Anatomy of the baculum-corpus cavernosum interface in the norway rat (Rattus norvegicus), and implications for force transfer during copulation

First description of the baubellum in the spectacled bear Tremarctos ornatus (Mammalia: Ursidae)

The baubellum (os clitoridis) is a bone found in the clitoris of many female eutherian mammals and is homologous to the baculum in males. In contrast to the baculum, the baubellum has received very little attention regarding its morphological or interspecific diversity, or on hypotheses for its function. The presence of the baubellum in bears (Ursidae) has only been established and mentioned in the literature for the Ursus genus, and not for the other genera of bears. Moreover, no scaled photographs are available for baubella of this clade, and the sizes reported vary between sources. We hereby present and describe the baubellum of a spectacled bear (Tremarctos ornatus), providing a detailed account of baubella in a basal ursid species. The baubellum of Tremarctos is slightly bowed dorsally, with two small prominences at the distal apex. The length of the Tremarctos baubellum in this study is comparable to that of Ursus americanus (American black bear). We infer the specific shape, with longitudinal ridges, of the baubellum in Tremarctos could indicate a discrete function during copulation or sexual arousal. However, future studies, especially regarding the associated soft tissues, will be required to confirm whether this is indeed the case. Our study expands the understanding of baubella within Ursidae, providing new data (including a three-dimensional model) that can be used to further explore the morphological diversity and function of this enigmatic extra-skeletal bone.

Fossil bacula of five species of Borophaginae (Family: Canidae): Implications for their reproductive biology

The baculum of mammals offers the opportunity to study the reproductive biology of extinct species given that it is a fossilizable part of the male genitalia and that its size and shape correlate with several aspects of the reproductive biology of extant mammals. Fossil bacula, however, are rare. Currently, bacula have been described from only two extinct species of canids, one from the subfamily Caninae and the other from the extinct subfamily Hesperocyoninae. Here, I describe the bacula of five extinct species of Borophaginae, each of which was found with other skeletal elements that have enabled identification to the species level. Two specimens (Aelurodon ferox and Aelurodon stirtoni) are largely complete, while the baculum from Carpocyon compressus is complete but still embedded in matrix that obscures some of its features. The bacula of Paratomarctus euthos and Desmocyon thomsoni are incomplete, but they provide useful information nonetheless. These borophagine bacula are similar to extant canines in being robust, having a urethral groove, and a simple distal end. These features suggest that the Borophaginae had long-lasting copulation and possibly spontaneous ovulation, similar to the extant canines. However, unlike the straight baculum of extant canines, borophagine bacula are ventrally curved (arched), which is also observed in the hesperocyonine baculum. The implication of this curvature for the reproductive biology of these animals remains unknown.

Co-exposures to trace elements and polycyclic aromatic compounds (PACs) impacts North American river otter (Lontra canadensis) baculum

Environmental loadings of polycyclic aromatic compounds (PACs) and trace elements are increasing in areas with marked oil and gas extraction, such as in the Athabasca oil sands region, Alberta, Canada. Some of these chemicals are recognized as potent endocrine disrupting compounds (EDCs). The impacts of co-exposure to PACs and metals on free-ranging wildlife is of considerable concern. River otters (Lontra canadensis) are sentinel species of aquatic ecosystem health. The baculum (penile bone) is an important part of the reproductive system in otters that ensures successful copulation. Although baculum health is critical to male reproductive success and is sensitive to exposure to EDCs, there is no information available regarding the impact of PAC and metal exposures on measures of baculum health. River otter baculum and livers were dissected from carcasses obtained from the fur trade. Trace element and PAC analyses were carried out in liver with matching baculums subjected to dimensional analysis, bone mineral density (BMD) and mechanical loading testing. Trace elements and select PACs exhibited both protective and deleterious effects on baculum bone health metrics. Alkylated four ring PACs were negatively associated with baculum bone material properties (ex: C4-Chrysene and C4-pyrene). The same compounds have been shown to exhibit strong anti-androgenic activities. Few comparable studies exist related to contamination and adverse effects of PACs in wild terrestrial mammals. Baculum health metrics may be an important tool to include in biomonitoring studies as to date, there are limited means to assess male reproductive performance in wildlife biomonitoring programs.

Postcopulatory sexual selection and the evolution of shape complexity in the carnivoran baculum

The baculum is an enigmatic bone within the mammalian glans penis, and the driving forces behind its often bizarre shape have captivated evolutionary biologists for over a century. Hypotheses for the function of the baculum include aiding in intromission, stimulating females and assisting with prolonged mating. Previous attempts to test these hypotheses have focused on the gross size of the baculum and have failed to reach a consensus. We conducted three-dimensional imaging and apply a new method to quantify three-dimensional shape complexity in the carnivoran baculum. We show that socially monogamous species are evolving towards complex-shaped bacula, whereas group-living species are evolving towards simple bacula. Overall three-dimensional baculum shape complexity is not related to relative testes mass, but tip complexity is higher in induced ovulators and species engaging in prolonged copulation. Our study provides evidence of postcopulatory sexual selection pressures driving three-dimensional shape complexity in the carnivore baculum.

