Ouch!! Gentlemen, fancy a bone in your penis? Seems a bit risky, given it could fracture during copulation. Even our near ancestors had such a bone. It has probably evolved several times, but what is its function?

The genitalia of male animals vary considerably in their morphology, which is widely attributed to sexual selection. A good example is the baculum or penile bone (os penis) of mammals. This isolated bone, which develops in connective tissue under the influence of androgens, is located in the distal end of the penis above the urethra. In some species, the females have a smaller equivalent in their clitoris, which is referred to as the baubellum. Arguably, the baculum is the most diverse bone of all, varying greatly in length and shape even between closely related species. Bacular morphology has therefore been frequently used in species diagnosis and taxonomy.

Taxonomic distribution of the baculum

The baculum is not present in all mammals, but only found in carnivores (Carnivora), rodents (Rodentia), bats (Chiroptera), primates (Primates) and some insectivores in the order Eulipotyphla. In all these orders, a penile bone occurs in some, but not in all species. For example, carnivores show a dichotomy with respect to baculum length. Whilst the baculum is greatly reduced or even absent in many cat-like carnivores (Feliformia), dog-like carnivores (Caniformia) possess an elongated penile bone. In primates, the baculum is present in most prosimians (but lacking in the genus Tarsius) and most New World monkeys, with the exception of howler monkeys (Alouatta), spider monkeys (Ateles), uakaris (Cacajao), bearded sakis (Chiropotes) and woolly monkeys (Lagothrix). Amongst the Old World monkeys and apes, the smallest bacula relative to body size are found in the great apes and humans are the only species without a baculum. In most bats, the baculum is small, but large in some members of the large-bodied genus Pteropus and completely absent in others (e.g. New World leaf-nosed bats in the family Phyllostomidae).

Evidence for convergence

The baculum evidently evolved several times, limiting its usefulness as a diagnostic taxonomic character. In caniforms in general and the weasel family (Mustelidae) in particular, there is pronounced bacular convergence. Several features of the penile bone have most likely been derived independently in distantly related species. These include lengthenings or shortenings, the development of the urethral groove and changes to the baculum’s head. Within the Mustelidae, for example, the bacular head of the extinct wolverine Plesiogulo marshalli is characterised by a wide leaf-shaped lobe and a finger-shaped outgrowth, as are those of two extant species from Asia, the back-striped weasel (Mustela strigidorsa) and the Malayan weasel (M. nudipes). Within the Caniforma, examples of very similar bacular morphologies include the bifurcated head of the racoon (Procyon lotor) and the European otter (Lutra lutra) penile bone.

Function of the baculum

So what is the penile bone for? It has been suggested that it might simply be a by-product of penile development with no adaptive function. The recurrent evolution of particular shapes in unrelated groups, however, indicates functionality. If the baculum was affected by sexual selection, this would help to explain its extreme morphological diversity. Furthermore, the baculum is likely to come with energetic costs, such as growth and maintenance (although these do not seem to be substantial), as well as the risk of infection or fracture (as observed in e.g. some mustelids). Hence, various hypotheses regarding adaptive baculum function have been proposed.

Mechanical support

The bony nature of the baculum has led to the suggestion that it could provide mechanical support to the penis during copulation. According to the “vaginal friction hypothesis”, the function of the baculum is to make the erect penis more rigid, thus facilitating intromission by overcoming the friction that is due to the relatively smaller size of the female vagina. Evidence for this hypothesis is limited. In the Norway rat (Rattus norvegicus), the microstructure of the baculum indicates that it is load-bearing and might increase penile stiffness during copulation, probably by force transfer to the hydrostatic corpus cavernosum. The baculum of dogs might also stiffen the penis. It has a lower mineral density and hence lower stiffness than skeletal bones, which might reduce the risk of fracture during copulation.

Stimulation of the female

The baculum could also serve to stimulate the female during copulation and trigger a hormonal response, which might then increase the male’s fertilisation success by facilitating sperm transport, preparing the uterus for implantation or inducing ovulation. In carnivores at least, there is not much support for this “induced ovulation hypothesis”. Canidae, which have large bacula, are spontaneous ovulators, while in contrast Felidae are characterised by induced ovulation and small bacula.

