A Long-Lost Bone

I’m missing a bone. You are, too, although which bone that is depends on your anatomical sex. For me and male readers of this post, it’s the baculum – the enigmatic “penis bone” found in the members of many mammals and not us. There is nothing bony about a human boner. But, through the winding path of evolution, female readers are lacking their own genital ossification that’s just as mysterious and has been rarely discussed – the os clitoridis.

The os penis and os clitoridis are osteological correlates of each other. They are the same bone, but in different form in each sex. And while the os clitoridis – sometimes called by the more elegant-sounding name “baubellum” – isn’t a feature of all mammal lineages, the bone has been found in a variety of species among distantly-related beasts. In a 1954 paper in which he lamented that the peculiar bone has “been only sporadically studied”, zoologist James Layne documented that the os clitoridis has been found in a variety of rodents, carnivorans, and primates – marmots, seals, cats, bats, bears, galagos, gibbons, and more had some sort of bone beneath the clitoris.

For his part, Layne focused on the curious genital structures of squirrels. The bones were quite small, often between one and three millimeters long, but many were just as oddly ornamented as the penis bones of males. Figured in a gallery of equivalent bones from other species in Layne’s paper, the os clitoridis of the Tropical ground squirrel (Notocitellus adocetus) is a flared, spurred spoon. The same bone in other squirrel species twist, taper, and flare in their own distinctive ways, and, often, closely resemble the os penis in male conspecifics. Experiments in the lab, and not just anatomy, have underscored that baubellem and baculum are different versions of the same bone.

Rats and mice, those icons of laboratory studies, are our guides here. Female mice have a clitoral bone, and female rats of some study strains have a small bone that corresponds to the tip of the baculum in male rats. Few seemed to pay much attention to this fact until, in their efforts to understand how different hormones affected tumor formation in the genital tracts of rats during the late 1960s, researchers Alfred Glucksmann and Cora Cherry found that injections of testosterone caused the female rats to form new genital bone. Following that thread, Glucksman and Cherry discovered that testosterone injections during the early days of life caused the female rats to grow a larger os clitoridis which closeled resembles the male counterpart. Working with other researchers, Glucksmann later found the same to be true for mice. “With prolonged androgen treatment”, osteologist Brian Hall has written about this find in scientific deadpan, “growth of these induced bones can be promoted and the distal element extended.”

Yet the connection between hormone and baubellem isn’t always so clear. For one thing, testosterone and other androgens seem to have nothing to do with why female fossa have disappearing clitoral bones. Fossa (Cryptoprocta ferox), made famous by the exceptionally annoying kid’s film Madagascar, are lithe, tawny carnivores closely related to civets and genets. And, apparently, they are the first recognized case of what zoologist Clare Hawkins and colleagues term “transient masculinization.”

So far as zoologists have documented, about nine mammal species have females that take on male characteristics during the course of their lives. The most famous example is the spotted hyena. Females of Crocuta crocuta are masculinized to the point where they develop a prominent pseudo-penis that, in one of the most startling examples of evolutionary jury-rigging, hyena mothers give birth through.

Young female fossa don’t have to cope with changes quite so extreme, but, as Hawkins and colleagues noted in a 2002 study, they produce a “a mildly pungent orange secretion” also found in mature males and have “an enlarged, spinescent clitoris supported by an os clitoridis” similar to the corresponding genital equipment of the opposite sex. Unlike the female hyenas, though, female fossa don’t retain these traits. They’re only masculinized for a short part of their lives, starting around seven months of age until the fossa’s second or third year, before the traits start to fade. Only a smaller form of the clitoral bone is left in adult female fossa. Why?

Sex hormones don’t appear to be the answer. From anatomical, genetic, and hormonal examinations of wild and captive fossa, Hawkins and coauthors couldn’t find any difference in androgen levels between juvenile and adult females. Nor was there any connection between androgen levels and the masculine traits – such as “os clitoridis length” and “secretion score” – of young female fossa. It could be that young female fossa have “target tissues” where the relatively low levels of androgens have more influence over anatomy, but, Hawkins and colleagues noted, no one knows whether this is the case.

Exactly how the temporary change transpires is unknown. Despite the black box of the carnivoran’s transient masculinization, though, Hawkins and collaborators offered two hypotheses as to why secreting orangish fluid and having a spiny clitoris supported by bone might be advantageous to young female fossa.

The male traits start to become prominent about the time female fossa are aggressively run off by their mothers and, even though they are not yet mature enough to breed, they may encounter the unwanted attention of a wandering male seeking mates during the very brief mating season. Such an encounter can be dire. A male could maul or even kill a young female fossa, so, Hawkins and coauthors propose, looking like a male during the vulnerable period in their lives “could allow them to escape detection or could signal to males that they are not a potential mate.” Then again, territorial adult females can be just as much as a threat as roving males, so perhaps young female fossa have a better chance of finding their own patch of forest if they disperse from home in disguise.

