Intromittent Organ Morphology and Biomechanics: Defining the Physical Challenges of Copulation

Evolution and Morphology of Genitalia in Female Amniotes

Despite their evolutionary and biomedical importance, studies of the morphology and function of female genitalia have continued to lag behind those of male genitalia. While studying female genitalia can be difficult because of their soft, deformable and internal nature, recent advances in imaging, geometric analyses of shape and mechanical testing have been made, allowing for a much greater understanding of the incredible diversity of form and function of female genitalia. Here we summarize some of these methods, as well as discuss some big questions in the field that are beginning to be examined now, and will continue to benefit from further work, especially a comparative approach. Topics of further research include examination of the morphology of female genitalia in situ, in-depth anatomical work in many more species, studies of the interplay between natural and sexual selection in influencing features of vaginal morphology, how these diverse functions influence the mechanical properties of tissues, and studies of clitoris morphology and function across amniotes. Many other research topics related to female genitalia remain largely unexplored, and we hope that the papers in this issue will continue to inspire further research on female genitalia.

A Review and Case Study of 3D Imaging Modalities for Female Amniote Reproductive Anatomy

Recent advances in non-invasive imaging methods have revitalised the field of comparative anatomy, and reproductive anatomy has been no exception. The reproductive systems of female amniotes present specific challenges, namely their often internal "hidden" anatomy. Quantifying female reproductive systems is crucial to recognising reproductive pathologies, monitoring menstrual cycles, and understanding copulatory mechanics. Here we conduct a review of the application of non-invasive imaging techniques to female amniote reproductive anatomy. We introduce the commonly used imaging modalities of computed tomography (CT) and magnetic resonance imaging (MRI), highlighting their advantages and limitations when applied to female reproductive tissues, and make suggestions for future advances. We also include a case study of micro CT and MRI, along with their associated staining protocols, applied to cadavers of female adult stoats (Mustela erminea). In doing so, we will progress the discussion surrounding the imaging of female reproductive anatomy, whilst also impacting the fields of sexual selection research and comparative anatomy more broadly.

Genome-wide molecular phylogenetic analyses and mating experiments which reveal the evolutionary history and an intermediate stage of speciation of a giant water bug

The intermediate stages of speciation are important for understanding the processes involved in the creation of biodiversity, and also comprise a number of interesting phenomena. However, difficulties are associated with dividing clear speciation stages because speciation is a continuous process. Therefore, the elucidation of speciation is an interesting and important task in evolutionary biology. We herein present an example of a species in an intermediate stage of speciation using the giant water bug Appasus japonicus (Heteroptera, Belostomatidae) that was investigated using mating experiments and phylogenetic analyses of the mtDNA COI (658 bp) and 16S rRNA (435 bp) regions, and nDNA SSR (13 loci) and its genome-wide SNPs (11,241 SNPs). The results of our phylogenetic analyses based on their mtDNA dataset and the genome-wide SNPs dataset strongly supported the paraphyly of the Japanese populations. Therefore, it is suggested that their ancestral lineage which being distributed in the Japanese Archipelago subsequently migrated to the Eurasian Continent (i.e., "back-dispersal" occurred). Furthermore, the results of the mating experiments suggested that among A. japonicus, even between closely related lineages, pre-mating reproductive isolation has been established by the differentiation of copulatory organ morphologies. In contrast, pre-mating reproductive isolation is not established in the absence of the differentiation of copulatory organ morphologies, even if genetic differentiation is prominent. These results suggested that their phylogenetic distance does not predict pre-mating reproductive isolation. Furthermore, in the present study, we present a clear example of pre-mating reproductive isolation driving speciation between closely related lineages.