Sperm competition success

Sperm competition, where sperm of different males compete for fertilisation of eggs in the female reproductive tract, is prominent in mammals where females mate with multiple males. The baculum could help males to increase their success in sperm competition in various ways. According to the “prolonged intromission hypothesis”, the baculum could directly facilitate sperm transport by keeping the urethral canal open and maintaining unhindered sperm flow. This should be especially beneficial in species with copulatory ties and long copulation times. Evidence for this hypothesis comes from a comparative study of primates. Those species with a relatively long baculum show prolonged intromission, whereas species with single or multiple brief intromissions have shorter bacula.

A long baculum could also help deliver sperm close to the site of fertilisation, displace or even remove sperm from rival males or facilitate the placement of copulatory plugs, which prevent further males from successfully copulating with a female. In a comparative analysis considering baculum length and relative testis size, a positive correlation was found in rodents and carnivores. Since testis size can be considered an indicator of sperm competition, with large testes implying intense sperm competition, baculum evolution might be related to the intensity of sperm competition at least in certain conditions.

Cryptic female choice

Another element of post-copulatory sexual selection is cryptic female choice, where females can “choose” between the sperm of multiple males, favouring that of high-quality males. There is evidence that females use the size of the penis as an indicator of male quality, and a similar function has been suggested for the baculum. In the muskrat (Ondatra zibethicus), baculum width varies considerably between males, and baculum dimensions show positive allometry (i.e. larger males have relatively larger bacula than smaller males) compared to non-sexual traits. It has hence been suggested that females might use the penile bone to evaluate male quality. Muskrats mate underwater, an environment that limits the availability of visual and olfactory cues and might therefore make it difficult for females to assess a male before copulation. If females were able to detect differences in the bacular morphology of different males in their reproductive tract during copulation, they could potentially exert cryptic female choice. This is in agreement with findings from other mammals that also mate aquatically (e.g. harp seals Phoca groenlandica) or underground (e.g. Cape dune mole rat Bathyergus suillus), where a pre-mating assessment of male quality might be tricky as well. Similarly, in bats males frequently copulate with females that are hibernating, which prevents them from actively choosing a mate. However, in the noctule bat (Nyctalus noctula), while the penis is under sexual selection and its length is correlated with indicators of male quality such as body size and mass, this is not the case for the baculum.

To further assess the role of the baculum, more direct evidence is needed that links baculum features to male fertilisation success. A recent study in the promiscuous bank vole (Myodes glareolus) has shown that bacular morphology is linked to male social status. Dominant males have wider (though not longer) bacula than subordinates, and there is evidence that the baculum is affected by sexual selection in this species. As a larger baculum might confer an advantage to the male in terms of sperm competition and/or cryptic female choice, this could explain the higher fertilisation success of dominant males compared with subordinate individuals.

All these hypotheses are non-exclusive, and none of them has proven fully satisfactory or well supported across taxa. According to Serge Larivière and Steven Ferguson, “the existence of the mammalian baculum remains one of the most puzzling enigmas of mammalian morphology” (2002, Mammal Review, vol. 32, p. 288). Baculum evolution has probably been affected by a number of factors, the importance of which should vary between the different mammalian groups. So it may be a mistake to assume that all bacula do the same thing.

Loss of the baculum in humans

Why has the baculum been lost in humans, while it is present in all the other Old World monkeys and apes? Evidence that this is indeed a secondary loss comes from some primate fossils from the Eocene, which possessed a prominent baculum, suggesting that this is an ancestral feature in primates. The reasons for this loss remain largely elusive, but are likely linked to changes in reproduction. Homo sapiens might simply have continued the general trend towards baculum reduction that is observed in the apes (it has been speculated that early hominids such as Homo erectus still possessed a baculum). This reduction could in turn be related to a reduction in intromission times and/or to an increase in body size. The loss of the baculum could also reflect a lower degree of sperm competition in humans compared with other primates (which is in accordance with e.g. the relatively small testes of human males). Richard Dawkins has furthermore proposed that the baculum might have disappeared in response to selection pressure from females. Females could assess the quality of a male from his ability to achieve a full and stiff erection even without a baculum, as this ability might be impaired by factors such as ill health or an inability to cope with stress.