The functional morphology of penile erection: tissue designs for increasing and maintaining stiffness

Inflatable penises have evolved independently at least fourtimes in amniotes, specifically in mammals, turtles, squamates,and the archosaurs. Males in these lineages therefore sharethe functional problem of building a penis out of soft and flexibletissues that can increase its flexural stiffness and resistbending during copulation. Research on penile erectile tissuesin mammals and turtles shows that these two taxa have convergentlyevolved an axial orthogonal array of collagen fibers to reinforcethe penis during erection and copulation; in both lineages,the collagen fibers in the array are crimped and folded in theflaccid penis. Collagen fiber straightening during erectionincreases the stiffness of the tissue and allows changes inpenile radius that increase its second moment of area: bothof these changes increase the flexural stiffness of the penisas a whole. And once erect, axial orthogonal arrays have thehighest flexural stiffness of any fiber arrangement. The highdegree of anatomical convergence (to the level of microanatomicalfeatures) within mammals and turtles suggests that the stiffnessrequirements for copulation produce an extremely restrictiveselective regime in organisms that evolve inflatable penises.     

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

The baculum is a nonappendicular bone found in the glans tissue of members of five orders of mammals. Its function during copulation is unknown. Anatomical examination of the baculum and corpus cavernosum in the Norway rat (Rattus norvegicus) shows that the two structures are connected by a layer of fibrocartilage, and that the distal tip of the corpus cavernosum swells during erection to surround the proximal end of the baculum. Microradiographs of bacula from sexually experienced males show that regions of the bone may be remodeling; these data suggest that the baculum is load-bearing. On the basis of this anatomy, I propose that the baculum increases the overall flexural stiffness of the penis during copulation by transferring bending and compressive forces from the distal end of the glans to the tensile wall of the corpus cavernosum. Forces on the distal end of the penis during copulation press the baculum against the corpus cavernosum, reducing its internal volume and increasing intracavernosal pressure and corpus cavernosum wall strains. Because the wall of the erect corpus cavernosum is reinforced with inextensible collagen fibers, an increase in wall strain will also increase wall tissue stiffness, and thereby increase the flexural stiffness of the corpus cavernosum.     

Modifications of Erectile Tissue Components in the Penis during the Fetal Period

Background

The penile erectile tissue has a complex microscopic anatomy with important functions in the mechanism of penile erection. The knowledge of such structures is necessary for understanding the normal physiology of the adult penis. Therefore, it is important to know the changes of these penile structures during fetal development. This study aims to analyze the development of the main components of the erectile tissue, such as collagen, smooth muscle fibers and elastic system fibers, in human fetuses.

Methodology/Principal Findings

We studied the penises of 56 human fetuses aged 13 to 36 weeks post-conception (WPC). We used histochemical and immunohistochemical staining, as well as morphometric techniques to analyze the collagen, smooth muscle fibers and elastic system fibers in the corpus cavernosum and in the corpus spongiosum. These elements were identified and quantified as percentage by using the Image J software (NIH, Bethesda, USA). From 13 to 36 WPC, in the corpus cavernosum, the amount of collagen, smooth muscle fibers and elastic system fibers varied from 19.88% to 36.60%, from 4.39% to 29.76% and from 1.91% to 8.92%, respectively. In the corpus spongiosum, the amount of collagen, smooth muscle fibers and elastic system fibers varied from 34.65% to 45.89%, from 0.60% to 11.90% and from 3.22% to 11.93%, respectively.

Conclusions

We found strong correlation between the elements analyzed with fetal age, both in corpus cavernosum and corpus spongiosum. The growth rate of these elements was more intense during the second trimester (13 to 24 WPC) of gestation, both in corpus cavernosum and in corpus spongiosum. There is greater proportional amount of collagen in the corpus spongiosum than in corpus cavernosum during all fetal period. In the corpus spongiosum, there is about four times more collagen than smooth muscle fibers and elastic system fibers, during all fetal period studied.     