Genital anatomy and copulatory interactions in the broad snouted Caiman (Caiman latirostris)

The broad snouted caiman is a crocodylian native to South America that is subject to extensive conservation management in both wild and farming environments. Although reproductive behaviors like egg laying and clutch care have been examined in this species, little else is known about their copulatory system. We examined the anatomy of male and female cloacal and genital tissues ex vivo to build hypotheses of their interactions during copulation and the effects of that interaction on insemination. Male phallic glans tissues were artificially inflated to expand into their copulatory state, allowing the examination and quantification of structural changes at the gross and tissue levels. Digital reconstruction of MRI stacks yielded three-dimensional tissue compartment specific glans models of the inflated state. Silicone molds of female cloacae and oviducts in conjunction with dissection and diceCT analysis allowed us to assess internal geometry and infer how male and female features interact in copulo. We observed glans expansion within the female proctodeum would result in a copulatory lock limiting deeper intromission or retraction. Intromission and subsequent creation of the copulatory lock produces extensive clitoral compression, providing a possible mechanism for female assessment of male copulatory performance. Further, glans expansion forms a distal lumen that positions the glans tip in or near the vaginal openings. A coiled, muscular vagina provides a possible mechanism for postcopulatory sexual selection by excluding semen. Together, the complex male-female interaction supports evidence for cryptic selection by female choice, which can act as a driver of genital coevolution.

Mechanical properties of a female reproductive tract of a beetle and implications for penile penetration

Coevolution of male and female genitalia is widespread in animals. Nevertheless, few studies have examined the mechanics of genital interactions during mating. We characterized the mechanical properties of the elongated female genitalia, the spermathecal duct, of the small cassidine beetle, Cassida rubiginosa. The data were compared with the mechanical properties of the elongated male genitalia, the flagellum. We analysed the material distributions of the spermathecal duct using a microscopy technique, established a tensile test setup under a light microscope and conducted tensile tests. Diameter and tensile stiffness gradients were present along the spermathecal duct, but its Young's modulus and material distribution were more or less homogeneous. The results confirmed the hypothesis based on numerical simulations that the spermathecal duct is more rigid than the flagellum. In the study species, the penile penetration force is simply applied to the base of the hyper-elongated flagellum and conveyed along the flagellum to its tip. Considering this simple penetration mechanism, the relatively low flexibility of the spermathecal duct, compared to the flagellum, is likely to be essential for effective penetration of the flagellum.

Glans inflation morphology and female cloaca copulatory interactions of the male American alligator phallus†

The phallic glans of the American alligator (Alligator mississippiensis) is the distal termination of the semen-conducting sulcus spermaticus and during copulation has the closest, most intimate mechanical interactions with the female urodeum, the middle cloacal chamber that contains the opening to the vaginal passages and oviducts. However, the details of this interface leading to insemination and gamete uptake are unclear. Here, we: (1) histologically characterize the underlying tissue types and morphologically quantify the shape changes associated with glans inflation into the copulatory conformation, (2) digitally reconstruct from MRI the 3D shape of functional tissue compartments, and (3) diffusible iodine-based contrast-enhanced computed tomography image the copulatory fit between male phallus and female cloaca. We discuss these results in relation to tissue type material properties, the transfer on intromittent forces, establishing potential copulatory lock, inflated glans volume scaling with body mass/length, the mechanics of semen targeting and insemination, and potential female cryptic choice impacting multiple clutch paternity. In part, this study further clarifies the phallic morphological variation observed among crocodylians and begins to investigate the role(s) these divergent male forms play during copulation interacting with female cloacal forms to increase reproductive success.

Morphological changes associated with Nile crocodile (Crocodylus niloticus) phallic glans inflation

The crocodylian phallic glans is the distal inflatable structure that makes the most direct contact with the female cloacal and associated reproductive tract openings during copulation. Therefore, its form and function directly impact female tissue sensory interactions and insemination mechanics. Compared to mammals, less is known about glans functional anatomy among other amniotes, including crocodylians. Therefore, we paired an ex vivo inflation technique with magnetic resonance imaging 3D-reconstructions and corresponding histological analyses to better characterize the morphological glans restructuring occurring in the Nile crocodile (Crocodylus niloticus) at copulation. The expansion of contiguous inflatable spongiform glans tissues is variably constrained by adjacent regions of dense irregular collagen-rich tissues. Therefore, expansion shows regional differences with greater lateral inflation than dorsal and ventral. Furthermore, this enlargement elaborates the cup-like glans lumen, dorsally reorients the glans ridge, stiffens the blunt and bifid glans tip, and putatively works to seal the ventral sulcus spermaticus semen conduit groove. We suggest how these dynamic male structures may interact with structures of the female cloacal urodeum and how these morphological changes, in concert with the varying material properties of the structural tissue compartments visualized in this study, aid copulatory gamete transfer and resulting fecundity. RESEARCH HIGHLIGHTS: Nile crocodile glans inflation produces a reproductively relevant copulatory structure directing insemination and female tissue interactions. Pairing magnetic resonance imaging 3D reconstruction with corresponding histology effectively studies functional anatomy.

