One of the most common mammals in Illinois is one you've probably never seen, and even if you did you likely mistook it for something else entirely.

Northern short-tailed shrews live across the state in both forested and open, grassy areas, according to the Illinois Department of Natural Resources. They aren't often seen because they spend much of their lives burrowing underground or in the leaf litter. And when they are spotted, it's easy to think these small, grayish creatures are another common mammal — mice.

These shrews are one of several different kinds of shrews living in the United States, characterized by their small size, long heads, small eyes and movable snouts, according to the Missouri Department of Conservation.

Northern short-tailed shrews live alone, coming together only to breed. They remain active all year long, spending most of their time underground. In winter, they dig their tunnels 10 inches to 20 inches below ground to remain below the frost line, or freezing depth, according to Northern Woodlands.

But these are just the routine facts about short-tailed shrews, which are truly fascinating creatures. Read on to learn what makes them so unique. 

They aren't closely related to mice

If a northern short-tailed shrew were to quickly cross your path, you'd probably think it was a mouse, but you'd actually be pretty far from the truth. Shrews aren't even rodents. Instead, shrews like the short-tailed shrew are classified as insectivores, a group of mammals that also includes moles and hedgehogs, according to the National Park Service.

While mice and shrews may seem similar at first glance because of their small size and grayish fur, they are actually quite different. Short-tailed shrews are smaller than mice — weighing less than an ounce — and with shorter tails too. They typically measure between 4 inches and 5 inches long with tails that are an inch long, according to Northern Woodlands. And they have small, beady eyes and pointy, pinkish noses. Compare that to some of our common mice species, like deer mice and white-footed mice, which have large eyes and ears and have tails about as long as their bodies. 

The animal that a short-tailed shrew most closely resembles is the meadow vole, another burrowing insectivore, Northern Woodlands reports. They are also sometimes confused with moles like the eastern mole, although moles are a little larger than shrews and have prominent noses that make them easy to identify.

They're one of only a few venomous mammals

You've heard of venomous snakes and spiders, but venomous mammals? They do exist, but there aren't many. In fact, the short-tailed shrew is the only venomous mammal in the entire United States, according to the Maryland Department of Natural Resources. 

The saliva of these shrews contains a neurotoxin that they can administer when biting their prey, the park service reports. The toxin is enough to either kill or paralyze their catch, making it easier to eat. And while the venom isn't enough to cause harm to large animals or humans, they do carry enough in their bodies to help them catch as many as 200 mice at a time. 

The venom in their saliva helps them kill or paralyze their prey, but it also contains a substance that helps them quickly digest protein and break down muscle tissue. This allows them to eat often without having to wait to digest their previous meal. Scientists and researchers are studying their venom in hopes that they can use it to develop drugs and other products that can be used to do everything from treat cancer to smooth wrinkles, according to the park service.

They use echolocation like bats

Bats aren't the only mammals to use echolocation to "see" their way around. Shrews do too. Northern short-tailed shrews have poor vision that doesn't allow them to differentiate anything more than light from dark, but this isn't a problem because they spend much of their time underground where it's dark. To help them navigate in these dark environs, they rely on echolocation just like bats.

Shrews' echolocation is not as precise as that of bats, but the premise is the same, the park service reports. They emit vocalizations at various pitches and gather input from the reverberations that are created. This helps them determine the features of a location, such as determining grass from dirt, and also helps them navigate through their habitat. It may also help them find prey.

Bats and shrews aren't the only mammals to use echolocation. Whales and dolphins do too, as do tenrecs, a shrew-like mammal that lives in Madagascar, according to BBC Wildlife. Some birds can echolocate as well, including the oilbird and some species of swiftlets. In addition, some humans who are blind have developed echolocation abilities.

They are fierce and ferocious

These shrews are technically omnivores, eating animals as well as plant matter, but their diet mainly consists of animal matter and they are built for preying on animals. As you might expect for an animal classified as an insectivore, short-tailed shrews do eat a lot of invertebrates, including centipedes, earthworms, snails and spiders as well as insect larvae. But they also eat small amphibians, birds, mammals and reptiles.  

While insects and other invertebrates may not put up much of a fight, some of their larger prey very well might. Remember, shrews are smaller than mice, so some of their prey are larger than them. To help them hunt, they have a set of 32 razor-sharp teeth that help them catch, crush and chew their prey, the park service reports. And that venomous neurotoxin certainly helps too.

Although shrews are fierce enough to prey on animals bigger than themselves, they are preyed on by many animals. But some of the animals that hunt shrews will leave their carcasses behind without feasting on their catch. Why? Because shrews can produce a foul-smelling musky odor that many mammals find off-putting. However, hawks, owls and snakes still eat shrews, according to the park service.

They have huge appetites

The heart of a short-tailed shrew beats between 800 and 1,300 times per minute, which is even faster than that of a hummingbird. To sustain themselves and keep up with their high metabolism, they have to eat many times throughout the day, typically every few hours, according to the University of Wisconsin-Milwaukee.

To maintain their metabolism, shrews have to eat about three times their body weight in food each day, the park service reports. If people had the same metabolism as a shrew, the average man would have to eat about 195 pounds of food a day. Because they have to eat so often and so much to sustain themselves, they sometimes kill more than they can eat in one meal and stash away their catch for later, when food is not as abundant, according to the park service. 

And in the winter, they eat about 40% more food than at other times of the year to help them maintain their body temperature, the Maryland natural resources department reports. However, many shrews don't survive winter because food is more scarce.