50% of sloth deaths occur during their once a week trip to the ground to dig a hole and defecate. They are near blind and can only see about 3 inches in front of them. They can’t self-regulate body temperatures very well. Therefore the temperature and humidity have to be perfect for them to survive. They use fermentation to digest food, so if they are fed too many fruits they may die from alcohol poisoning. They're so lazy that despite the female being in heat, the male sometimes just falls asleep/lacks interest, leading to a huge lack of reproduction. You have to whisper while you're with them otherwise they may get too stressed and have a non-symptomatic heart attack and die.
2. Kakapo is a large nocturnal ground-dwelling parrot. If attacked, the bird will remain motionless or will jump on some high branch and throw itself, which is weird, since they can't fly. Only around 100 of them are left in the wild and still, it rigorously adheres to a spectacularly convoluted and almost entirely ineffective mating ritual. It wobbles through the valleys of New Zealand dangerously unafraid of its predators. Males have a mating call that can be heard four miles away, but females can't tell where it's coming from.
3. Evolution made cicadas fatter, delicious and numerous. Predators eat cicadas till they're stuffed, and whatever left is enough to lay eggs for the next generation. The next generation emerges after 13 or 17 years because these prime integers rarely sync with other integers making it rare that they will breed in sync with predators, so predators won’t be able to adapt and prepare themselves for the next buffet. Therefore cicadas' entire survival strategy is to get eaten.
4. A wild pig species named babirusa can grow long tusks and if they don’t grind their tusks, it will curve back, penetrate its skull and into its brain, killing it slowly and painfully.
5. Male bat bugs have sharp needle-like penises, which they use to stab the female bat bugs right in the stomach, even though they have a vagina. Sperm is injected directly into the bloodstream, which isn't even where it needs to go. They have been observed to even stab other males. Both genders have evolved “false vaginas” in the middle of their abdomens to make this fun hobby less painful.
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Banana slugs which are hermaphrodites can be 6-8 inches and their erect penis can be just as long. In addition, their penises emerge from their heads. After mating, banana slugs eat each other’s penises.
7. Female spotted hyenas have the largest clitoris of all mammals. It is shaped like and larger than the male hyena’s penis. This is referred to as “pseudo-penis”. This means that it is really hard for the males to have sex with a female hyena who isn't receptive to it. It does have drawbacks though, in that they give birth to live young who have to come through that long and not too stretchy pseudo-penis. It is not uncommon for the pseudo-penis to split during childbirth and for the mother to bleed out. A lot of hyena cubs die before they're actually birthed because they suffocate on their way out.
8. Mayflies have a lifespan of only a few minutes and rarely live past a day. A male mayfly has two penis-like organs and the female has two sexual openings.
9. A small mouse-like Australian creature called antechinus literally f*ck themselves to death. The male stops eating and sleeping when mating season starts and he can mate with females for up to 14 hours at a time. He exhausts himself so thoroughly that his body starts to fall apart. His fur falls off. He bleeds internally. His immune system fails to fight off incoming infections, and he becomes riddled with gangrene. By the end of the mating season, physically disintegrating males may run around frantically searching for last mating opportunities.
10. Greenland sharks have the longest known lifespan of all vertebrate. They have an average lifespan of 400 years. Unfortunately for them, they spend most of this long lifespan swimming blind despite being born with fully functioning eyes. Majority of these sharks are blind due to the fact that they have parasites hanging from their eyes. These pinkish-white parasites are called copepods, and they attach themselves to the Greenland sharks’ cornea, rendering them blind.
Peppered moths evolved to blend in with tree bark. Then the industrial revolution covered all the trees with soot, so they stood out like a sore thumb and got eaten by birds. This created a selection pressure towards black pigment making them darker. They are now lighter again after pollution regulations cut down sooty coal emissions.
12. Sea cucumbers spit out their guts via their anus as a defense mechanism. Also since pearlfish has no defense mechanism of their own, they hide from predators by burrowing into a sea cucumber's anus. While inside, it may eat the cucumber's gonads. Although the cucumber normally defends itself by squirting out its guts, the pearlfish does not trigger this response, possibly because it isn't worth the trouble.
13. Stephens Island Wren had no natural predators, so evolution rendered it flightless as it didn’t need to escape. Humans arrived on the island and one year after discovery and soon after its declaration as a new species, it went extinct. The lighthouse keeper's cat killed every single one of them. It is the only species to be eradicated by a single, lone entity.
14. Australian jewel beetles are threatened because they try to have sex with beer bottles that look/feel like the female Australian jewel beetle and they subsequently die of sun exposure.
15. Deathwatch beetles start their life as a pupa emerging from an egg and eat wood for 10 to 15 years, at the end of which they emerge as an adult beetle. At this point, they are blind and have less than a week to find a mate and have sex. So the male starts banging his head against the wood and hopes for a female to answer his calls. If he is lucky and finds a female, there are chances that she may reject him.
16Crab Hacker Barnacle
Crab hacker barnacle is a species of barnacle that is a parasite found in crabs. The female will attack a crab, movie inside it, push out a sac that contains its eggs. It then controls the mind of the crab into caring for its egg sack as its own, and when the time comes the crab shoots them out through pulses creating a large cloud of parasites.
17. The ophiocordyceps unilateralis is a parasitical fungus that infects and exerts mind-control over certain species of ants. It will hijack an ant’s central nervous system and force the ant to climb a stalk of vegetation and anchor itself. The fungus then sprouts a stalk from the ant’s head, releasing spores that infect ants below. It can destroy entire ant colonies. In response, ants have evolved the ability to sense that a member of the colony is infected. Healthy ants will then carry the dying one far away from the colony in order to avoid the fungal spore exposure.
18. A giraffe starts its life by falling 2 meters (6ft) to the ground. Because they need to get blood to their brains through their long neck, they have a huge heart. Consequently, their blood pressure is extremely high, the highest of any animal on the planet at 280/180 and their heart rate rests at about 170bpm. The Humr people of Sudan consume a hallucinogenic drink; which is created from the liver and marrow of giraffes.
19. Bedbugs reproduce via a process known as “traumatic insemination” where the male pierces the abdomen of a female with his lance-like penis and injects his seed into the wound.
20. The duck-billed platypus has a gene mix of birds, reptiles, and mammals and genetically has 25 possible sexes. The Platypus is one of the few mammals that lay eggs. They lactate but have no nipples, hunt via electricity, and their venom has 80 different kinds of toxins.
Despite being commonly referred to as "mosquito eaters" and resembling giant mosquitos, adult crane flies are anatomically incapable of harming the insect. In fact, their lifespans are so short (less than a week) they typically don't eat anything at all.
22. The tapir is a large, herbivorous mammal which looks similar to a pig. They are so well endowed that they can step on their penises while mating and trip as a result.
23. The tripod fish walks around the bottom of the ocean floor on their three fins looking for a partner to reproduce with. If they can't find a partner, which is almost always as the ocean floor is giant and completely dark, they reproduce with themselves as they are hermaphrodites.
24. The New Mexico whiptail is a female-male only lizard that reproduces asexually via parthenogenesis (eggs don't need to be fertilized). Despite reproducing asexually, and being an all-female species, the whiptail still engages in mating behavior with other females of its own species, giving rise to the common nickname “lesbian lizards.”
25. Immediately after sexual intercourse, a female praying mantis will rotate her head 180° and eat the head of the male. Therefore male praying mantis has an extra “brain” in its rear, which controls the necessary motions for copulation, so it can continue mating after the female has eaten his head.