Why do guys get sleepy after sex?

Oxytocin and vasopressin. Now you know. But here is some more informative info.

Orb spiders are weird

The male has 2 penises and the female has 2 openings for impregnation. The male detaches its penis after insertion in the female and this allows it to fight off other suitors better. Even after this act of valor the male orb spider is lucky to avoid being eaten by his selected mate. What price love? Meanwhile the orifice of the female is blocked to other males.

Explore the wacky world of weird wildlife willies. Here's an example to whet your appetite.

Argentine blue-bill lake duck

The Argentine lake duck has something to quack about. Its penis is a whopping 17 inches long, proportionally the longest of all vertebrates. It's especially impressive among birds: Most male birds don't have penises, and instead mate by briefly touching genital openings with females. The Argentine lake duck has a corkscrew-shaped appendage, which mirrors the female’s oppositely corkscrewed vagina. They really do get screwed! No one knows why this duck evolved a gigantic member, but some researchers have speculated that the giant penis may be an example of "runaway" sexual selection, where female preference drives male anatomy to extremes, as in the peacock's tail.

There are pictures.