What Do Dogs and Other Mammals Have That Humans Don’t? A Penis Bone
In a place before time, life somehow emerged on Earth. It grew and flourished. It changed. It split, and some of it went its different ways. There was life in the water and life on the land. Some life stayed in the water, and some emerged onto land. Some parts returned to the water, and other parts of it took to the skies. All of it evolved and kept on growing and changing. Natural selection kicked in, and the strong survived. By the later stages of this progression some mammals developed a penis bone, and others didn’t. This article aims to answer the question why.
Why Don’t Humans Have a Penis Bone?
The question doesn’t have an obvious answer, because the truth is that we just don’t know. And you know what happens when humans are confronted with a mystery, don’t you? We can’t stand it. Why doesn’t all the water drain off the edge if the Earth is flat? Raindrops keep falling on my head. Where do they come from? Why do those damn apples keep falling on my head? I know where they come from. But why me? And then the question at hand: Why don’t humans have a penis bone?
But, what is a penis bone? The official name for the penis bone is baculum. The plural form is bacula. The word derives from the Greek baklon, which means stick. It then came to us through the Latin baculum, which means staff or stick. The penis bone is also referred to as the penile bone, os penis, os genitale, or os priapi.
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Penis bone of the Japanese macaque. (Didier Descouens / CC BY-SA 4.0)
The label is descriptive. A penis bone is a bone in the penis. It belongs to a classification of heterotopic bones or bones that are dissociated from the skeleton. In most cases, it rests in the abdomen until sexual arousal causes it to extend within the erectile tissue of the penis. A specific description of appearance is impossible. The shape of a baculum varies from species to species. Some have a smooth rounded tip, while the tips of others look like teeth, barbs, an ice cream scoop, or even a pitchfork.
Forbes reported that the largest baculum was discovered in an extinct walrus and measured 4.5 feet (137 cm). Modern walruses still have big bacula. They measure around 2 feet (0.6 meters) and resemble a club. But, if the walrus is making you feel inadequate or wanting, consider that some primates got the short end of the stick with penis bones that only measure 0.079 inches (2 mm) to 0.51 inches (13 cm). It could be worse.
The anatomy of a dog showing the location and shape of the canine penis bone. (decade3d / Adobe Stock)
What Does a Penis Bone Do?
Sometimes a penis bone will be something for some of its possessors, but it can’t be everything for all its possessors. The following categories are possible functions for a penis bone, but an article byy Anna Nele Herdina et. al. published in the Journal of Anatomy explained that variations in applicable functions will vary among species.
The penis bone provides rigidity to the penis. The implication is that rigidity is necessary for the male organ to penetrate the vagina. That isn’t hard to understand, given the tight spaces where it works. It is the most obvious explanation of the baculum’s function. Anyone who breeds dogs understands the canine mating process. The male’s penis is flaccid when it enters the female. Without the penis bone, entry would not be possible. The canine penis does not become erect until after penetration is achieved.
While all that sounds good, it may not be true all the time or as accurate in some animals. The tip of the baculum is often clearly designed to facilitate penetration, such as a wedge or pointed tip. However, such a purpose is not entirely applicable across all species.
An article by Charles Long and Theodore Frank in the Journal of Mammalogy explained that in ground squirrels, the comb-like structure of the baculum seems to be designed more for the stimulation or agitation of the vagina than for penetration. In addition, Inverse clarified that some researchers assert that the rigidity provided by the baculum serves to keep it firmly implanted once coupling has been achieved more than being a tool to achieve penetration. It seems as if deciding what the baculum is for is rather hard.
If you ever wondered, I’m that person who has like 1 1/2 mimosas and shows everyone pictures of ground squirrel baculums. Baculua? What’s the plural of baculum? Spaghetti scoops? pic.twitter.com/dgLkru1Bhn
— Melissa, Queen of the Sloths(@slothgirlmel) June 19, 2021
Placed directly over the urethra, the penis bone protects the urethra. The arrangement sometimes seems strategic. It is possible, without it, due to the pressure of the male erection and the constriction of the female organ, for the urethra to be damaged.
