Original cartoon by Alexandra Martin
Source: Original cartoon by Alexandra Martin

Many male mammals have a rod-like bone (baculum) in the penis. Strikingly, it is unconnected to any other bone and varies wildly between species. Unusually, men get by without a baculum. Rigid erection is achieved exclusively through pressure of blood pumped into sponge-like tissues. But men’s lack of a baculum is actually far from unique. For starters, no egg-laying monotreme or marsupial has one; it occurs only among placental mammals and then only in some groups: primates, rodents, bats, carnivores, insectivores and a few others. Many placentals are baculum-free: tree-shrews, pangolins, most endemic African species, and all hoofed mammals. Dolphins and whales also have no baculum. The oft-cited gigantic “baculum” of the blue whale is just the dried, boneless penis. (Check out the prominent display in the Icelandic Phallological Museum.) It was long thought that rabbits and their relatives (lagomorphs) also lack a penis bone, but one pika species is now known to have one.

//creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons. Illustration of orangutan penis adapted from a figure in Weber (1927).
Above: Penis bone (baculum) of a beaver (Natural History Museum of Toulouse, former collection of Armand de Montlezun). Below: Longitudinal section of orangutan penis showing location of the baculum.
Source: Beaver baculum adapted from a photograph by Didier Descouens [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons. Illustration of orangutan penis adapted from a figure in Weber (1927).

Origins of penis bones among mammals

Humans are not even alone among primates in lacking a baculum. Although most non-human primates have one, tarsiers do not and neither do two groups of New World monkeys. But a penis bone is present in all other New World monkeys and is universal among lemurs and lorises and in Old World monkeys and apes. Because 95% of primates do possess a baculum, it was probably already present in their common ancestor and then secondarily lost in four lineages, including ours. The sister-group principle reinforces this conclusion. It is theoretically possible that a penis bone was initially absent in the primate cluster containing tarsiers, monkeys, apes and humans, but then evolved independently in various descendants. However, a baculum is universal in the sister group  —  lemurs and lorises  —  so that is the likely ancestral primate condition.

Adapted from a figure in Schultz et al. (2016).
Evolutionary tree of primates, showing presence/absence of the baculum. Red = baculum present; blue = baculum absent; mauve = equivocal. It is highly likely that the baculum was independently lost in two separate groups of New World monkeys and humans.
Source: Adapted from a figure in Schultz et al. (2016).

Because possession of a baculum is patchily distributed among placental mammals, it has long remained unclear whether the bone was already present in their common ancestor or evolved independently in multiple lineages. Two recent sophisticated comparative analyses of penis bones across mammals now provide a new perspective. In the first, Nicholas Schultz and colleagues examined presence/absence of the baculum in almost a thousand mammal species across an evolutionary tree. Their analysis indicated at least nine independent origins and ten independent losses. They inferred independent origins in at least two primate lineages, one leading to lemurs and lorises and the other to monkeys, apes and humans. Accordingly, three independent losses led to baculum absence in saki monkeys, spider monkeys and humans. Multiple independent origins of the baculum could indicate differing selection pressures, perhaps explaining why it has been difficult to find a consistent explanation for patchy occurrence of the baculum among mammals.

Adapted from a figure in Brindle & Opie (2016).
Evolutionary tree of mammals, showing presence/absence of the baculum. Red = baculum present in all species; pink = baculum present in most species; mauve = baculum present in a few species; grey = baculum absent. The baculum is universally absent in monotremes and marsupials, but it is present in many placental mammals, either as a retention from their common ancestor or following independent development in multiple lineages.
Source: Adapted from a figure in Brindle & Opie (2016).

Matilda Brindle and Christopher Opie subsequently published an even more sophisticated comparative analysis of presence/absence of penis bones across mammals. Ancestral states were inferred across a supertree containing over 5000 extant mammal species. This approach confirmed the inference that the common ancestor of all mammals lacked a baculum, but indicated its presence in both ancestral primates and ancestral carnivores. So penis bones first originated after placentals diverged from other mammals but before ancestral primates and carnivores emerged.

Function(s) of penis bones

With a clearer picture of the likely origins of penis bones in mammals, we can more confidently re-examine possible functional explanations. Schultz and colleagues noted four hypotheses put forward to explain baculum evolution: (1) mechanical support and protection of the urethra during copulation; (2) triggering ovulation or implantation; (3) enhancing sperm competition; (4) signalling male quality. Another suggestion is that the baculum might counter increased friction of the relatively small vaginal opening in species with markedly bigger males than females. In fact, most attention has focussed on the first hypothesis, which predicts an association between baculum development and extended copulation. But analyses designed to test this hypothesis have yielded conflicting results. In 1987, Alan Dixson reported that the baculum tends to be longer in primate species with extended copulation, and he later published a similar result for carnivores. Yet in 2002 Serge Larivière and Steven Ferguson reported that baculum length was unconnected with copulation time in a study of 52 carnivore species. Although they used a potentially superior statistical technique, they examined only a relatively small sample of North American carnivores. In 2004 Dixson and Matt Anderson published results from a refined statistical analysis of data for over 300 species of bats, carnivores and primates, revealing that copulation time is significantly correlated with baculum length.

