The world's largest bacteria is so big you can see it with a magnifying glass, not even a microscope, but a Sherlock Holmes style magnifying glass. It's called Thiomargarita namibiensis, in case anyone was wondering.

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There is this parasite called Cymothoa exigua that enters the gills of a fish, eats their tongue, and then replaces it. It's pretty darn freaky

Here is the wiki about it.

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Sloth hands work opposite to ours. They have to exert energy to open their “fist” and relax to close it. This is how they can hang from trees while they’re sleeping and not fall off.

Dragonflies have the most successful kill rate of any creature on earth.

Some squid have toroid shaped (donut) brains, and their esophagus (throat) passing through it. If they eat something too big they can get brain damage.

Some species of burrowing tarantula let tiny frogs live in their burrows. The frogs are kept safe by the big, mean spider, and in exchange, they keep the burrow free of pests too small for the tarantula to deal with. This is pretty much how cats were domesticated.

Tiny frogs are tarantula cats.

Crows are absurdly intelligent. We all know they have very good problem solving skills, like throwing a nut onto a road and picking the remains when the lights turn red.

But they have social structure. They punish crows who have stolen food from younger crows. They avoid areas where a lot of crows have died. And they don't caw during funerals.

And when a crow dies, its buddies come examine the corpse to see how it died so they can avoid a similar fate. Yes, crows understand the concept and are afraid of death and try to avoid it at all costs.

A horse can outrun a bear.

If you see a mountain lion, it has already decided to eat you.

If you're camping and you hear an animal moving around, it's probably a skunk or a beaver or a porcupine. Bears, moose, and other large animals are surprisingly sneaky.

There's an insect called the scorpionfly that impresses mates by bringing them prey to eat. The bigger the meal, the better.

Only, some scorpionflies aren't that great at catching food. So some of these males will imitate females, and wait for other males to bring them their gifts. Then they take the gift, fly away, and give it to an actual female.

Seals will get seasick if you put them on a boat.

A green sea turtle can swim faster than Usain Bolt can sprint. That's right: an animal out there with a house for a body can swim at speeds we don’t allow in playground zones -- 35MPH.

Naked mole rats are very weird animals. They don't feel pain at all, except for chilli. They can survive 30 minutes without oxygen. Mole rats can't regulate their body temperature, but their incisors can work independently. No one has ever found cancer in a mole rat; they are thought to be immune to it.

They can live for 30 years.

The American bison is the only animal in which both lungs share a common cavity. In every other animal the lungs are separated. What this means is that when shot by an arrow, both lungs would collapse and the buffalo would suffocate quickly. (For other animals, being shot in the chest would only collapse one lung, and they would at least have a chance.) That's why such a large animal could be taken down with relatively small weapons.

There aren't a lot of bees in Australia, so for pollination, plants rely on birds. Birds see the colour red better, which in turn increases the flora reproduction rate -- this is why a lot of plants are red in Australia.

The Greenland shark reaches sexual maturity at 150 years old and can live as long as 300-500 years. It always fascinates me that theres something that can live that long.

Gorillas will pleasure themselves after they win a fight against another Gorilla.

Some stores will only sell gerbils in pairs because they get very lonely. I think it's in Switzerland where it's forbidden by law to only keep one guinea pig. The loneliness is considered animal cruelty.

Porcupines climb trees and come down backwards and use their tail to feel for the ground. Sometimes their tail will hit a branch and they will think it is the ground and so they will jump off and impale themselves. Over time they evolved to have antibiotics in their quills so when they fall out of trees they don't die.

Tasmanian devils can carry a type of cancer that is contagious to all others of their species. There was a serious epidemic in Australia not too long ago. They’ve created a vaccine though -- really interesting stuff.

Snapping/pistol shrimps and their colonies are so loud that they can interfere with sonar used by other animals and submarines.

They also create tiny but incredibly powerful implosions through the sheer force of their pincers closing ludicrously fast, which is cool.