Female fossa changes could be attributable to both scenarios. The immature civets face dangers from males and females alike. But, beyond the enigmatic mechanics of the anatomical change, we still don’t know why young female fossa evolved such mimicry. Nor is this the only mystery the os clitoridis embodies. Why some of our primate relatives have genital bones, while we lack them, is an evolutionary conundrum just as vexing.

For my part, I’m glad I don’t have a baculum. I can’t imagine that I would have been keen on playing soccer or sparring in Taekwondo classes if there was a possibility that I’d be carted off to the emergency room with a shattered penis bone. (Although, speaking of such injury, there are pathological cases of penis bones forming in humans, including after trauma such a kick or gunshot wound to the area. Perhaps I should have reconsidered my childhood activities, after all.) But I still want to know why the genital bone – in both female and male humans – disappeared without a trace.

We’re not unique among primates in lacking osteological genital curiosities. In a review of human reproduction from a primatological perspective, anthropologist Robert Martin noted that tarsiers don’t have penis or clitoris bones, nor do several genera of New World monkeys. And even in primates that have them, such as our close ape relatives, the genital bones have shrunk to a negligible, almost unnoticeable size. While os penis and os clitoridis bones are prominent in ring-tailed lemurs, for example, they’ve almost entirely disappeared in gibbons and chimpanzees.

In primates, at least, a baculum is hypothesized to help support the penis during long bouts of sex between individuals that rarely encounter each other during the mating season. Males that are often around their mates, the following argument goes, can copulate more frequently for shorter amounts of time and therefore don’t require osteological assistance for their erections. Perhaps. There are other hypotheses out there, not all of which are mutually exclusive. But what, then, are we to make of the baubellem? The baculum and baubellum are old bones that have been retained in some forms of placental mammal and lost in others over millions and millions of years. And while a great deal of ink has been spilled over the function of the baculum and our lack, I haven’t been able to find so much as even a minor degree of interest into why the baubellum evolved and exists in many mammal species.

Even in reduced form, a clitoral bone is present in an array of female primates. And when we look beyond our primate kin – among Layne’s squirrels, for one – female mammals have os clitoridis of prominent size and intricately complex morphology. So much so that zoologists have occasionally pondered using the bones to tell species apart. But beyond such utility, the clitoral bone is often ignored.

There’s an assumption that the clitoral bone is functionally unimportant and just a case of females developing a masculine trait – a reverse of males having nipples. But I believe the bone is more unstudied than insignificant. The importance of the clitoral bone in the life of female fossa, for one, is a clue that there is more to the curious bone than researchers have appreciated. Given how little we know about this varied skeletal feature, we’d be terribly foolish to relegate the os clitoridis as a masculine leftover of only passing interest. All the same, the os clitoridis, so inscrutable, is a totem of our shared history with our therian relatives. Tracing back what we’re missing, we walk along the pathway of our deep history – a mysterious absence guides us back through evolutionary enigmas barely considered.

[Top image by Flickr user Nathan Rupert.]

References:

Glucksmann, A., Cherry, C. 1972. The hormonal induction of an os clitoridis in the neonatal and adult rat. Journal of Anatomy. 112, 2: 223-231

Glucksmann, A., Ooka-Souda, S., Miura-Yasugi, E., Mizuno, T. 1976. The effect of neonatal treatment of male mice with antiandrogens and of females with androgens on the development of the os penis and os clitoridis. Journal of Anatomy. 121, 2: 363-370

Hall, B. 2005. Bones and Cartilage: Developmental and Evolutionary Skeletal Biology. San Diego: Elsevier Academic Press. p. 344

Hawkins, C., Dallas, J., Fowler, P. Woodroffe, R., Racey, P. 2002. Transient masculinization in the fossa, Cryptoprocta ferox (Carnivora, Viverridae)Transient masculinization in the fossa, Cryptoprocta ferox (Carnivora, Viverridae)Transient masculinization in the fossa, Cryptoprocta ferox (Carnivora, Viverridae). Biology of Reproduction. 66: 610-615

Layne, J. 1954. The os clitoridis of some North American Sciuridae. Journal of Mammalogy. 35, 3: 357-366

Martin, R. 2007. The evolution of human reproduction: A primatological perspective. Yearbook of Physical Anthropology. 50: 59-84

Murakami, R., Mizuno, T. 1984. Histogenesis of the os penis and os clitoridis in rats. Development, Growth, and Differentiation. 26, 5: 419-426

Stockley, P. 2012. The baculum. Current Biology. 22, 24: R1032