The innervation of the male copulatory organ of spiders (Araneae) - a comparative analysis

Background

Nervous tissue is an inherent component of the many specialized genital structures for transferring sperm directly into the female’s body. However, the male copulatory organ of spiders was considered a puzzling exception. Based on the recent discovery of nervous tissue in the pedipalps of two distantly related spider species, we investigated representatives of all major groups across the spider tree of life for the presence of palpal nerves. We used a correlative approach that combined histology, micro-computed tomography and electron microscopy.

Results

We show that the copulatory organ is innervated in all species investigated. There is a sensory organ at the base of the sperm transferring sclerite in several taxa and nervous tissue occurs close to the glandular tissue of the spermophor, where sperm are stored before transfer.

Conclusions

The innervation of the copulatory organ by the bulb nerve and associated efferent fibers is part of the ground pattern of spiders. Our findings pave the way for unraveling the sensory interaction of genitalia during mating and for the still enigmatic mode of uptake and release of sperm from the male copulatory organ.

Genital coupling, morphology and evolution of male holding structures in Cicadinae (Hemiptera: Cicadidae)

Male and female genitalia include some of the most complex and morphologically diverse structures in Metazoa. Ornamentations in genitalia have been studied in several groups, and a variety of functional roles have been proposed. Although complex features of the genitalia have been observed in internal genitalia in cicadas, their functions have not yet been elucidated. These ornamentations, together with precopulatory sexual selection, make cicadas good models for evolutionary studies on genital coupling. We explore the structural interaction of male and female genitalia in Guyalna bonaerensis (Berg) (Cicadinae) and the morphology of male ornamentations in Cicadinae generally. We group these ornamentations into two traits according to their inferred function: anchoring or gripping. We analyse the theca and vesica of 24 species and perform ancestral trait reconstruction under maximum likelihood and stochastic mapping on a Bayesian tree. Ornamentations of the male vesica and the female seminal ampoule possibly ensure male attachment by working as an active lock to avoid the premature termination of intercourse. These ornamentations emerged independently in different lineages in Cicadinae, reinforcing the suggestion that they are important adaptations to achieve complete copulation. Our results foster questions for the field of sexual selection and associated mechanisms shaping the evolution of male and female genitalia.

Biomechanical properties of female dolphin reproductive tissue

Whales, dolphins, and porpoises have unusual vaginal folds of unknown function(s) that are hypothesized to play an important role in sexual selection. The potential function of vaginal folds was assessed by testing the mechanical properties of common bottlenose dolphin (Tursiops truncatus) reproductive tract tissues in 6 different regions and across age classes in post-mortem specimens. We assessed the regional (local) and overall effective elastic modulus of tissues using indentation and tensile tests, respectively. We explore the non-linear mechanical response of biological tissues, which are not often quantified. Indentation tests demonstrated that sexual maturity state, tissue region, force history, and force magnitude values significantly affected the measured effective elastic modulus. Tissue was stiffest in the vaginal fold region and overall stiffer in sexually immature compared to mature animals, likely reflecting biomechanical adaptations associated with copulation and parturition. Tensile tests showed that only tissue region significantly affected the effective modulus. Our data support the hypothesis that vaginal folds function as mechanical barriers to the penis and may provide females with mechanisms to reduce copulatory forces on other reproductive tissue. STATEMENT OF SIGNIFICANCE: Cetaceans have unusual folds of vaginal wall tissue that appear to evolve under sexual selection mechanisms and present physical barriers to the penis during copulation. We explore the biomaterial properties of vaginal fold tissue, how it varies from other reproductive tract tissues, and ontogenetic patterns. We demonstrate that vaginal folds can withstand higher mechanical forces and respond in a manner conducive to dissipating copulatory forces to other reproductive tissues. This study yields exciting insights on how female genital tissue may function during copulation, and is the first to do so in any vertebrate species. Additionally, we provide an example for testing biological tissues, non-linear properties, and materials with uneven surface structure and uneven thickness.

