Male genital organs of the black-crowned dwarf marmoset (Callibella humilis)

This study provides the first morphological description of the male genital organs of black-crowned dwarf marmoset (Callibella humilis). Internal and external genital organs were similar to other species of non-human primates. However, some shape and size differences were noted. The penile shaft was laterally flattened and comprised of two corpus cavernosus. These were split by a septum up to the level of the glans and then merged to form a single structure. The glans penis had small, keratinized spicules arising from epidermal or dermal projections. The small, fully ossified and well-mineralized penile bone, located at the distal end of the penis, consisted of a bone matrix surrounded by sparse osteoblasts and well-developed internal osteocytes. The penile urethra was lined with columnar pseudostratified epithelium, with areas of simple cuboidal epithelium. The testes were small relative to the body compared to other primate species. The findings of this work may support comparative studies of primate reproductive ecology.

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.

Testing Morphological Relationships Between Female and Male Copulatory Structures in Bats

The lower reproductive tract of female mammals has several competing functions including mating, tract health maintenance, and parturition. Diverse vaginal anatomy suggests interactions between natural and sexual selection, yet despite its importance, female copulatory morphology remains under-studied. We undertook a comparative study across the species-rich mammalian order Chiroptera (bats) with a focus on the suborder Yangochiroptera (Vespertilioniformes) to examine how female vaginal features may have coevolved with male penis morphology to minimize mechanical damage to their tissues during copulation. The penis morphology is diverse, presenting great potential for post-copulatory sexual selection and coevolution with the female morphology, but vaginas have not been carefully examined. Here we test the hypotheses that vaginal thickness and collagen density have coevolved with features of the male penis including the presence of spines and a baculum. We present histological data from females of 24 species from 7 families of bats, and corresponding data on male penis anatomy. We also examine the role of phylogenetic history in the morphological patterns we observe. We found evidence that female vaginal thickness has coevolved with the presence of penile spines, but not with baculum presence or width. Collagen density did not appear to covary with male penile features. Our findings highlight the importance of considering interactions between the sexes in influencing functional reproductive structures and examine how these structures have been under selection in bats.

Different traits at different rates: The effects of dynamic strain rate on structural traits in biology

Phenotypic diversity is influenced by physical laws that govern how an organism's morphology relates to functional performance. To study comparative organismal biology, we need to quantify this diversity using biological traits (definable aspects of the morphology, behavior, and/or life history of an organism). Traits are often assumed to be immutable properties that need only be measured a single time in each adult. However, organisms often experience changes in their biotic and abiotic environments that can alter trait function. In particular, structural traits represent the physical capabilities of an organism and may be heavily influenced by the rate at which they are exposed to physical demands ('loads'). For instance, materials tend to become more brittle when loaded at faster rates which could negatively affect structures trying to resist those loads (e.g., brittle materials are more likely to fracture). In the following perspective piece, we address the dynamic properties of structural traits and present case studies that demonstrate how dynamic strain rates affect the function of these traits in diverse groups of organisms. First, we review how strain rate affects deformation and fracture in biomaterials and demonstrate how these effects alter puncture mechanics in systems such as snake strikes. Second, we discuss how different rates of bone loading affect the locomotor biomechanics of vertebrates and their ecology. Through these examinations of diverse taxa and ecological functions, we aim to highlight how rate-dependent properties of structural traits can generate dynamic form-function relationships in response to changing environmental conditions. Findings from these studies serve as a foundation to develop more nuanced ecomechanical models that can predict how complex traits emerge and, thereby, advance progress on outlining the Rules of Life.

Gender Bias in the Study of Genital Evolution: Females Continue to Receive Less Attention than Males

The evolution of female genitalia has historically received less attention than male reproductive organs. Several papers have underscored the disparities in research efforts, but have calls for change resonated with the scientific community and rectified the skew? A literature review was conducted of journal articles published between 2013 through 2021 that explore genital evolution to determine if gender bias (sex of research subject) and imbalance (sex of researcher) have changed. Of the 334 articles that specifically explored genital evolution, first authors of both sexes published on female genitalia less than half as often as male genitalia, although the majority of authors published on genitalia of both sexes. First authors of both sexes mentioned females after males substantially more often than females before males. Female first authors published the most about genital evolution in all taxa except for insects and arachnids. Female first authors published in high impact journals marginally less often than male first authors. Articles about genital evolution across taxa generally had high impact factors, but how impact factors and number of citations varied by the sex of the subject was not clear. Although the number of studies exploring genital co-evolution between the sexes has increased across taxa and years, female genitalia continue to be researched less often than male genitalia when only one sex is investigated. Both female and male scientists are publishing in the field of genital evolution, although research on female subjects continue to lag behind males, demonstrating continued bias within the discipline.