Formation, structure, and function of extra-skeletal bones in mammals

This review describes the formation, structure, and function of bony compartments in antlers, horns, ossicones, osteoderm and the os penis/os clitoris (collectively referred to herein as AHOOO structures) in extant mammals. AHOOOs are extra-skeletal bones that originate from subcutaneous (dermal) tissues in a wide variety of mammals, and this review elaborates on the co-development of the bone and skin in these structures. During foetal stages, primordial cells for the bony compartments arise in subcutaneous tissues. The epithelial-mesenchymal transition is assumed to play a key role in the differentiation of bone, cartilage, skin and other tissues in AHOOO structures. AHOOO ossification takes place after skeletal bone formation, and may depend on sexual maturity. Skin keratinization occurs in tandem with ossification and may be under the control of androgens. Both endochondral and intramembranous ossification participate in bony compartment formation. There is variation in gradients of density in different AHOOO structures. These gradients, which vary according to function and species, primarily reduce mechanical stress. Anchorage of AHOOOs to their surrounding tissues fortifies these structures and is accomplished by bone-bone fusion and Sharpey fibres. The presence of the integument is essential for the protection and function of the bony compartments. Three major functions can be attributed to AHOOOs: mechanical, visual, and thermoregulatory. This review provides the first extensive comparative description of the skeletal and integumentary systems of AHOOOs in a variety of mammals.

Physicochemical and biochemical spatiotemporal maps of a mouse penis

Spatiotemporal mechanobiology resulting in penile pathologies continues to be investigated using small scale animals models such as mice. However, species-dependent functional biomechanics of a mouse penis, is not known. In this study, spatial mapping of a mechanosensitive transcription factor, scleraxis (Scx), at ages 4, 5, 6 weeks, and 1 year were generated to identify mechanoactive regions within penile tissues. Reconstructed volumes of baculum collected using micro X-ray computed tomography illustrated significantly increased baculum length with decreased porosity, and increased mineral density (p < 0.05) with age. The bony-baculum was held centrally in the Scx positive corpus cavernosum glandis (CCG), indicating mechanoactivity within the struts in a 6 week old mouse. The struts also were stained positive for fibrillar proteins including collagen and elastin, and globular proteins including protein gene product 9.5, and α-smooth muscle actin. The corpus cavernosum penis (CCP) contained significantly (p < 0.05) more collagen than CCG within the same penis, and both regions contained blood vessels with equivalent innervation at any given age. Comparison of volumes of flaccid and erect penile forms revealed functional characteristics of the CCP. Results of this study provided insights into biomechanical function of the CCG; in that, it is a high-pressure chamber that stiffens the penis and is similar to the human corpus cavernosum.

Cross-Sectional Geometry and Scaling in the Baculum of Cuban Hutias (Rodentia: Capromyidae)

Bacula from 61 individual hutia (Rodentia) from five species were studied. The purpose was to investigate cross-sectional geometry as an indicator of mechanical behavior in order to answer questions around the origin and maintenance of the mammalian baculum. From images of the apical and basal cross sections, the following variables were calculated: perimeter, cross-sectional area, maximum second moment of area, and polar moment. An allometric analysis showed that these variables were related to body size. The orientation of the maximum second moment of area was analyzed by means of circular statistics. This orientation was transverse in both the apical and basal cross sections. Values for the second moment of area and polar moment, obtained from the predicted value of the allometric equations, showed that either the bending moment or the twisting moment of the baculum must be relatively low in hutias, compared with those of the radius in the same species. The results of the second moment of area predict that the main bending stress acting on the baculum is transverse. At the same time, shear stress would not be negligible. Anat Rec, 303:1346-1353, 2020. © 2019 American Association for Anatomy.

The baubellum is more developmentally and evolutionarily labile than the baculum

Understanding the evolutionary forces that influence sexual dimorphism is a fundamental goal in biology. Here, we focus on one particularly extreme example of sexual dimorphism. Many mammal species possess a bone in their penis called a baculum. The female equivalent of this bone is called the baubellum and occurs in the clitoris, which is developmentally homologous to the male penis. To understand the potential linkage between these two structures, we scored baculum/baubellum presence/absence across 163 species and analyzed their distribution in a phylogenetic framework. The majority of species (N = 134) shared the same state in males and females (both baculum and baubellum present or absent). However, the baubellum has experienced significantly more transitions, and more recent transitions, so that the remaining 29 species have a baculum but not a well-developed baubellum. Even in species where both bones are present, the baubellum shows more ontogenetic variability and harbors more morphological variation than the baculum. Our study demonstrates that the baculum and baubellum are generally correlated across mammals, but that the baubellum is more evolutionarily and developmentally labile than the baculum. The accumulation of more evolutionary transitions, especially losses in the baubellum, as well as noisier developmental patterns, suggests that the baubellum may be nonfunctional, and lost over time.

