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What is a baculum? -

The baculum, or the penile bone, is a bone that resides in some mammals, including many primates, rodents, bats, carnivores, and some insectivores.

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The penile bone -

Cats and dogs also have a baculum (pictured is an X-ray of a canine). But it's not present in all mammals, and humans don't have this unusual bone either.

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Etymology -

The word baculum means "stick" or "staff" in Latin and originates from the Greek baklon (stick).

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Does size matter? -

How long is a baculum? Well, that depends on the mammal in question. It can be as long as a finger in a monkey. Or it can measure 2 ft (0.6 m) in a walrus. Pictured is a zoologist wielding a large walrus baculum during a presentation on the subject.

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Baculum timeline -

The baculum was first identified in the 17th century. Its exact functional significance was unclear. Scientists were stumped by the fact that some primates had the feature while others didn't. For answers, researchers traced the bone's evolutionary history through time.

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Evolution -

They found that the penile bone evolved in mammals more than 95 million years ago, during the Cretaceous Period.

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Primates and baculum -

Further study revealed that the baculum was present in the first primates that emerged 50 million years ago, during the Early Eocene Epoch.

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Different sizes -

Scientists concluded that the baculum was present in the most recent common ancestor of all primates and carnivores. And from the Eocene Period onwards, the baculum became larger in some animals and smaller in others.

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Stump-tailed macaque -

For example, the stump-tailed macaque, an animal that weighs just 22 lbs (10 kg), has an extremely long baculum for its size.

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Collared mangabey -

In fact, the stump-tailed macaque's penile bone is five times the size of the baculum in the collard mangabey (pictured), which is a larger monkey.

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Mating strategies -

Variations in size and shape apart, the penile bone allows a male to mate for a prolonged time with a female. How is this so? Well, it's all about mating strategies.

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"Prolonged intromission" -

According to studies, penile bone length is longer in males that engage in what's known as "prolonged intromission." In other words, the act of penetration that lasts more than three minutes.

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Keeping ahead of the competition -

Longer intromission is a strategy that helps the male impregnate the female while keeping her away from competing males.

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Increased sexual activity -

In primates especially, the presence of a penile bone was most strongly correlated with increased intromission duration.

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Polygamous mating practices -

In fact, prolonged intromission times often occur in species with polygamous mating practices, where multiple males mate with multiple females. Chimpanzees and bonobos are species that typically follow this practice.

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Lessening the odds -

In turn, increased intromission duration creates an intense competition for fertilization. To lessen the odds, males look to reduce a female's access to additional mates by spending more time having sex with her themselves.

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Keeping up appearances -

Throughout all this feverish activity, the baculum provides structural support for male animals engaged in prolonged intromission.

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Homo erectus -

Researchers believe humans likely lost their penile bones when monogamy emerged as the dominant reproductive strategy during the time of Homo erectus about 1.9 million years ago.

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Monogamous relationships -

By engaging in a monogamous relationship—having only one partner at any one time—the mating strategy during this period changed. Males no longer felt the need to prolong their own act of intercourse because the threat of competition from other lustful hopefuls had diminished.

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Disappearance of the penile bone -

Ancestral humans' upright posture may also have played a part in the disappearance of the penile bone. But the exact reason why human males lack a baculum remains a mystery.

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A myriad of forms -

A particularly striking characteristic of the baculum is its extreme anatomical diversity. The penile bone of different species comes in a myriad of forms, varieties in length, thickness, curvature, and complexity of shape. The baculum of large carnivores for example, animals such as bears, are relatively simple in appearance.

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Odd-looking features -

Squirrels, on the other hand, are possessed of odd-looking bacula with spoon-shaped distal ends and tooth-like projections.

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Baculum beasts -

The previously mentioned walrus and other pinnipeds such as seals and sea lions are the baculum beasts, possessed of enormous penile bones.

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The female baubellum -

In case you're wondering, the female equivalent of the baculum is the os clitoridis, also called the baubellum. It is absent from the human clitoris, but present in the clitoris of some primates, such as the ring-tailed lemur.

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A bone to pick -

In the Far East, mammals' members are considered a delicacy in some countries, eaten for their supposed libido-boosting remedies. Here, a waitress displays a dog's baculum in a Beijing restaurant.

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A biblical euphemism? -

While the etymology of the world baculum has been traced back to ancient Greece, it's been suggested by some scholars that the "rib" in the story of Adam and Eve is actually a mistranslation of a biblical Hebrew euphemism for baculum. In other words, Eve was not created from Adam's rib, but his penile bone.

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Cultural significance -

And in Native Alaskan cultures, for example the Iñupiat (pictured), oosik is a term used to describe the bacula of walruses, seals, sea lions, and polar bears.

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Good luck charm -

Meanwhile in Hoodoo—a form of African-American folk magic—the raccoon baculum is sometimes worn as an amulet for love or luck. 

Sources: (Smithsonian Magazine) (The Royal Society) (Current Biology) (Haaretz) 

See also: Good luck charms, bad luck signs!

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