Cornification and classical versus nonclassical androgen receptor signaling in mouse penile/preputial development

Mouse penile development is androgen-dependent. During development of male and female external genitalia, an internal ectodermal epithelial structure forms called the preputial lamina. At puberty the male preputial lamina canalizes to create the preputial space, effectively splitting into two layers: (a) the epithelial lining of the prepuce and (b) the surface epithelium of the penis. The female preputial lamina does not canalize, and instead remodels into the inverted U-shaped clitoral lamina of the adult female mouse. Androgen-dependent penile development was studied in transgenic mice with pathway-selective AR mutant transgenes through which AR signaling was activated either via the classical (AR-C) or the nonclassical pathway (AR-NC). Penile development and canalization of the preputial lamina was observed in AR-C and wild-type male mice naturally having both AR-C and AR-NC pathways. Conversely, clitoral development occurred in AR null (lacking both AR-C and AR-NC pathways) and AR-NC mice. The process of canalization of the preputial lamina seen in wild-type, AR-C and AR-C/AR-NC male mice involved cornification of the preputial lamina which involved up-regulation of keratin 10 and loricrin. Such up-regulation of these epidermal proteins was absent in the developing and adult clitoral lamina seen in wild-type female mice and AR-NC and AR null male (XY) mice. Thus, signaling through AR-C is sufficient to initiate and promote penile development and canalization of the preputial lamina, a process involving epithelial cornification.

The ultimate database to (re)set the evolutionary history of primate genital bones

Scientific literature concerning genital bones in primates consists of both ancient works (dating back to the nineteenth century) and more recent revisions/meta-analyses, which, however, are not always so detailed or exhaustive. Based on a thorough analysis, several conflicting data, inaccurate references, and questionable claims have emerged. We generated a binary matrix of genital bone occurrence data, considering only data at the species level, based on (1) a rigorous literature search protocol, (2) raw data (collected exclusively from primary literature), (3) an updated taxonomy (often tracing back to the species taxonomic history) and (4) new occurrence data from scanned genitals of fresh and museum specimens (using micro-computed tomography-micro-CT). Thanks to this methodological approach, we almost doubled available occurrence data so far, avoiding any arbitrary extension of generic data to conspecific species. This practice, in fact, has been recently responsible for an overestimation of the occurrence data, definitively flattening the interspecific variability. We performed the ancestral state reconstruction analysis of genital bone occurrence and results were mapped onto the most updated phylogeny of primates. As for baculum, we definitively demonstrated its simplesiomorphy for the entire order. As for baubellum, we interpreted all scattered absences as losses, actually proposing (for the first time) a simplesiomorphic state for the clitoral bone as well. The occurrence data obtained, while indirectly confirming the baculum/baubellum homology (i.e., for each baubellum a baculum was invariably present), could also directly demonstrate an intra-specific variability affecting ossa genitalia occurrence. With our results, we established a radically improved and updated database about the occurrence of genital bones in primates, available for further comparative analyses.

Female reproductive senescence across mammals: A high diversity of patterns modulated by life history and mating traits

Senescence patterns are highly variable across the animal kingdom. However, while empirical evidence of actuarial senescence in vertebrates is accumulating in the wild and life history correlates of actuarial senescence are increasingly identified, both the extent and variation of reproductive senescence across species remain poorly studied. Here, we performed the first large-scale analysis of female reproductive senescence across 101 mammalian species that encompassed a wide range of Orders. We found evidence of reproductive senescence in 68.31 % of the species, which demonstrates that reproductive senescence is pervasive in mammals. As expected from allometric rules, the onset of reproductive senescence occurs later and the rate of reproductive senescence decreases with increasing body mass and delayed age at first reproduction. Moreover, for a given pace of life, females displaying a high level of multiple mating and/or with induced ovulation senesce earlier than females displaying a low level of multiple mating and/or with spontaneous ovulation. These results suggest that both female mating behavior and reproductive physiology shape the diversity of reproductive senescence patterns across mammals. We propose future avenues of research regarding the role played by environmental conditions or reproductive features (e.g. type of placentation) on the evolution of reproductive senescence.

The hidden ageing costs of sperm competition

Ageing and sexual selection are intimately linked. There is by now compelling evidence from studies performed across diverse organisms that males allocating resources to mating competition incur substantial physiological costs, ultimately increasing ageing. However, although insightful, we argue here that to date these studies cover only part of the relationship linking sexual selection and ageing. Crucially, allocation to traits important in post-copulatory sexual selection, that is sperm competition, has been largely ignored. As we demonstrate, such allocation could potentially explain much diversity in male and female ageing patterns observed both within and among species. We first review how allocation to sperm competition traits such as sperm and seminal fluid production depends on the quality of resources available to males and can be associated with a wide range of deleterious effects affecting both somatic tissues and the germline, and thus modulate ageing in both survival and reproductive terms. We further hypothesise that common biological features such as plasticity, prudent sperm allocation and seasonality of ejaculate traits might have evolved as counter-adaptations to limit the ageing costs of sperm competition. Finally, we discuss the implications of these emerging ageing costs of sperm competition for current research on the evolutionary ecology of ageing.