Immunohistochemical analysis of smooth muscle cells and volumetric density of the elastic system fibers of wild boar (Sus scrofa) penis

The purpose of the present study was to verify the smooth muscle cell distribution and elastic system fibers volumetric density (Vv) in the corpus spongiosum and corpus cavernosum of the wild boar penis. Adult wild boars (n=13) were used. The penile mid shaft fragments were fixed with 4% phosphate buffered formalin solution and/or Bouin's liquid during 24-48 h, and processed using standard histological techniques. The sections were stained with Weigert's Resorcin-Fucsin with previous oxidation. The elastic system fibers Vv was determined in 25 random fields of each fragment using M42 test system. For immunohistochemical analysis, monoclonal anti-alpha actin smooth muscle was used. The histochemical methods detected elastic system fibers in both corpus spongiosum and corpus cavernosum of all animals. The elastic fibers Vv average was 36.6%+/-0.9 for corpus spongiosum and 11.7%+/-0.5 for corpus cavernosum. Through immunocitochemistry, a small quantity of smooth muscle cells was visualized in intimate relationship with blood vessels wall. The great amount of elastic fibers and the smooth muscle cell distribution beneath the endothelium suggest that these fibers may have an important role in penile erection process in the penis of wild boars.     

Experimental investigation of collagen waviness and orientation in the arterial adventitia using confocal laser scanning microscopy

Mechanical properties of the adventitia are largely determined by the organization of collagen fibers. Measurements on the waviness and orientation of collagen, particularly at the zero-stress state, are necessary to relate the structural organization of collagen to the mechanical response of the adventitia. Using the fluorescence collagen marker CNA38-OG488 and confocal laser scanning microscopy, we imaged collagen fibers in the adventitia of rabbit common carotid arteries ex vivo. The arteries were cut open along their longitudinal axes to get the zero-stress state. We used semi-manual and automatic techniques to measure parameters related to the waviness and orientation of fibers. Our results showed that the straightness parameter (defined as the ratio between the distances of endpoints of a fiber to its length) was distributed with a beta distribution (mean value 0.72, variance 0.028) and did not depend on the mean angle orientation of fibers. Local angular density distributions revealed four axially symmetric families of fibers with mean directions of 0°, 90°, 43° and ?43°, with respect to the axial direction of the artery, and corresponding circular standard deviations of 40°, 47°, 37° and 37°. The distribution of local orientations was shifted to the circumferential direction when measured in arteries at the zero-load state (intact), as compared to arteries at the zero-stress state (cut-open). Information on collagen fiber waviness and orientation, such as obtained in this study, could be used to develop structural models of the adventitia, providing better means for analyzing and understanding the mechanical properties of vascular wall.     

Scanning microscopy of collagen in the basement lamella of normal and regenerating frog tadpoles

The arrangement of collagen fibers over the body surface in the basement lamella of Pseudaeris and Xenopus tadpoles is described. It can be viewed by scanning microscopy after removal of epidermis and basal lamina by trypsin treatment of alcohol fixed tissue. The orthogonal array is modified in regions where fiber direction changes extensively such as the base of the ventral fin or the posterior part of the head. In these regions “exceptional points” in the orthogonal pattern occur, as described by Rosin (1946). The pattern is bilaterally symmetrical. In the region of the nasal opening the orthogonal pattern is replaced by a mat of randomly oriented fibers. In tail regeneration the wound area is marked by aberrant disposition of collagen anteriorly then a mat of randomly disposed fibers followed posteriorly with a sharp transition to the orthogonal pattern of the regenerate. No fiber terminations could be seen in normal or regenerating regions of the lamella.     

Right-handed penises of the earwig Labidura riparia (Insecta, Dermaptera, Labiduridae): evolutionary relationships between structural and behavioral asymmetries

The number of penises vary in the insect suborder Forficulina (order Dermaptera; earwigs). Males of the families Diplatyidae, Pigidicranidae, Anisolabididae, Apachyidae, and Labiduridae have two penises (right and left), while those of the Spongipohridae, Chelisochidae, and Forficulidae have a single penis. The proposed phylogenetic relationships among these families suggest that the single‐penis families evolved from an ancestor possessing two penises. To date, examinations of double‐penis earwig species have found that only a single penis is used per single copulation. These diversities in structural and behavioral aspects of genitalia raises the following intriguing questions: How are the two penises used? Why did a penis degenerate in several earwig families, and which one was lost? To address these questions, structural and behavioral asymmetries were examined in detail for a representative species Labidura riparia (Labiduridae). Although there was no detectable morphological differentiation between the right and left penises, male L. riparia predominantly used the right one for insemination. This significant “right‐handedness” developed without any experience of mating and was also manifested in the resting postures of the two penises when not engaged in copulation. However, surgical ablation of the right penis did not influence the insemination capacity of males. In wild‐caught males, only about 10% were left‐handed; within this group, abnormalities were frequently observed in the right penis. These lines of evidence indicate that the left penis is merely a spare intromittent organ, which most L. riparia males are likely never to use. Additional observations of five species of single‐penis families revealed that the left penis degenerated in the common ancestor of this group. Considering the proposed sister relationship between the Labiduridae and the single‐penis families, it is possible that such behavioral asymmetries in penis' use, as observed in L. riparia, are parental to the evolutionary degeneration of the infrequently used left penis. J. Morphol., 2006. © 2006 Wiley‐Liss, Inc.     