The Baculum was Gained and Lost Multiple Times during Mammalian Evolution

The rapid evolution of male genitalia is a nearly ubiquitous pattern across sexually reproducing organisms, likely driven by the evolutionary pressures of male-male competition, male-female interactions, and perhaps pleiotropic effects of selection. The penis of many mammalian species contains a baculum, a bone that displays astonishing morphological diversity. The evolution of baculum size and shape does not consistently correlate with any aspects of mating system, hindering our understanding of the evolutionary processes affecting it. One potential explanation for the lack of consistent comparative results is that the baculum is not actually a homologous structure. If the baculum of different groups evolved independently, then the assumption of homology inherent in comparative studies is violated. Here, we specifically test this hypothesis by modeling the presence/absence of bacula of 954 mammalian species across a well-established phylogeny and show that the baculum evolved a minimum of nine times, and was lost a minimum of ten times. Three different forms of bootstrapping show our results are robust to species sampling. Furthermore, groups with a baculum show evidence of higher rates of diversification. Our study offers an explanation for the inconsistent results in the literature, and provides insight into the evolution of this remarkable structure.

Testing hypotheses of bat baculum function with 3D models derived from microCT

The baculum (os penis) has been extensively studied as a taxon-specific character in bats and other mammals but its mechanical function is still unclear. There is a wide consensus in the literature that the baculum is probably a sexually selected character. Using a novel approach combining postmortem manipulation and three-dimensional (3D) imaging, we tested two functional hypotheses in the common noctule bat Nyctalus noctula, the common pipistrelle Pipistrellus pipistrellus, and Nathusius' pipistrelle Pipistrellus nathusii: (i) whether the baculum can protect the distal urethra and urethral opening from compression during erection and copulation; and (ii) whether the baculum and corpora cavernosa form a functional unit to support both the penile shaft and the more distal glans tip. In freshly dead or frozen and thawed bats, we compared flaccid penises with artificially 'erect' penises that were inflated with 10% formalin. Penises were stained with alcoholic iodine and imaged with a lab-based high-resolution x-ray microtomography system. Analysis of the 3D images enabled us to compare the changes in relative positions of the baculum, corpora cavernosa, urethra, and corpus spongiosum with one another between flaccid and 'erect' penises. Our results support both functional hypotheses, indicating that the baculum probably performs two different roles during erection. Our approach should prove valuable for comparing and testing the functions of different baculum morphologies in bats and other mammals. Moreover, we have validated an essential component of the groundwork necessary to extend this approach with finite element analysis for quantitative 3D biomechanical modeling of penis function.

On the socio-sexual behaviour of the extinct Ursid Indarctos arctoides: an approach based on its baculum size and morphology

The fossil bacula, or os penis, constitutes a rare subject of study due to its scarcity in the fossil record. In the present paper we describe five bacula attributed to the bear Indarctos arctoides Depéret, 1895 from the Batallones-3 site (Madrid Basin, Spain). Both the length and morphology of this fossil bacula enabled us to make interpretative approaches to a series of ecological and ethological characters of this bear. Thus, we suggest that I. arctoides could have had prolonged periods of intromission and/or maintenance of intromission during the post-ejaculatory intervals, a multi-male mating system and large home range sizes and/or lower population density. Its size might also have helped females to choose from among the available males.

The canine baculum: the structure and mechanical properties of an unusual bone

The baculum is an extraskeletal bone located in the penis of a few species in several orders of mammals such as carnivores, insectivores, rodents, bats and primates. This study aims to describe the structure, architecture and mechanical properties of the canine baculum. To this end canine bacula from castrated and uncastrated dogs were collected and examined by light microscopy, micro-computed tomography (microCT) scanning, histological staining, and mechanical testing. Their mineral density and mechanical properties were compared with those of a typical skeletal bone (the radius) in the same dog. Furthermore, a numerical model of a representative baculum was created and its mechanical performance analyzed using the finite element method, in order to try to elucidate its function. Examination of light microscopy images of transverse sections shows that the baculum consists of a typical sandwich structure, with two cortical plates separated, and joined, by loose cancellous bone. MicroCT scans reveal that the mineral density is lower in the baculum than in the radius, both in castrated as well as in uncastrated dogs, resulting in much lower stiffness. Castration was found to decrease the mineral density in both the baculum and the radius. The most likely function of the baculum of the dog is to stiffen the penis to assist intromission, and its much lower mineral density compared to that of the radius may be a mechanism designed to decrease the stiffness somewhat, and thus reduce the risk of fracture during copulation.

