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.
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.
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.
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.
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.
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.
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.
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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.
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.
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.