Communication between the corpus cavernosum penis and the corpus spongiosum penis in a bull diagnosed by modified contrast cavernosography: A case report

Traditional methods of penile cavernosography failed to reveal a vascular defect or shunt from the corpus cavernosum penis of a 6-yr-old Brahman bull with a history of acquired failure of penile erection. When a tourniquet was applied to the penis and the contrast medium was introduced into the corpus cavernosum penis under pressure, however, a communication between the corpus cavernosum penis and corpus spongiosum penis was demonstrated. In a normal bull, cavernosography with the introduction of contrast medium under pressure produced a normal radiographic appearance. It is suggested that modification of accepted techniques of cavernosography to incorporate injection of contrast medium under pressure is useful in diagnosing this particular type of vascular abnormality, and may also aid in the detection of smaller vascular shunts from the corpus cavernosum penis to the superficial penile vasculature.     

Endogenous urotensin II selectively modulates erectile function through eNOS

Background

Urotensin II (U-II) is a cyclic peptide originally isolated from the neurosecretory system of the teleost fish and subsequently found in other species, including man. U-II was identified as the natural ligand of a G-protein coupled receptor, namely UT receptor. U-II and UT receptor are expressed in a variety of peripheral organs and especially in cardiovascular tissue. Recent evidence indicates the involvement of U-II/UT pathway in penile function in human, but the molecular mechanism is still unclear. On these bases the aim of this study is to investigate the mechanism(s) of U-II-induced relaxation in human corpus cavernosum and its relationship with L-arginine/Nitric oxide (NO) pathway.

Methodology/Principal Findings

Human corpus cavernosum tissue was obtained following in male-to-female transsexuals undergoing surgical procedure for sex reassignment. Quantitative RT-PCR clearly demonstrated the U-II expression in human corpus cavernosum. U-II (0.1 nM–10 µM) challenge in human corpus cavernosum induced a significant increase in NO production as revealed by fluorometric analysis. NO generation was coupled to a marked increase in the ratio eNOS phosphorilated/eNOS as determined by western blot analysis. A functional study in human corpus cavernosum strips was performed to asses eNOS involvement in U-II-induced relaxation by using a pharmacological modulation. Pre-treatment with both wortmannin or geldanamycinin (inhibitors of eNOS phosphorylation and heath shock protein 90 recruitment, respectively) significantly reduced U-II-induced relaxation (0.1 nM–10 µM) in human corpus cavernosum strips. Finally, a co-immunoprecipitation study demonstrated that UT receptor and eNOS co-immunoprecipitate following U-II challenge of human corpus cavernosum tissue.

Conclusion/Significance

U-II is endogenously synthesized and locally released in human corpus cavernosum. U-II elicited penile erection through eNOS activation. Thus, U-II/UT pathway may represent a novel therapeutical target in erectile dysfunction.     

Ca2+-activated Cl- channels in corpus cavernosum smooth muscle: a novel mechanism for control of penile erection.

Little is known of the excitatory mechanisms that contribute to the tonic contraction of the corpus cavernosum smooth muscle in the flaccid state. We used patch-clamp electrophysiology to investigate a previously unidentified inward current in freshly isolated rat and human corporal myocytes. Phenylephrine (PE) contracted cells and activated whole cell currents. Outward current was identified as large-conductance Ca(2+)-activated K(+) current. The inward current elicited by PE was dependent on the Cl(-) gradient and was inhibited by niflumic acid, indicative of a Ca(2+)-activated Cl(-) (Cl(Ca)) current. Furthermore, spontaneous transient outward and inward currents (STOCs and STICs, respectively) were identified in both rat and human corporal myocytes and derived from large-conductance Ca(2+)-activated K(+) and Cl(Ca) channel activity. STICs and STOCs were inhibited by PE and A-23187, and combined 8-bromoadenosine cAMP and 8-bromoadenosine cGMP decreased their frequency. When studied in vivo, chloride channel blockers transiently increased intracavernosal pressure and prolonged nerve-evoked erections. This report reveals for the first time Cl(Ca) current in rat and human corpus cavernosum smooth muscle cells and demonstrates its key functional role in the regulation of penile erection.     

