Arctic ground squirrels are — for part of the year, at least — quite literally the coolest mammals on Earth. During hibernation, which can last up to eight months in the frigid Alaskan winter, these squirrels lower their core body temperature to approximately 26 degrees Fahrenheit — below the freezing point of water. They’re the only known warm-blooded mammals capable of supercooling their bodies this way.

They achieve this feat of suspended animation by slowing their metabolisms to a fraction of the normal rate. Their bodies become so cold, and their breathing and heartbeat so slow, that it’s hard to tell if they’re still alive. Many of their brain cells and synapses die off during this process but rapidly regenerate as the animals emerge from hibernation.

The blue whale — the largest animal that’s ever lived — has a heart befitting its monumental size. Weighing around 400 pounds and measuring roughly the size of a golf cart, it’s the largest heart of any animal on Earth. 

When a blue whale dives in search of food, this mighty organ slows its rhythm to just two beats per minute. This extremely low heart rate is a remarkable physiological adaptation that helps the whale conserve oxygen, allowing the massive creature to reach depths of more than 1,000 feet and remain underwater for around 30 minutes. When it returns to the surface, its heart rate surges back up to around 25 to 37 beats per minute as the whale takes on fresh oxygen.

It pays to be the biggest creature around, and at 10 feet tall on their hind legs and weighing up to 1,300 pounds, polar bears are king. As the world’s largest land carnivores, they have no natural predators, instead spending their days as the toughest hunters around, stalking seals across sea ice and diving into frigid waters in search of their next meal.

That doesn’t mean polar bears are invincible. Cubs are occasionally vulnerable to wolves or even other polar bears, but healthy adults have little to fear from other wildlife. Their greatest threat today isn’t another predator at all — it’s the rapid loss of Arctic sea ice. As the ice disappears, polar bears lose the hunting grounds they depend on, making it harder to catch enough seals to survive.

If you’ve ever found yourself trapped in a staring contest with a goat you might have noticed something fairly unusual. Goats have horizontal, rectangular pupils. They’re not alone, either. Sheep, deer, horses, and many other grazing animals share this oblong eye shape — and for good reason. Horizontal pupils give herbivores a panoramic view of the world, allowing them to scan for predators without lifting their heads from the ground while they graze. Their pupils even rotate as they tilt their heads, helping keep the horizon in focus. 

Predators, by contrast, often have vertical slit pupils that improve depth perception and help them judge the distance to their prey. Humans fall somewhere in between, with round pupils that provide balanced, even vision and accurate depth perception — much more suitable for a species that relies on hands rather than hooves.

Much like human fingerprints, the pattern of ridges and grooves on a dog’s nose is believed to be unique to each individual pup. Kennel clubs have used nose prints as a form of identification for decades, and recent research has begun to put that long-held belief to the test. One study found that a puppy’s nose print is fully developed by the time they’re two months of age, and remains stable as the dog grows. These prints were run through a biometric algorithm and shown to be different from every other examined dog’s.

What about paw prints? Dogs’ paw pads also contain distinctive ridges, loops, and whorls, but scientists haven’t yet determined whether those patterns are unique enough to reliably identify individual dogs. For now, if you’re looking for a canine equivalent of a fingerprint, the nose is still the best bet.

Cats have a reputation for being sweet on their own terms, but there’s one kind of sweetness they never experience. Unlike humans and most other mammals, cats can’t taste sweet flavors at all. Because their wild ancestors evolved as strict carnivores, detecting sugar from fruit or other carbohydrates wasn’t truly necessary. Over time, the gene responsible for producing one of the receptors needed to detect sweetness disappeared.

Cats also have far fewer taste buds than humans — around 500 compared to our 9,000. That doesn’t mean food is flavorless to them. They can still detect sour, salty, bitter, and umami, and may even enjoy foods we find surprisingly bland. So if your cat ignores a marshmallow but comes running for tuna, it’s simply tasting the world exactly as evolution intended.