagine seeing that same rat walking around with the sprung trap impotently hanging from it. This story might sound like something out of a horror movie, but it's what actually happened in the Bengtsson-Korsas household. The family had already exhausted their main anti-rodent option. Their cat, Enok, had suddenly become terrified of the kitchen.
The cat was so intimidated by the rat that he didn't even want to go into the kitchen anymore. The family next tried a standard trap. They learned one of the many reasons why more advanced and quick-kill traps are a better option. But researchers would eventually get a chance to carefully examine this peculiar rat. More and more sightings of this particular breed would show after that initial sighting in 2014. Today it's known as the Swedish viking rat. The largest example to date is an impressive 24 inches long.
The Swedish viking rat's size is only one of its amazing properties. The rat is also immune to at least one commonly used poison. The idea of a rat nearly the size of a cat who's also immune to poison is a sobering concept. But how did such an oddity suddenly appear out of nowhere?
The Bosavi woolly rat remained unknown for so long due to its secluded environment inside a volcanic crater. But a middle class family's kitchen is the exact opposite of such a secluded environment. The answer seems to circle back to the nature of evolution. Examples like the coypu show that a rodent with the rat's general body shape can thrive in the modern world. A rat simply needs to have the right genetic mutation to spur further growth.
It's just such a mutation which seems to have given rise to these big rats. The University of Huddersfield published a study which looked into the Swedish viking rat's biology. And it appears that the both the rat's size and immunity to poison arose from a single mutation. A mutation which was then passed on to the first rat's descendants. From there it's spread as these larger animals outcompete their smaller kin.
Bringing the Past, Present and Future Together
Thankfully, the chances that you'll encounter rats on the scale of a Bosavi woolly rat are very low. But that doesn't mean you should let a rat problem escalate. Even a smaller rat can become a huge problem as they start to create a colony in your home. This makes it vitally important to use a high-quality trap before they have a chance to reproduce. Modern high-tech traps will even work if you do happen to have rats on par with the Swedish viking rat.