The female orgasm and the homology concept in evolutionary biology

The definition of homology and its application to reproductive structures, external genitalia, and the physiology of sexual pleasure has a tortuous history. While nowadays there is a consensus on the developmental homology of genital and reproductive systems, there is no agreement on the physiological translation, or the evolutionary origination and roles, of these structural correspondences and their divergent histories. This paper analyzes the impact of evolutionary perspectives on the homology concept as applied to the female orgasm, and their consequences for the biological and social understanding of female sexuality and reproduction. After a survey of the history of pre-evolutionary biomedical views on sexual difference and sexual pleasure, we examine how the concept of sexual homology was shaped in the new phylogenetic framework of the late 19th century. We then analyse the debates on the anatomical locus of female pleasure at the crossroads of theories of sexual evolution and new scientific discourses in psychoanalysis and sex studies. Moving back to evolutionary biology, we explore the consequences of neglecting homology in adaptive explanations of the female orgasm. The last two sections investigate the role played by different articulations of the homology concept in evolutionary developmental explanations of the origin and evolution of the female orgasm. These include the role of sexual, developmental homology in the byproduct hypothesis, and a more recent hypothesis where a phylogenetic, physiological concept of homology is used to account for the origination of the female orgasm. We conclude with a brief discussion on the social implications for the understanding of female pleasure derived from these different homology frameworks.

Characteristics of menstrual cycles with or without intercourse in women with no known subfertility

STUDY QUESTION

Does sexual intercourse enhance the cycle fecundability in women without known subfertility?

SUMMARY ANSWER

Sexual intercourse (regardless of timing during the cycle) was associated with cycle characteristics suggesting higher fecundability, including longer luteal phase, less premenstrual spotting and more than 2 days of cervical fluid with estrogen-stimulated qualities.

WHAT IS KNOWN ALREADY

Human females are spontaneous ovulators, experiencing an LH surge and ovulation cyclically, independent of copulation. Natural conception requires intercourse to occur during the fertile window of a woman's menstrual cycle, i.e. the 6-day interval ending on the day of ovulation. However, most women with normal fecundity do not ovulate on Day 14, thus the timing of the hypothetical fertile window varies within and between women. This variability is influenced by age and parity and other known or unknown elements. While the impact of sexual intercourse around the time of implantation on the probability of achieving a pregnancy has been discussed by some researchers, there are limited data regarding how sexual intercourse may influence ovulation occurrence and menstrual cycle characteristics in humans.

STUDY DESIGN SIZE DURATION

This study is a pooled analysis of three cohorts of women, enrolled at Creighton Model FertilityCare centers in the USA and Canada: 'Creighton Model MultiCenter Fecundability Study' (CMFS: retrospective cohort, 1990-1996), 'Time to Pregnancy in Normal Fertility' (TTP: randomized trial, 2003-2006) and 'Creighton Model Effectiveness, Intentions, and Behaviors Assessment' (CEIBA: prospective cohort, 2009-2013). We evaluated cycle phase lengths, bleeding and cervical mucus patterns and estimated the fertile window in 2564 cycles of 530 women, followed for up to 1 year.

PARTICIPANTS/MATERIALS SETTING METHODS

Participants were US or Canadian women aged 18-40 and not pregnant, who were heterosexually active, without known subfertility and not taking exogenous hormones. Most of the women were intending to avoid pregnancy at the start of follow-up. Women recorded daily vaginal bleeding, mucus discharge and sexual intercourse using a standardized protocol and recording system for up to 1 year, yielding 2564 cycles available for analysis. The peak day of mucus discharge (generally the last day of cervical fluid with estrogen-stimulated qualities of being clear, stretchy or slippery) was used to identify the estimated day of ovulation, which we considered the last day of the follicular phase in ovulatory cycles. We used linear mixed models to assess continuous cycle parameters including cycle, menses and cycle phase lengths, and generalized linear models using Poisson regression with robust variance to assess dichotomous outcomes such as ovulatory function, short luteal phases and presence or absence of follicular or luteal bleeding. Cycles were stratified by the presence or absence of any sexual intercourse, while adjusting for women's parity, age, recent oral contraceptive use and breast feeding.

MAIN RESULTS AND THE ROLE OF CHANCE

Most women were <30 years of age (75.5%; median 27, interquartile range 24-29), non-Hispanic white (88.1%), with high socioeconomic indicators and nulliparous (70.9%). Cycles with no sexual intercourse compared to cycles with at least 1 day of sexual intercourse were shorter (29.1 days (95% CI 27.6, 30.7) versus 30.1 days (95% CI 28.7, 31.4)), had shorter luteal phases (10.8 days (95% CI 10.2, 11.5) versus 11.4 days (95% CI 10.9, 12.0)), had a higher probability of luteal phase deficiency (<10 days; adjusted probability ratio (PR) 1.31 (95% CI 1.00, 1.71)), had a higher probability of 2 days of premenstrual spotting (adjusted PR 2.15 (95% CI 1.09, 4.24)) and a higher probability of having two or fewer days of peak-type (estrogenic) cervical fluid (adjusted PR 1.49 (95% CI 1.03, 2.15)).