Regulation of intracellular Ca2+ release in corpus cavernosum smooth muscle: synergism between nitric oxide and cGMP

Tonic contraction of corpus cavernosum smooth muscle cells (SMCs) maintains the flaccid state of the penis, and relaxation is initiated by nitric oxide (NO), leading to erection. Our aim was to investigate the effect of NO on the smooth muscle cellular response to adrenergic stimulation in corpus cavernosum. Fura-2 fluorescence was used to record intracellular Ca²⁺ concentration ([Ca²⁺]_i) from freshly isolated SMCs from rat and human. Phenylephrine (PE) transiently elevated [Ca²⁺]_i in the presence and absence of extracellular Ca²⁺, indicating release from intracellular stores. Whereas the NO donor S-nitroso-N-acetylpenicillamine (SNAP) with sildenafil citrate (SIL) caused no change in basal [Ca²⁺]_i, the PE-induced rise of [Ca²⁺]_i was reversibly inhibited by 27 ± 7% (n = 21, P < 0.005) in rat and by 55 ± 15% (n = 9, P < 0.01) in human SMCs. SNAP and SIL also reduced the contractile response to PE. To investigate the mechanism, we applied mediators alone or in combination. The soluble guanylyl cyclase inhibitor ODQ reduced the effect of SNAP and SIL. SIL, cGMP analogs, and NO donors without SIL did not reduce the PE-induced rise of [Ca²⁺]_i. However, the combination of 8-bromo-cGMP with SNAP reduced the Ca²⁺ peak by 42 ± 9% (n = 22, P < 0.01). Our results demonstrate that NO and cGMP act synergistically to reduce Ca²⁺ release from intracellular stores. Reduction of intracellular Ca²⁺ release may contribute to relaxation of the corpus cavernosum, leading to erection. calcium stores; nitric oxide; sildenafil citrate; inositol 1,4,5-trisphosphate receptor     

Lead acetate may cause erectile dysfunction by modulating NO/cGMP pathway in rat corpus cavernosum

Despite the fact that metal toxicity has been widely reported in industrial toxicological studies, very little has been reported about the effect of lead exposure on erectile function. This study investigated the effect of lead on erectile function in rats and aimed to preliminarily test the mechanisms by which it might affect erection. Rats were injected with lead acetate (0.25–2 mg/kg) intraperitoneally for 21 days. Intracavernosal pressure/mean arterial pressure (ICP/MAP) next to nerve stimulation; nitrite/nitrate; malonaldehyde; and reduced glutathione levels and superoxide dismutase activity in the corpus cavernosum, kidney, and brain were measured in addition to creatinine, urea, and testosterone. For acute studies, rats were injected intravenously with lead acetate, and then ICP/MAP was recorded for 45 min. Subacute treatment significantly reduced erection with significant elevation of malonaldehyde and reduction of nitrite/nitrate levels in the corpus cavernosum. In acute studies, lead (2 and 5 mg/kg) reduced neurogenic erections by 28.42?±?3.76 and 96.84?±?8.52 %, respectively, an effect that was masked in the presence of NG-nitro-l-arginine, tetraethyl ammonium, or methylene blue, but not zinc protoporphyrine, and reversed by vitamin C and partially by sildenafil. Lead acetate may inhibit the erectile process in rats. Besides its prooxidant effect and consequent inactivation of nitric oxide, lead may negatively modulate the action of nitric oxide on guanylate cyclase and potassium channels.     