Genital morphology associated with mating strategy in the polymorphic lizard, Uta stansburiana

Sexual selection can lead to rapid evolution of sexual traits and striking morphological diversity across taxa. In populations where competition for mates is intense, males sometimes evolve distinct behavioral strategies along with morphological differences that help them secure mating opportunities. Strong postcopulatory selection and differential resource allocation across male strategy type can result in strategy-specific differences in sexual traits, such as sperm morphology, ejaculate components, and testis size. Some polymorphic species also have strategy-specific genital morphology. Thus far, among vertebrates, this has only been observed in fish. Here, we present the first morphological description of the intromittant copulatory organ, the hemipenis, of the three mating types of the side-blotched lizard, Uta stansburiana, from a population that exhibits alternative mating strategies. We found that the isometrically scaling hemipenis was shortest in the nonterritorial (yellow) morph that sneaks copulations with other males' mates. Although the hemipenes were generally the same shape across morphs, the usurping territorial (orange) morph had a significantly wider apical horn than the nonterritorial sneaker morph. Sneaker males also had smaller relative body masses than both the mate-guarding (blue) morph and the usurper morph, and shorter tibia than the usurper morph. This study using a small sample of males suggests that strong sexual selection may drive genital trait differentiation across morphs within populations of terrestrial vertebrates.

Testing hypotheses for the function of the carnivoran baculum using finite-element analysis

The baculum (os penis) is a mineralized bone within the glans of the mammalian penis and is one of the most morphologically diverse structures in the mammal skeleton. Recent experimental work provides compelling evidence for sexual selection shaping the baculum, yet the functional mechanism by which this occurs remains unknown. Previous studies have tested biomechanical hypotheses for the role of the baculum based on simple metrics such as length and diameter, ignoring the wealth of additional shape complexity present. For the first time, to our knowledge, we apply a computational simulation approach (finite-element analysis; FEA) to quantify the three-dimensional biomechanical performance of carnivoran bacula (n = 74) based upon high-resolution micro-computed tomography scans. We find a marginally significant positive correlation between sexual size dimorphism and baculum stress under compressive loading, counter to the 'vaginal friction' hypothesis of bacula becoming more robust to overcome resistance during initial intromission. However, a highly significant negative relationship exists between intromission duration and baculum stress under dorsoventral bending. Furthermore, additional FEA simulations confirm that the presence of a ventral groove would reduce deformation of the urethra. We take this as evidence in support of the 'prolonged intromission' hypothesis, suggesting the carnivoran baculum has evolved in response to pressures on the duration of copulation and protection of the urethra.

Penetration mechanics of a beetle intromittent organ with bending stiffness gradient and a soft tip

Hyper-elongated structures and their penetration are widespread among insects, for example, intromittent organs, ovipositors, and piercing-sucking mouthparts. The penetration of thin structures with high aspect ratio without buckling and rupturing is mechanically very challenging. However, this problem is economically solved in nature, and the solutions might be helpful for, for example, in the development of harmless catheters. We focus on the penetration process of a hyper-elongated structure of a cassidine beetle intromittent organ, termed a flagellum. We applied a three-point bending test for the flagellum to measure its bending stiffness along the entire flagellum. We demonstrated the bending stiffness gradient, in which the basal half is relatively stiff and the apical half is softer, whose good performance during copulation had been previously numerically demonstrated. The stiffness gradient is the result of the flagellum shape, which is cylindrical and tapered toward the tip. Moreover, the curved tip comprises a harder outer curve and a softer inner curve. Considering the findings of preceding studies, the flagellum works in the following way: (i) the bending stiffness gradient supports the flagellum, easily fitting to a shape of a highly coiled spermathecal duct, (ii) the stiffness property of the very tip may make the tip tougher, and (iii) the curled tip and homogeneously cylindrical shape of the organ help the very tip to fit the shape of the spermathecal duct of the female. Our study shows that the apparently simple flagellum penetration is achieved with numerous elaborate mechanical adaptations.