LIMITATIONS REASONS FOR CAUTION

Our study participants were geographically dispersed but relatively homogeneous in regard to race, ethnicity, income and educational levels, and all had male partners, which may limit the generalizability of the findings. We cannot exclude the possibility of undetected subfertility or related gynecologic disorders among some of the women, such as undetected endometriosis or polycystic ovary syndrome, which would impact the generalizability of our findings. Acute illness or stressful events might have reduced the likelihood of any intercourse during a cycle, while also altering cycle characteristics. Some cycles in the no intercourse group may have actually had undocumented intercourse or other sexual activity, but this would bias our results toward the null. The Creighton Model FertilityCare System (CrM) discourages use of barrier methods, so we believe that most instances of intercourse involved exposure to semen; however, condoms may have been used in some cycles. Our dataset lacks any information about the occurrence of female orgasm, precluding our ability to evaluate the independent or combined impact of female orgasm on cycle characteristics.

WIDER IMPLICATIONS OF THE FINDINGS

Sexual activity may change reproductive hormonal patterns, and/or levels of reproductive hormones may influence the likelihood of sexual activity. Future work may help with understanding the extent to which exposure to seminal fluid, and/or female orgasm and/or timing of intercourse could impact menstrual cycle function. In theory, large data sets from women using menstrual and fertility tracking apps could be informative if women can be appropriately incentivized to record intercourse completely. It is also of interest to understand how cycle characteristics may differ in women with gynecological problems or subfertility.

STUDY FUNDING/COMPETING INTERESTS

Funding for the research on the three cohorts analyzed in this study was provided by the Robert Wood Johnson Foundation #029258 (Creighton Model MultiCenter Fecundability Study), the Eunice Kennedy Shriver National Institute of Child Health and Human Development 1K23 HD0147901-01A1 (Time to Pregnancy in Normal Fertility) and the Office of Family Planning, Office of Population Affairs, Health and Human Services 1FPRPA006035 (Creighton Model Effectiveness, Intentions, and Behaviors Assessment). The authors declare that they have no conflict of interest.

TRIAL REGISTRATION NUMBER

N/A.

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.

Female Genital Variation Far Exceeds that of Male Genitalia: A Review of Comparative Anatomy of Clitoris and the Female Lower Reproductive Tract in Theria

A review of the literature on the anatomy of the lower female genital tract in therian mammals reveals, contrary to the general perception, a large amount of inter-specific variation. Variation in female external genitalia is anatomically more radical than that in the male genitalia. It includes the absence of whole anatomical units, like the cervix in many Xenarthra, or the absence of the urogenital sinus (UGS), as well as the complete spatial separation of the external clitoral parts from the genital canal (either vagina or UGS). A preliminary phylogenetic analysis shows two patterns. Some morphs are unique to early branching clades, like the absence of the cervix, while others arose multiple times independently, like the flattening out or loss of the UGS, or the extreme elongation of the clitoris. Based on available information, the ancestral eutherian configuration of the external female genitalia included a cervix, a single vaginal segment, a tubular UGS, and an unperforated clitoris close to the entrance of the genital canal. The evidence for either bilobed or unitary glandes clitorides is ambivalent. Despite the wealth of information available, many gaps in knowledge remain and will require a community-wide effort to come to a more robust model of female genital evolutionary patterns.

Masturbation in the Animal Kingdom

Masturbation is one of the most common sexual behaviors in humans. It is also a phylogenetically widespread trait of various other mammalian and some non-mammalian species. Several hypotheses have been proposed aiming to explain the function of masturbation in primates and other species. These were mainly based on observations of nonhuman primates such as rhesus macaques or bonobos and rodents such as African ground squirrels. Based on these observations various scholars suggested that masturbation improves ejaculate quality, decreases the risk of contracting sexually transmitted infections or is merely a by-product of sexual arousal and thus an alternate outlet to copulation. While these theories may explain some facets of masturbation in some species, they do not explain why masturbation is so widespread and has developed in various species as well as our hominid ancestors. Moreover, the research on which these theories are based is scarce and heavily focused on male masturbation, while female masturbation remains largely unexplored. This sex difference may be responsible for the one-sided theorizing that attributes a specific biological benefit to masturbation. We propose that the widespread prevalence of masturbation in the animal kingdom may be better explained by viewing masturbation as a primarily self-reinforcing behavior that promotes pleasure both in human and in nonhuman species.

Sexual selection and the ascent of women: Mate choice research since Darwin

Darwin's theory of sexual selection fundamentally changed how we think about sex and evolution. The struggle over mating and fertilization is a powerful driver of diversification within and among species. Contemporaries dismissed Darwin's conjecture of a "taste for the beautiful" as favoring particular mates over others, but there is now overwhelming evidence for a primary role of both male and female mate choice in sexual selection. Darwin's misogyny precluded much analysis of the "taste"; an increasing focus on mate choice mechanisms before, during, and after mating reveals that these often evolve in response to selection pressures that have little to do with sexual selection on chosen traits. Where traits and preferences do coevolve, they can do so whether fitness effects on choosers are positive, neutral, or negative. The spectrum of selection on traits and preferences, and how traits and preferences respond to social effects, determine how sexual selection and mate choice influence broader-scale processes like reproductive isolation and population responses to environmental change.