阴茎勃起及勃起功能障碍的研究进展

勃起功能障碍的基础研究须近十年来取得了较大进展,一氧化氮－cGMP(NO-cGMP)通路的发现使得阴茎平滑肌松驰的机制进一步阐明。一氧化氮合酶（NOS）、磷酸二酯酶（PDEs）的研究为勃起功能障碍的临床治疗提供了坚实的基础,进而促使了万艾可的问世。目前,勃起功能障碍的基因治疗停留在实验室阶段,但随着分子生物学的深入研究,转基因疗法可能成为临床上治疗勃起功能障碍的有效方法之一。     

Mechanical properties of pufferfish (Lagocephalus gloveri) skin and its collagen arrangement

Skin is a biological material the mechanical properties of which are dependent on the constituents from which it is assembled. Skin, the outer covering of animals is made up of collagen fibers arranged in more or less ordered arrays. Pufferfish skin provides a rigid framework to support the body contents and a flexible covering to allow whatever changes are necessary for the remarkable inflation mechanism. Here, we describe the structure and tensile properties of the dorsal and ventral skin of the pufferfish, Lagocephalus gloveri Abe and Tabeta, 1983. The ultimate tensile strength of ventral skin was found to be around two times higher than that of the dorsal skin. It was observed that the dorsal skin could resist more deformation than the ventral skin. The collagen fibers were arranged in different ordered arrays for ventral and dorsal skin and the concentration of fibers was found to be more in ventral than dorsal skin. This provides more stiffness to ventral skin. Scanning electron microscopy studies of the ventral skin showed a unidirectional arrangement of the collagen fibers, which provides more stretching capacity. Dorsal skin, on the other hand, has an orthogonal arrangement of fibers. The present study thus showed that the mechanical behavior of the skin of L. gloveri is strongly influenced by the concentration and arrangement of collagen fibers.     

Study of neuropeptide Y-containing nerve fibers in the human penis

Summary Neuropeptide Y 1–36 (IR-NPY) immunoreactive nerve-fiber processes have been observed in tunicae of veins and arteries and in smooth muscles of the human penis taken at autopsy or during surgery by use of light-and electron-microscopic immunohistochemical techniques. Numerous IR-NPY nerve fibers were mostly concentrated in the inner part of the adventitia close to the media of the arterial and venous vessels and among the intracavernous smooth muscle cells. IR-NPY nerve fibers were less abundant in veins than in arteries. Positive somata were not observed in the penises. At the ultrastructural level, IR-NPY were localized exclusively in large, dense granules of nerve terminals by means of the postembedding immunogold technique. In the deep dorsal vein, IR-NPY nerve fibers were also located in the media formed by an outer circular and an inner longitudinal layer. In the intracavernous and dorsal arteries, they showed the highest density in the inner part of the adventitia. In the corpora cavernosa and in the corpus spongiosum, IR-NPY nerve processes were intermingled between the smooth-muscle fibers around the sinusoid spaces. IR-NPY nerve fibers were present in the cavernous nerves close to the central arteries. The urethra did not show any IR-NPY-positive nerve fibers. This peculiar distribution of IR-NPY nerve fibers suggested that they could participate in regulating arterial and venous blood flow and intracavernous smooth-muscle tone. NPY may therefore be of importance in some of the mechanisms of penile erection especially during detumescence.     

Penile erection: possible role for vasoactive intestinal polypeptide as a neurotransmitter.

Concentrations of vasoactive intestinal polypeptide were measured in blood drawn from the cavernous spaces of corpus cavernosum of the human penis during tumescence and erection, and the effect of injecting the polypeptide into the cavernous spaces was studied. A significant release of the polypeptide was shown during tumescence and erection. Injection of exogenous vasoactive intestinal polypeptide induced erection. These findings support the concept of vasoactive intestinal polypeptide as a neurotransmitter in penile erection and suggest that it might have a clinical use in patients suffering from erectile dysfunction.     

The state of water on hydrated collagen as studied by pulsed NMR

The spin-lattice relaxation time (T₁) of water adsorbed on collagen fibers was determined at six frequencies and temperatures varying from 25° to ?80°C. Care was taken to eliminate the contributions to the signal of protons other than those in the adsorbed water. Quantitative calculations were made on T₁ and the results were compared with the experimental data. It is suggested that a maximum of about 0.50–0.55 g water per g collagen forms a hydration layer, which cannot be frozen down to ?90°C and exhibits a distribution of motional correlation times. For collagen samples containing a larger quantity of adsorbed water, the additional water molecules behave like ordinary isotropic water, having a single correlation time and a freezing temperature of about ?10°C.     