Herdina et. al. argue that the baculum protects the length of the urethra from collapse, the distal end from being crimped, and the opening of the urethra from being pinched. The baculum guards against permanent damage to the urethra and assures it will be an optimally functional tool and a clear pathway for the delivery of sperm. However, when you think you have it all figured out, there is often an exception to the rule. The baculum of the great ape is small and located near the tip of the penis. It doesn’t protect the entire length of the urethra.
It acts as a guide for placing semen close to the cervix. According to The Daily Mail, the honey badger has a baculum with a tip shaped like an ice cream scoop. The scoop seems to be designed to cup the cervix and provide accurate delivery of sperm. Since the female will mate with more than one male, the shape also seems designed to scoop out any rival’s sperm.
This mating strategy is called offensive sperm competition. It stands to reason that any species employing such a strategy, in either a courteous or an offensive manner, would see a stable or increasing evolution of the baculum. However, just the opposite is true. The complexity of the baculum increases in monogamous groups where sperm competition is absent, and complexity decreases in polygamous and highly competitive animals.
And if you think the modern walrus baculum is impressive... there is a mystery walrus from the fossil record of Japan known ONLY by it's penis bone. It's the giant one on the bottom, modern walrus in the middle for scale. 12 million years old, baseball bat sized: pic.twitter.com/8xKsua4Izj
— Robert Boessenecker, Doctor of Whaleontology™️ (@CoastalPaleo) June 15, 2019
It stimulates the female to ovulate. Ice cream scoops, saws, teeth, barbs, shovels, pitchforks, nail pullers, blunt tips, pointy tips, skinny, fat, wide, narrow, long, short…. One theory is that the shapes and ends of bacula have evolved into the design that effectively stimulates ovulation during mating for each species. That’s not too hard to believe, is it?
It overcomes vaginal friction. This theory, described in Scientific American, is a bit rough around the edges. It argues that the baculum is designed to work like a shoehorn and ensure that the male can penetrate the vagina.
It allows lengthy intromission. Intromission is another way to describe how long sex lasts. It is more than the time between the penetration of the female to the point of male ejaculation. It is the measure of the entire time the pair stays coupled together. Lengthy intromission is a strategy during mating when the time the male couples with the female is extended to prevent her from mating with rivals and provide his sperm as much time as possible to fertilize an egg.
If we return to canine mating, a 30-minute intromission period is not unusual, and it may last as long as an hour or so. However, the male ejaculates multiple times, the first being early on during intromission. By contrast, Intromission in humans may be as short as one or two minutes with a male who exhibits premature ejaculation. An extended period of intromission is 7 to 13 minutes, claims the Between Us Clinic. What’s that? I’m hearing lots of bubbles popping.
Artic Fox baculum. (Travis / CC BY-NC 2.0)
Has Anyone Seen a Penis Bone? I Seem to Have Lost Mine
The Guardian reported that the development of the penis bone is seen in mammals about 95 million years ago, while it developed in primates about 50 million years ago. Evolution seems to have run wild with the penis bone after its evolutionary origins because it began to change with each species, becoming longer, shorter, wider, and skinnier from one species to another.
All primates, except for spider monkeys and humans, have penis bones. What type of extraordinary forces influenced the loss of the penis bone in humans? One particularly poetic explanation is that the bone that God removed from Adam, commonly known as Adam’s Rib, was, in fact, the penis bone. That would account for the same number of ribs in male and female humans and the loss of the male baculum.
Phew. That explains that then, right? Unfortunately, it doesn’t. It doesn’t explain why a human female lacks a bone in her clitoris. The structure is called a baubellum. It is the female counterpart to the male baculum. If Adam had a baculum, Eve would have had a baubellum and would probably retain it. Only she didn’t and doesn’t. Unless she did, and she lost it, too. But that is another story.
Another reasonable hypothesis suggests that Mother Nature decided a baculum for a human male wasn’t a good idea. Imagine the early caveman. He hits a female on the head with a club and drags her off by her hair, only to be confronted by another male. All sorts of hand-to-hand combat takes place. The outcome is determined by some groin kicks and groin punches which leave one of the males with a shattered baculum. Imagine that. Think a broken jaw hurts?
That leads to a different idea about how evolution conspired to deny a baculum to the human male. Evolution decided humans don’t need a penis bone, so get out of here, go home, and be glad for what you’ve got.