Non-pharmacological and non-surgical strategies to promote sexual recovery for men with erectile dysfunction

Erectile dysfunction (ED), the most commonly reported sexual problem for men, reduces the quality of life for both patients and their partners. Even when physiologically effective, long-term adherence to ED treatments is poor. We review here the implication of having patients' partners involved in ED treatment, starting with treatment selection. We suggest that having partners engaged from the outset may promote an erotic association of the treatment with the partner, i.e., conceptually linking the aid to the sexual pleasure that the partner provides. We hypothesize that this erotic association should enhance the sexual aid's effectiveness and might potentially help improve long-term adherence. The primary focus of this review, though, is non-pharmacological and non-surgical options for maintaining sexual activity for men with ED. Though not ED treatments per se, anecdotal data suggest that these options may be effective for some patients and their partners in regaining a satisfying sex life. The aids discussed include external penile prostheses, penile sleeves, and penile support devices. These devices can allow men to participate in penetrative sexual intercourse despite moderate to severe ED. External penile prostheses can be personalized so they match in size and shape a man's normal full erection. Penile sleeves can similarly be customized with a lumen that fits best a patient's penis for optimal tactile stimulation. We review how multi-sensory integration can enhance sexual arousal for men who use such devices, allowing them to achieve orgasm despite intractable ED. Patients are not always advised within ED clinics about these options nor why and how they can facilitate non-erection dependent sexual recovery. Clinicians need to be aware of these devices and their positive attributes, so they can objectively counsel and encourage couples to explore their use as an alternative to more invasive treatments. The most commonly promoted non-medical ED aid offered to patients is the vacuum erection device. We discuss how erections achieved with the vacuum erection device have a "hinge effect", that is an underappreciated barrier to the effectiveness of the erection. With a hinged erection, the penis points downward rather than upward. We show how the normal kinematics of the penis during coitus is not strictly linear (i.e., not uniaxial; not just in-and-out), and is impeded by hinging. Positional adjustment, such as the receptive partner being on top, may help overcome this problem for some couples. Lastly, we suggest that, in the case where ED can be anticipated from a pending medical treatment, such as a prostatectomy, pre-habilitative approaches may potentially improve adherence to sexual aid use in the long-term. In conclusion, non-pharmacological and non-surgical options for sexual recovery are available. Scientific studies on the effectiveness of these interventions in restoring satisfying sex are warranted.

Eversion and withdrawal of an intromittent organ before sexual maturation prepares male beetles for copulation

Some species of criocerine beetles have a hyper-elongated part of the intromittent organ called a flagellum. In resting position, the flagellum is stored in a specialized internal sac in the intromittent organ. This specialized state of the flagellum and internal sac is indispensable during copulation for flagellar insertion into the female spermathecal duct for sperm transfer. However, the morphogenesis of the flagellum does not generate the active state of the flagellum; rather, the flagellum is generated in an inactive and completely coiled state. After eclosion, males of Lema coronata evert and withdraw the internal sac multiple times before sexual maturation, without mounting a female. This behaviour serves to uncoil the flagellum and guide it into the active state with the aid of surface structures on the internal sac. A closely related species, Lema dilecta, also has a long flagellum and undergoes the same behaviour to place the flagellum in the active position. However, some other species of criocerine beetles with much shorter flagella can attain the active state without exhibiting this behaviour. Based on a previously proposed phylogenetic tree, we discuss the evolutionary history of the hyper-elongation of the flagellum and associated behaviour.