Vascular anatomy of the gills of the stingarees Urolophus mucosus and U. paucimaculatus (Urolophidae,Elasmobranchii)

The branchial vascular anatomy of Urolophus mucosus and U. paucimaculatus was studied by scanning electron microscopical examination of critical-point-dried tissue or of vascular corrosion casts. The vasculature could be divided into arterioarterial and arteriovenous pathways, which channel the flow of blood through the gills. The arterioarterial pathway consists of an afferent branchial artery which gives rise to afferent distributing arteries that run through the tissues of the interbranchial septum and supply the afferent filament arteries of several filaments. Afferent filament arteries open regularly into a corpus cavernosum in the core of the filament; unlike other elasmobranchs no septal corpora cavernosa are found. At the tip of the filament, channels of the corpus cavernosum connect to a channel which passes across the distal end of the filament from afferent to efferent side. This channel always connects to the afferent filament artery, and in many filaments it connects to the efferent filament artery as well. In addition, a vascular arcade connects all the afferent filament arteries along the entire length of each hemibranch. The filament corpus cavernosum supplies the secondary lamellae. The lamellae drain into efferent lamellar arterioles which in turn drain into the efferent filament artery and the efferent branchial artery. The vascular anatomy of the arteriovenous pathway is similar to that described in other elasmobranchs and consists of arteriovenous anastomoses, found only arising from efferent arterial circulation, and the venolymphatic system, which is composed of the central venous sinus and the companion vessels.     

Effect of lysyl oxidase (LOX) on corpus cavernous fibrosis caused by ischaemic priapism

Penile fibrosis caused by ischemic priapism (IP) adversely affects patients’ erectile function. We explored the role of lysyl oxidase (LOX) in rat and human penes after ischemic priapism (IP) to verify the effects of anti‐LOX in relieving penile fibrosis and preventing erectile dysfunction caused by IP in rats. Seventy‐two rats were randomly divided into six groups: control group, control + β‐aminopropionitrile (BAPN) group, 9 hrs group, 9 hrs + BAPN group, 24 hrs group, and 24 hrs + BAPN group. β‐aminopropionitrile (BAPN), a specific inhibitor of LOX, was administered in the drinking water. At 1 week and 4 weeks, half of the rats in each group were randomly selected for the experiment. Compared to the control group, the erectile function of IP rats was significantly decreased while the expression of LOX in the corpus cavernosum was significantly up‐regulated in both 9 and 24 hrs group. Proliferated fibroblasts, decreased corpus cavernosum smooth muscle cells/collagen ratios, destroyed endothelial continuity, deposited abnormal collagen and disorganized fibers were observed in IP rats. The relative content of collage I and III was not obviously different among the groups. β‐aminopropionitrile (BAPN) could effectively improve the structure and erectile function of the penis, and enhance recovery. The data in this study suggests that LOX may play an important role in the fibrosis of corpus cavernosum after IP and anti‐LOX may be a novel target for patients suffering with IP.     

Fitness advantages of the biased use of paired laterally symmetrical penises in an insect

The evolution of laterality, that is the biased use of laterally paired, morphologically symmetrical organs, has attracted the interest of researchers from a variety of disciplines. It is, however, difficult to quantify the fitness benefits of laterality because many organs, such as human hands, possess multimodal functions. Males of the earwig Labidura riparia (Insecta: Dermaptera: Labiduridae) have morphologically similar laterally paired penises, only one of which is used for inseminating the female during a single copulation bout, and thus provide a rare opportunity to address how selection pressure may shape the evolution of population‐level laterality. Our population studies revealed that in 10 populations, located at 2.23–43.3° north, the right penis is predominantly used for copulating (88.6%). A damaged penis was found in 23% of rare left‐handers, suggesting that the left penis can function as a spare when the right one is damaged. By pairing L. riparia females with surgically manipulated males, we found that males forced to use the right penis outperformed left‐handed males in copulation (the probability of establishing genital coupling during the 1‐hr observation period: odds ratio [OR] of 3.50) and insemination (probability of transferring a detectable amount of sperm: OR of 2.94). This right‐handed advantage may be due to the coiled morphology of the sperm storage organ with a right‐facing opening. Thus, female genital morphology may play a significant role in the evolution of handedness and may have acted as a driving force to reduce penis number in related taxa.