Flatworms fence with their genitals. The loser gets stabbed with the winner's appendage and becomes pregnant. They are actually hermaphroditic, and being the one to have to carry babies is very taxing, so they will fight to not have to do that!

Stoats hunt for fun, and live in the dens of things they kill. They also do a hypnotizing dance to disorient their prey. It looks like a cross between a one-man mosh pit and a seizure.

The Warrior Wasp's sting was described by biologist Justin Schmidt (Creator of the Schmidt Sting Pain Index) as: "Torture. You are chained in the flow of an active volcano. Why did I make this list?" It was also considered "traumatically painful" on the Starr Sting Pain Scale.

According to Coyote Peterson (Brave Wilderness), who made videos of himself being stung by the most painful insects in the world, the Warrior Wasp's sting isn't nearly as bad as the Bullet Ant. Being stung by the Bullet Ant feels like you're being shot and the pain can last for up to 24 hours. It hurts to even think about it.

Both an animal and a plant fact.

​Avocados were evolved to be eaten by the giant ground sloth. Imagine a sloth, but so massive it could only live on the ground. The flesh attracted the sloth and the very large seed was "designed" to travel through their intestinal tract for re-seeding or whatever plants do.

Humans killed the sloth but loved the avocado. We therefore domesticated the avocado to increase the yummy flesh. The pit has gotten smaller but is still very large. Whenever you eat an avocado, think of the long extinct giant sloth.

Another cool fact: scientists in south America have been studying massive, symmetrical cave systems in the jungle that are anywhere from 30 to 300 feet deep with different branches and chambers. Because of their shape (perfectly oval-shaped and symmetrical), they couldn't have been formed by water flow, or nearly any other geological process. However, by looking at the markings on the walls of these tunnels, the scientists have determined that they were carved out over many generations by ground sloths.

Different species created different sizes, with some being 3 feet wide, and others over 10 feet wide, and hundreds of feet deep. It's all very interesting.

Ants breed and domesticate aphids around their colonies so they can drink their milky secretions. Aphids are ant cows. So ladybugs, then, would be like ant chupacabras.

There are also a variety of ants called slave making ants that will go scout around for other ant colonies nearby. Then they go on a little ant crusade and invade another colony. They capture the larva of the target colony and leave with them. The larva are then tricked into thinking they're originally from the slavers colony and will serve  it how they normally would have their real colony.

A snail has over 2,000 teeth. And they can bite you. I have been bitten by a snail.

Ducks are extremely aggressive with crazy inflatable corkscrew genitals, so the females have evolved corkscrewed genitals of their own that corkscrew the other way. The females also have "pockets" in there to store unwanted seed before it gets too far.

Long story short: most duck sex is not consensual, but there are very few babies as a result.

SPIDER FACT: Did you know that spider webs are initially liquid? Upon coming into contact with the air, the web hardens, creating a substance that is 5x stronger than steel. It is believed that, if a spider could produce threads as thick as a pencil, they would be strong enough to stop a plane in flight.

However, farming silk from spiders is largely impossible, both due to the quantity of the silk, and due to the fact that the spiders attack one another. For this reason, scientists have genetically engineered goats with spider DNA that can produce silk through their milk. They are called spider goats, and they are chimeras — a creature with the genetic information of two animals. In this case, the genetic information comes from two different species.

There's a parasite that needs to get inside a warm-blooded animal. Problem is, it lives in fresh water. So it waits to get swallowed by a fish. Then it "takes over" a part of the host's brain, adjusting its behavior so the fish is no longer afraid of shadows. The fish is then more likely to get eaten by a heron (large bird that stalks freshwater fish). That way the parasite gets inside its warm-blooded host and does lord knows what.

Here's a bonus one. Researchers took a leech, put it in a maze and put food at the other end. Eventually the leech figured out the route. They then liquidized the leech, fed that leech to other leeches, and those leeches were able to complete the maze on the first try. They had "acquired" the other leech's memory

People thought the okapi (sort of mix between a horse and a giraffe) was a hoax until it was photographed.

