1. Antechinus, a tiny Australian marsupial, breeds so hard in a few weeks that it essentially screws itself to death.
2. Female Chinese Fruit Bats contort themselves to give the male fellatio while they're having sex.
3. The short-beaked echidna's balls increase in size by 200% when mating season begins.
4. Male honey bees' genitals explode at the end of sex, leaving the tip of their penis in the female as a sexual plug.
5. The male anglerfish bites into the female one and slowly rots away until it's just a pair of gonads that's conjoined to her.
This is a reconstruction of what it would look like if humans mated like this.
The thing is, male anglerfish need to do this because otherwise they'll die. Well, die sooner.
Basically, they are a) rubbish at catching prey and b) usually have underdeveloped digestive systems. When the bite you see above takes place, an enzyme is released that digests both the skin of the male's mouth and the female's body.
In probably the weirdest sexual quid pro quo you'll read about today, the male receives nutrients through the shared circulatory system, and in return he gives the female his sperm. He'll stay "alive" just as long as the female lives and can take part in multiple spawnings.
But why? Well it's probably because there's very little mate choice for the males. So on balance, it's likely to improve lifetime fitness, and it means they get more bang for their buck. In the sperm stakes.