The baculum, also known as the os penis, or penis bone, is something of a mystery. Read on to find out what it is, what it's for, and why you might want to wear one around your neck.

1. THEY COME IN ALL SHAPES AND SIZES.

Most male mammals have penis bones, but no two species' are alike. They vary widely in shape and size. Some are spiked or barbed; some are curved. They're not proportional to an animal's size, either; a gorilla's baculum is just a few millimeters long, but a dog's can reach three inches.

2. NOBODY REALLY KNOWS WHAT THEY'RE FOR.

The baculum helps keep its owner's penis stiff, but the exact purpose is still unclear. Scientists have theorized that it might make it easier for a male animal to maneuver inside his mate. Others think the bone may stimulate the female reproductive tract, encouraging her ovaries to release eggs. Still others say it could help animals mate for longer periods of time. The bacula of promiscuous animals are usually bigger than those of animals who are monogamous, so it may have something to do with sperm competition. A bush baby that can mate for hours stands a much better chance of becoming a father than one that can't.

3. THEY CAN FETCH A PRETTY PRICE AT AUCTION.

In 2007, the fossilized penis bone of a walrus was auctioned off for $8000 to the company that runs the Ripley’s Believe It or Not museums. At four-and-a-half-feet long, the bone is believed to be the largest mammal penis fossil in the world. The final selling price was actually a disappointment to the auction house, which thought it could sell the bone for $16,000. "I'm glad it's going to a museum and not a private collection," the auctioneer told the Associated Press. "It's definitely something everyone should see once in their life."

4. BUT YOU CAN PROBABLY AFFORD ONE.

Even if your penis bone budget is a little more modest, you don’t have to go empty handed. Taxidermists sell raccoon and skunk bacula on the Internet for just a few cents apiece. You can even buy them on Amazon, although based on the reviews (“It is about half the size of a normal adult coon dick toothpick”), you may not want to.

5. THEY MAKE LOVELY NECKLACES.

Raccoon penis bones are considered by some in the South to be lucky charms. The little bones are also known as coon dogs, Texas toothpicks, and Abrahams. Southerners wear them on necklaces, keep them in their pockets, and even include them in bridal bouquets. The trend went Hollywood in 2004, when Sarah Jessica Parker and Vanessa Williams were both seen wearing raccoon baculum earrings.

6. WHEN THE PENIS BONES WERE HANDED OUT, SOME SPECIES, INCLUDING HUMANS, GOT STIFFED.

Most male mammals have bacula, but there are a few notable exceptions: lagomorphs (rabbits and hares), ungulates (deer, camels, rhinos, hippos, horses, pigs, and giraffes), marsupials (koalas, kangaroos, and quokkas), monotremes (platypuses), elephants, hyenas, cetaceans (whales and dolphins), and, of course, humans are boneless in the nether regions. You will notice that most of Earth’s largest animals, including whales and elephants, have no bacula, which is why that auctioned walrus weenie bone is the biggest we have.

7. NOBODY KNOWS WHY THE HUMAN PENIS IS BONELESS.

We're an outlier. With the exception of spider monkeys, all other primate species have bacula. Some scientists believe that our evolution into (generally) monogamous animals reduced competition, which did away with the need for the fatherhood sex marathon which may be enabled by a penis bone. Other scholars take a more metaphorical approach. A 2001 letter to the editor in the American Journal of Medical Genetics, jointly written by a professor of biology and a professor of biblical literature, theorized that Adam's Rib—you know, the one that became Eve—was actually his penis bone.

8. THAT'S FINE. WE DON'T NEED 'EM.

Clearly, humans are doing just fine without bacula. The human penis can stiffen even without a bone; arousal cues increased blood flow into the spongy tissue of the penis, where it is temporarily trapped. The increased pressure creates an erection. And while the bone itself may be gone, men do have a structure like it. In 2005, a Taiwanese urologist discovered what he called a “distal ligament” at the tip of the penis. Like the baculum, the ligament helps keep the urethra open, which acts as a "supporting trunk" and ensures an open pipeline for sperm. It may be that humans never lost the penis bone—it just turned into something else.