Tissue clearing-based method for unobstructed three-dimensional imaging of mouse penis with subcellular resolution

Although mice are widely used to elucidate factors contributing to penile disorders and develop treatment options, quantification of tissue changes upon intervention is either limited to minuscule tissue volume (histology) or acquired with limited spatial resolution (MRI/CT). Thus, imaging method suitable for expeditious acquisition of the entire mouse penis with subcellular resolution is described that relies on both aqueous- (clear, unobstructed brain imaging cocktails and computational analysis) and solvent-based (fluorescence-preserving capability imaging of solvent-cleared organs) tissue optical clearing (TOC). The combined TOC approach allows to image mouse penis innervation and vasculature with unprecedented detail and, for the first time, reveals the three-dimensional structure of murine penis fibrocartilage.

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.

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.

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.

Paternity data and relative testes size as measures of level of sperm competition in the Cercopithecoidea

Historically, the empirical study of the role of sperm competition in the evolution of sexual traits has been problematic through an enforced reliance on indirect proxy measures. Recently, however, a procedure was developed that uses paternity data to measure sperm competition level directly in terms of males/conception (i.e., the number of males that have sperm present in a female's ampulla at conception). When tested on apes and humans (Hominoidea) this measure proved not only to correlate significantly with the traditionally used measure of relative testes size but also to offer a number of advantages. Here we provide a second test of the procedure, this time using paternity data for the Old World monkeys (Cercopithecoidea). We calculate sperm competition levels (males/conception) for 17 species of wild and free-ranging cercopithecoids and then analyze the data against measures of relative testes size. Calculated sperm competition levels correlate strongly with relative testes size both with and without phylogenetic control at both the species and generic levels. The signal-to-noise ratios inherent in both the past measure of relative testes size and the new measure of sperm competition level from paternity data are discussed. We conclude that although both measures are appropriate for the future study of the role of sperm competition in the evolution of sexual traits, when paternity data are available they provide the more direct and meaningful analytical tool. Not least, they potentially allow a first empirical analysis of the role of sperm competition in the evolution of relative testes size that could then be compared with the wealth of theoretical analyses that already exist.

Making a point: shared mechanics underlying the diversity of biological puncture

A viper injecting venom into a target, a mantis shrimp harpooning a fish, a cactus dispersing itself via spines attaching to passing mammals; all these are examples of biological puncture. Although disparate in terms of materials, kinematics and phylogeny, all three examples must adhere to the same set of fundamental physical laws that govern puncture mechanics. The diversity of biological puncture systems is a good case study for how physical laws can be used as a baseline for comparing disparate biological systems. In this Review, I explore the diversity of biological puncture and identify key variables that influence these systems. First, I explore recent work on biological puncture in a diversity of organisms, based on their hypothesized objectives: gripping, injection, damage and defence. Variation within each category is discussed, such as the differences between gripping for prey capture, gripping for dispersal of materials or gripping during reproduction. The second half of the Review is focused on specific physical parameters that influence puncture mechanics, such as material properties, stress, energy, speed and the medium within which puncture occurs. I focus on how these parameters have been examined in biology, and how they influence the evolution of biological systems. The ultimate objective of this Review is to outline an initial framework for examining the mechanics and evolution of puncture systems across biology. This framework will not only allow for broad biological comparisons, but also create a baseline for bioinspired design of both tools that puncture efficiently and materials that can resist puncture.

Postcopulatory sexual selection influences baculum evolution in primates and carnivores

The extreme morphological variability of the baculum across mammals is thought to be the result of sexual selection (particularly, high levels of postcopulatory selection). However, the evolutionary trajectory of the mammalian baculum is little studied and evidence for the adaptive function of the baculum has so far been elusive. Here, we use Markov chain Monte Carlo methods implemented in a Bayesian phylogenetic framework to reconstruct baculum evolution across the mammalian class and investigate the rate of baculum length evolution within the primate order. We then test the effects of testes mass (postcopulatory sexual selection), polygamy, seasonal breeding and intromission duration on the baculum in primates and carnivores. The ancestral mammal did not have a baculum, but both ancestral primates and carnivores did. No relationship was found between testes mass and baculum length in either primates or carnivores. Intromission duration correlated with baculum presence over the course of primate evolution, and prolonged intromission predicts significantly longer bacula in extant primates and carnivores. Both polygamous and seasonal breeding systems predict significantly longer bacula in primates. These results suggest the baculum plays an important role in facilitating reproductive strategies in populations with high levels of postcopulatory sexual selection.