Histomorphology of the penis bone (Baculum) in the gray long-eared bat Plecotus austriacus (Chiroptera, Vespertilionidae)

For the first time, the histomorphology of the penis bone of a bat (Plecotus austriacus) was examined in detail. From Plecotus austriacus, 14 whole penes and 11 isolated bacula were studied and compared to bacula of Plecotus auritus and Plecotus macrobullaris. The baculum was located on specimen microradiographs and in micro-CT images in the tip of the penis. Using serial semithin sections and surface-stained, undecalcified ground sections, the types of bone and other tissues constituting the baculum were examined by light microscopy. 3D reconstructions were generated from the serial semithin sections and from micro-CT images. The shaft and the proximal branches of the Y-shaped baculum form a tubular bone around a medullary cavity. Since the small diameter of this channel and the main lamellar bone around it resemble a Haversian canal, the baculum is equivalent to a single-osteon bone. Several oblique nutrient canals enter this medullary cavity in the shaft and branches. All ends of the baculum consist predominantly of woven bone. The collagen fiber bundles of the tunica albuginea of both corpora cavernosa insert via fibrocartilage into the woven bone of the branches. Thus, the microscopic structures support the hypothesis that the baculum functions as a stiffening element in the erect penis. In this study, several microscopic imaging techniques were evaluated for displaying the microscopic structures of the baculum. Specimen microradiography, but especially micro-CT proved to be suitable nondestructive methods for accurate and reproducible demonstration and comparison of the three-dimensional structures of the baculum in different bat species.

Male Genital Morphology and Function: An Evolutionary Perspective

PURPOSE

The genitals are at the forefront of evolution. Advantageous features of genital form and function determine which individuals pass their genes to future generations. The male genitalia are primarily for sperm deposition but additional functions of the penis and its accessory structures are being identified.

MATERIALS AND METHODS

The literature regarding genital evolution was reviewed.

RESULTS

This review explains key theories regarding the mechanisms of genital evolution. Also discussed are the anatomical and functional adaptations among different species that have evolved to optimize fertilization success. Finally, these evolutionary influences are discussed in the context of clinically relevant human genital morphology and function.

CONCLUSIONS

The function of the genitalia extends beyond simple sperm transfer. Genital morphology evolves to provide the primary and secondary functionality that is critical for successful mating. By considering the form and function of human genital morphology in an evolutionary context we gain not only an appreciation for its design, but also the potential for a deeper understanding of our treatment goals.

Sexual Selection and Genital Evolution in Mammals: A Phylogenetic Analysis of Baculum Length

Studies in invertebrate taxa suggest that postcopulatory sexual selection is an important factor in genital evolution. However, despite wide interspecific variation in genital morphology, evidence for an influence of sexual selection on mammalian genitalia is equivocal. Here I conduct phylogenetically controlled comparative analyses across four mammalian orders to assess how one aspect of this variation--male genital length--scales with (a) male body mass and (b) relative testis mass, the latter providing an index of the intensity of sperm competition. In all four orders, baculum (=os penis) length is found to scale only weakly with male body mass. Both baculum and glans penis length in rodents and baculum length in carnivores are found to vary positively with relative testis mass. In contrast, there is no evidence to support an association between baculum length and relative testis mass in either bats or primates. These results suggest that postcopulatory sexual selection influences genital length in at least some mammals, but significant questions remain both as to why selection on the baculum should differ between mammalian groups and as to the precise mechanistic basis through which males benefit from increased genital length.

Penises as Variable-Volume Hydrostatic Skeletons

Penises are inflatable intromittent organs that transfer sperm to a female during copulation. Most of the time, males store their penises in a flexible detumesced state, but they can rapidly inflate them with blood when an opportunity for reproductive behavior arises. In mammals, the primary erectile tissue is called the corpus cavernosum; its anatomy is a close match to a model hydroskeleton reinforced by an axial orthogonal fiber array. The wall of the corpus cavernosum contains layers of highly organized collagen fibers arranged at 0 degrees and 90 degrees to the penile long axis. Flaccid wall tissue is folded. Collagen fiber straightening during erection expands the tunica albuginea and increases both its stiffness and its second moment of area. These changes make the entire penis larger and harder to bend. Axial orthogonal fiber reinforcement affects the mechanical behavior of the erect corpus cavernosum, making it resistant to tensile, compressive, and bending forces.