In parts of Sumatra it is believed that orangutans can speak but refuses to do so because they don't want to work.

Squirrels are responsible for thousands of new trees every year. They collect and bury their nuts all over the place so they'll have food to last thru winter, but they forget about most of them.

Scientists thought the platypus was a joke until they send a dead one back to be studied. One of the scientists of the time actually took a knife to the dried pelt to search for stitches. They thought it had to be a prank with different animals sewn together.

Fish have been seen using tools -- deliberately hitting a clam with a rock to get it to open so they can eat it. Some fish (eels and groupers) form hunting partnerships where they communicate across the species barrier with specific signals to put their individual strengths to work (the eels chase prey out of crevices to where the groupers are waiting in open water). They also get fooled by illusions in the same way we do, meaning that their brains are processing and interpreting their environment in a similar way to ours.

Woodpeckers' tongues wrap around their skull to help absorb the impact of smashing their face into wood repeatedly.

Opossums were originally found in the eastern and central parts of the United States until the 1930s when they were intentionally transported to the western portion to be used as food during the Great Depression.

Spay and neuter your pets, but especially your ferrets.

Coral is an animal, related to jellyfish. Many coral species have a symbiotic relationship with a microorganism called zooxanthellae, which lives in their tissue and photosynthesizes like a plant, converting light into organic energy. Corals also deposit calcium carbonate and build huge geological structures, called reefs. The most massive structure ever created by any living organism on planet earth is a coral reef.

Corals are like a cross between animals, plants, and rocks, and they’re incredibly important for the health of our oceans, because reefs serve as a “nursery” for many, many marine species. Save the reefs.

Humans and dogs are the only two different animals that share a bond that we do. "Behavior research supports the recent neuroscience too. According to Andics, dogs interact with their human caregivers in the same way babies do their parents. When dogs are scared or worried, they run to their owners, just as distressed toddlers make a beeline for their parents. This is in stark contrast to other domesticated animals: Petrified cats, as well as horses, will run away.

"Dogs are also the only non-primate animal to look people in the eyes. This is something Andics, along with other researchers, discovered about a decade ago when he studied the domestication of wolves, which he thought would share that trait. They endeavored to raise wolves like dogs. This is a unique behavior between dogs and humans — dogs seek out eye contact from people, but not their biological dog parents."

Source.

Fishkeeping is a gateway to an untapped goldmine of obscure, useless, and easily demonstrable animal facts. Freshwater in particular is great for this, because there's a lot of weird stuff that ends up in the hobby, and you just tend to learn about it as you get more into it.

• Did you know that Electric Catfish exist? They're weird looking and a tad stupid, but they do in fact generate offensive electric shocks.

• Electric eels are not in fact eels, but rather South American knifefish, a group which contains several species of electricity-generating fish, though none as potent.

• Bichirs are a group of primitive freshwater fish that have basic lungs, lobed, limb-like paired fins, and - while in the larval state - feathery external gills like a salamander.

Hippopotamus milk is pink. So is their sweat. It often looks like they're sweating blood.

On ostrich farms, some farmers have a hard time with breeding because the ostrich is more attracted to humans than other ostriches.

That's right: somewhere out there an ostrich wants to make sweet love to you.

Crocodiles never die of old age. They continue to live and grow for as long as there is a sustainable food source around. Most crocs die because they're killed by humans or they grow to big and die of starvation because they can't eat enough to sustain themselves.

Here's a fact you'll want to know if you're a dog owner -- or just a dog-lover. The reason dogs sneeze when they're playing with humans is to show that they are in fact playing, and not being aggressive. It's their way of saying, "Just joshing!"

Sharks have existed on the planet longer than trees.

That still blows my mind. Like, you think of early life on this planet. What is there? Plants, moss, grasses, vines, bushes, and trees. But oh, no, trees haven’t been around forever. They had to evolve from other plants. And while those pre-tree plants were doing their thing SHARKS WERE ALREADY SWIMMING IN THE OCEAN EXACTLY THE SAME AS THEY ARE NOW!