The comprehensive approach adopted by Brindle and Opie threw valuable new light on possible functions. Having reconstructed baculum evolution across mammals, they proceeded to test associations between baculum length and testis weight, polygamous mating, seasonal breeding and copulation time in primates and carnivores. No significant association was found between relative testis size and baculum length in either group, suggesting that baculum size is unconnected with sperm competition. However, in both primates and carnivores baculum length was significantly correlated with extended copulation. In primates, baculum length was also significantly associated with polygamous mating and breeding seasonality.

Adapted from a figure in Stallman & Harcourt (2006).
Logarithmic plot of copulation time against body weight in mammals. Each point represents average figures for an entire order (major group). Note that marsupials are all characterized by remarkably long copulation times.
Source: Adapted from a figure in Stallman & Harcourt (2006).

Despite the encouraging advances achieved by recent broad-ranging comparative studies, problems remain. Firstly, all such reconstructions rely on the assumption that evolution involves a minimal number of changes. That is not always true. With simple features such as baculum size or copulation time, independent changes in the same direction can easily occur. This is just one reason why reference to fossil evidence is needed. After examining only living species, Brindle and Opie inferred that the baculum was “fairly small” in ancestral primates. Unfortunately, because it is unattached to any other bone, the baculum is rarely preserved in fossil mammals. But the renowned 47-million-year-old Messel site in Germany has yielded two very similar fossilized penis bones of the primate Europolemur. Relative to body size, they are the largest recorded for any primate, suggesting that decrease in size occurred during evolution of modern primates. The inference that baculum size is closely connected with copulation time is also more complicated than it seemed. A comparative analysis of copulation times in mammals published by Robert Stallmann and Alexander Harcourt in 2006 revealed a tendency to decline with increasing body size. Many mammals that lack a baculum  —  notably hoofed mammals, cetaceans and elephants  —  are in fact quite large-bodied. So that fits. Yet the study also showed that marsupials, which consistently lack a penis bone, tend to have much longer copulation times than placentals. Last but not least, it is unfortunate that baculum length has been the standard measure of size in comparative studies. Two experimental investigations of mice that linked baculum size to male reproductive success both indicated that baculum width, not length, is decisive.

Adapted from a figure in Martin (2007).
Logarithmic plot of baculum length against body weight in primates. Relative to body size, two penis bones of the 47-million-year-old fossil primate Europolemur from Messel (Germany) are the largest known from any primate. Note that all great apes (the four points on the extreme right of the graph) are characterize by a relatively small baculum.
Source: Adapted from a figure in Martin (2007).

So why do men lack a penis bone?

The latest findings enhance our understanding of men’s lack of a baculum in several respects. Secondary loss has clearly occurred. All Old World monkeys and apes have a penis bone, so complete elimination of the human baculum occurred at some point during the past 8 million years, after our lineage diverged from that leading to bonobos and chimpanzees. However, great apes have a relatively small baculum, indicating that reduction was already underway before humans evolved. This is perhaps attributable to increased body size of the common ancestor of great apes and humans, probably accompanied by a reduced copulation time. Note that a five-country survey published by Marcel Waldinger and colleagues in 2005 yielded an overall median duration of only 5.4 minutes for human coitus.

Recent findings also confirm that possession of a baculum is broadly associated with longer copulation times, at least in primates and carnivores. But this is no more than a general rule with many exceptions. For instance, bonobos and chimpanzees both have penis bones, but their copulations are much briefer than in humans, each lasting only a few seconds. Loss of the human baculum is hence not fully explained by relatively brief coitus. Other factors must be involved. Nonetheless, absence of a baculum and short coitus duration can be added to the extensive list of features indicating that our species is not adapted for direct sexual competition between men. (See my August 7, 2013 blog post Sperm Wars: Dispatch From a Conscientious Objector.)

References

Brindle, M. & Opie, C. (2016) Postcopulatory sexual selection influences baculum evolution in primates and carnivores. Proceedings of the Royal Society of London B 283:20161736:1-7.

Dixson, A.F. (1987) Baculum length and copulatory behavior in primates. American Journal of Primatology 13:51-60. 

Dixson, A.F. & Anderson, M.J. (2004) Sexual behavior, reproductive physiology and sperm competition in male mammals. Physiology & Behavior 83:361-371. 

Larivière, S. & Ferguson, S.H. (2002) On the evolution of the mammalian baculum: vaginal friction, prolonged intromission or induced ovulation? Mammal Review 32:283-294.

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

Schultz, N.G., Lough-Stevens, M., Abreu, E., Orr.T. & Dean, M.D. (2016) The baculum was gained and lost multiple times during mammalian evolution. Integrative & Comparative Biology 56:1-13.

Simmons, L.W. & Firman, R.C. (2014) Experimental evidence for the evolution of the mammalian baculum by sexual selection. Evolution 68:276-283.

Stallmann, R.R. & Harcourt, A.H. (2006) Size matters: the (negative) allometry of copulatory duration in mammals. Biological Journal of the Linnean Society 87:185-193.

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 11:66:1-6.

Waldinger, M.D., Quinn, P., Dilleen, M., Mundayat, R., Schweitzer, D.H. & Boolell, M. (2005). A multinational population survey of intravaginal ejaculation latency time. Journal of Sexual Medicine 2:492-497. 

Weber, M. (1927) Die Säugetiere: Einführung in die Anatomie und Systematik der recenten und fossilen Mammalia. vol. 1: Anatomischer Teil. (2nd edition). Jena: Gustav Fischer Verlag.

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