A flurry of media interest heralded striking results from new research into the baculum, a separate bone that fortifies the penis of many mammals. Paula Stockley and colleagues experimentally examined the relationship between the penis bone and breeding success in mice. They simulated a natural context in which dominant males can sire offspring with several unrelated females. Four breeding groups, each established with several founders of both sexes, were maintained for 4-5 months in extensive outdoor enclosures. Males were then removed randomly to compare baculum size to breeding success determined from DNA markers. In addition to expected associations with testis size and overall body size, breeding success was significantly correlated with the width (but not length) of the penis bone. This is the first direct experimental evidence that baculum size is connected with reproductive success.
Natural history of penis bones
Penis bones of mammals are unusual in two ways: They are never attached to any other part of the skeleton and show extraordinary variation in size and shape between species. But knowledge is scarce, so it is perhaps hardly surprising that some media coverage has been wide of the mark. In particular, it has been implied that it is really rare for mammals to lack a penis bone. While most bats, rodents, carnivores, insectivores and primates indeed have a baculum, that accounts for only a quarter of mammal groups. Bats and rodents are particularly numerous, so it is technically possible that “most” mammals have a penis bone. But many species do not. Rabbits, treeshrews, elephants, sea cows, hoofed mammals (even-toed and odd-toed), dolphins and whales all lack a baculum, as do all marsupials. So humans are far from alone in lacking a baculum.
The author holding the fossilized penis bone of a walrus (Photo by Lu Yao)
The baculum is particularly prominent in many carnivores. In a big dog, it can be four inches long, but the acknowledged record holder is the walrus, with a penis bone measuring thirty inches or more. This awe-inspiring bone, known as an oosik
in Alaska, has been widely used for carvings and rituals. Speaking of rituals, the originally all-male Tetrapods Club in London (to which I belonged long enough to vote for the admission of women) symbolically used a walrus baculum as a gavel. But there are exceptions even among carnivores. Hyaenas completely lack a baculum, and it is small or even absent in members of the cat family.
Penis bones in primates
In fact, penis bones are also patchily distributed in primates. It was presumably present in ancestral primates and then retained, although downsized, in the ancestors of higher primates. Among lower primates, it is present and often large in lemurs and lorises but absent from tarsiers. Most higher primates have a baculum, but certain New World monkeys, such as howlers and spider monkeys, are exceptions like humans. All Old World monkeys and apes have one. It is small in some species yet large in others such as mandrills and several macaques, probably as a secondary development. Marked differences in baculum size between macaque species have been attributed to differences in mating behavior. As all apes have only a small baculum, complete loss during human evolution ended a reduction process that was already under way.
I once mentioned to David Hosken—an authority on penis bones in bats—that the absence of a baculum in humans is an intriguing evolutionary puzzle. He laconically replied: “Speak for yourself!” In fact, comparative studies by primatologist Alan Dixson revealed that a long baculum is generally linked to prolonged mating. So it is reasonable to suggest that mating in early primates, which had a large baculum, lasted longer than in modern primates. As mating time became shorter, penis bones became smaller, eventually disappearing completely in the human lineage.
But other commentaries have overlooked a key point. By and large, comparative studies of bats, carnivores and primates have focused on baculum length, whereas Stockley and colleagues link reproductive success to its width. This discrepancy may explain inconsistencies in previous findings and should spark renewed research.
Adam’s rib explained?
Beyond its relevance to the evolution of human copulation, the lack of a baculum may also throw new light on the origins of the biblical tale that Eve was created from one of Adam’s ribs. Biologist Scott Gilbert and biblical scholar Ziony Zevit introduced a new angle. The Hebrew word tzela, translated as “rib” in the English version of the Bible, has several meanings, including a kind of strut. Gilbert and Zevit proposed that tzela possibly referred to the baculum, which is indeed absent, rather than to one of the ribs, which are all present and correct in a man’s skeleton. Still, it is not clear how the originators of the biblical tale could have known that humans are unusual in lacking a baculum. Among domestic beasts, all hoofed mammals also lack one. As noted, it is prominent in dogs, yet quite small in cats. Thus any notion that the human penis lost its bone could only have come from a comparison with dogs.
Hot tip for readers: It is easy to order a raccoon penis bone over the Internet. Just type in “mountain man toothpick.”
Dixson, A.F. (1987) Baculum length and copulatory behavior in primates. American Journal of Primatology 13:51-60.
Dixson, A.F. (1995). Baculum length and copulatory behaviour in carnivores and pinnipeds (Grand Order Ferae). Journal of Zoology, London 235:67-76.
Gilbert, S.F. & Zevit, Z. (2001) Congenital human baculum deficiency: The generative bone of Genesis 2 : 21-23. American Journal of Medical Genetics 101:284-285.
Hosken, D.J., Jones, K.E., Chipperfield, K. & Dixson, A.F. (2001) Is the bat os penis sexually selected? Behavioral Ecology and Sociobiology 50:450-460.
Ramm, S.A. (2007) Sexual selection and genital evolution in mammals: A phylogenetic analysis of baculum length. American Naturalist 169:360-369.
Stockley, P. (2012) The baculum. Current Biology 22:R1032-R1033.
Stockley, P., Ramm, S.A., Sherborne, A.L., Thom, M.D.F., Paterson, S. & Hurst, J.L. (2013) Baculum morphology predicts reproductive success of male house mice under sexual selection. BMC Biology 11:66 .
See also (particularly recommended):
Map of Life - "Baculum (penile bone) in mammals"