Human mating back in the Cave Age was whatever the dating rituals dictated. Maybe they met by the river, ate some dates, and went to her cave. One thing we do know, there was never any sperm competition among humans. The female produces an egg, and once a sperm penetrates it, it seals itself off from other sperm. First come, first served. Males didn’t need to remain coupled with a female to prevent other males from mating with the female to advance his genetic lineage. They mated quickly enough, separated, and he rolled over and went to sleep. A baculum wasn’t needed, so evolution took it away.
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An article in the Journal of Andrology highlights that there is some evidence that a baculum, or penis bone, was present in humans during the evolutionary progression. The human penis seems to have retained some remnants of an ancestral baculum. The baculum of a human seems to have been replaced by a distal ligament in the glans, which keeps the urethra open during a discharge of sperm. The ligament still retains structural and interlocking components of collagen types I and III common to the baculum of other species. Then again, the fossil record is silent regarding the presence of a penis bone in humans.
The answers don’t pop up out of anywhere. As far as the baculum and the human male are concerned, we can eliminate the idea that they needed it and didn’t have it. What we don’t know is if humans had it and didn’t need it or if they never had it and never missed it.
Top image: Brown bear penis bones. Source: Didier Descouens / CC BY-SA 4.0
By Forest Bauer
Avery, D. 16 October 2020. “Honey badger is found to have a penis bone shaped like an 'ice cream scooper' that experts believe is used to clear out its rivals' semen” in The Daily Mail. Available at: https://www.dailymail.co.uk/sciencetech/article-8848263/Honey-badger-penis-bone-shaped-like-ice-cream-scooper-clears-rivals-semen.html
Brindle, M. 14 December 2016. “Why Humans Have No Penis Bone” in Scientific American. Available at: https://www.scientificamerican.com/article/why-humans-have-no-penis-bone/
Carosi, M, Scalici, M. & Linn, G. S. 16 April 2017. “Baubellum (Os Clitoridis)” in The International Encyclopedia of Primatology. Available at: https://doi.org/10.1002/9781119179313.wbprim0215
Herdina, A. N. et. al. 6 February 2015. “Testing hypotheses of bat baculum function with 3D models derived from microCT” in Journal of Anatomy. Vol. 226, Issue 3, pp. 229 – 235. Available at: https://doi.org/10.1111/joa.12274
Horowitz, K. 2 October 2015. “8 Hard Facts About the Penis Bone” in Mental Floss. Available at: https://www.mentalfloss.com/article/69282/8-hard-facts-about-penis-bone
Hsu, G-L., et. al. 2005. “Distal ligament in human glans: a comparative study of penile architecture” in Journal of Andrology, 26 (5): 624-8. Available at: https://doi.org/10.2164/jandrol.04145
Langham, R. Y. 10 November 2021. “How Long Does Sex Usually Last? Probably Not as Long as You’d Think!” in Between Us Clinic. Available at: https://www.betweenusclinic.com/premature-ejaculation/how-long-does-sex-last/
Long, C. A. & Frank, T. February 1968. “Morphometric Variation and Function in the Baculum, with Comments on Correlation of Parts” in Journal of Mammalogy. Available at: https://doi.org/10.2307/1377725
Montanari, S. 22 June 2016. “Why Do Chimps Have Penis Bones When Humans Don’t?” in Forbes. Available at: https://www.forbes.com/sites/shaenamontanari/2016/06/22/the-unusual-history-of-the-mammalian-penis-bone/?sh=326b2d67051b
Sample, I. 14 December 2016. “Why don't humans have a penis bone? Scientists may now know” in The Guardian. Available at: https://www.theguardian.com/science/2016/dec/14/why-dont-humans-have-a-penis-bone-scientists-may-now-know-baculum
Tayag, Y. 18 September 2018. “Scientists Learn Why Some Animals (But Not Humans) Still Have a Penis Bone” in INVERSE. Available at: https://www.inverse.com/article/49107-penis-bone-study-baculum
Zimmer, C. 8 August 2013. “A Most Interesting Bone” in National Geographic. Available at: https://www.nationalgeographic.com/science/article/a-most-interesting-bone