The effect of baculum shape and mating behavior on mating-induced prolactin release in female house mice

Male genitalia are subject to rapid divergent evolution, and sexual selection is believed to be responsible for this pattern of evolutionary divergence. Genital stimulation during copulation is an essential feature of sexual reproduction. In mammals, the male intromittent genitalia induces a cascade of physiological and neurological changes in females that promote pregnancy. Previous studies of the house mouse have shown that the shape of the baculum (penis bone) influences male reproductive success and responds to experimentally imposed variation in sexual selection. Here, we test the hypothesis that the baculum is subject to sexual selection due to a stimulatory function during copulation. We selected male and female house mice (Mus musculus domesticus) from families with breeding values at the extremes of baculum shape and performed two series of experimental matings following which we examined the concentration of prolactin in the blood of females either 15 (“early”) or 75 (“late”) min after ejaculation. Our results provide evidence of a mating-induced release of prolactin in the female house mouse early after ejaculation, the level of which is dependent on an interaction between the shape of the baculum and male sexual behavior. Our data thereby provide novel insight into the mechanism(s) of sexual selection acting on the mammalian baculum.

G-spot: Fact or Fiction?: A Systematic Review

Introduction

The G-spot, a putative erogenous area in the anterior vaginal wall, is a widely accepted concept in the mainstream media, but controversial in medical literature.

Aim

Review of the scientific data concerning the existence, location, and size of the G-spot.

Methods

Search on Pubmed, Pubmed Central, Cochrane, clinicaltrials.gov and Google Scholar from inception to November 2020 of studies on G-spot's existence, location and nature. Surveys, clinical, physiological, imaging, histological and anatomic studies were included.

Main Outcome Measure

Existence, location, and nature of the G-spot.

Results

In total, 31 eligible studies were identified: 6 surveys, 5 clinical, 1 neurophysiological, 9 imaging, 8 histological/anatomical, and 2 combined clinical and histological. Most women (62.9%) reported having a G-spot and it was identified in most clinical studies (55.4% of women); in 2 studies it was not identified in any women. Imaging studies had contradictory results in terms of its existence and nature. Some showed a descending of the anterior vaginal wall, that led to the concept of clitourethrovaginal complex. In anatomic studies, one author could systematically identify the G-spot, while another group did not find it. Studies on innervation of the vaginal walls did not systematically identify an area with richer innervation.

Conclusion

The different studies did systematically agree on the existence of the G-spot. Among the studies in which it was considered to exist, there was no agreement on its location, size, or nature. The existence of this structure remains unproved.

Vieira-Baptista P, Lima-Silva J, Preti M, et al. G-spot: Fact or Fiction?: A Systematic Review. Sex Med 2021;9:100435.

Sporadic anovulation is not an important determinant of becoming pregnant and time to pregnancy among eumenorrheic women: A simulation study

BACKGROUND

Attaining pregnancy is conditional upon a series of complex processes, including adequately timed intercourse, ovulation, fertilisation, and implantation. Anovulation is a first-line treatment target for couples with difficulty conceiving and is frequently examined in studies of fecundability.

OBJECTIVES

To identify whether sporadic anovulation is an important determinant of cumulative pregnancy rates and time to pregnancy among fertile women with regular menstrual cycles.

METHODS

We simulated cumulative pregnancy rates and time to pregnancy for 12 consecutive menstrual cycles among 100 000 women based on data-driven probabilities of implantation, fertilisation, ovulation, and intercourse occurring in the fertile window. We assumed anovulation probabilities of 1%, 8%, or 14.5% and intercourse averaging once per week, every other day, or daily. The model incorporated reductions in implantation and fertilisation rates for successive cycles of non-pregnancy.

RESULTS

After 12 cycles, a reduction in the per cycle incidence of anovulation from 14.5% to 1% resulted in a 4.0% higher cumulative pregnancy rate (86.7% vs 90.7%) and similar time to pregnancy (1-cycle median difference). In contrast, increasing mean unscheduled sexual intercourse frequency from weekly to every other day was associated with a 5-cycle median reduction in time to pregnancy (weekly: 7 cycles; every other day or daily: 2 cycles) and a 28.9% increase in the cumulative pregnancy rate (weekly: 59.9%, every other day: 88.8%; daily: 91.6%).

CONCLUSIONS

In presumed fertile women with regular menstrual cycles, routine investigation of anovulation may not be an informative outcome in studies of fecundability, and routine testing to ensure ovulation and treatment of anovulation are unlikely to be medically necessary. While biomarkers or cervical fluid may help time intercourse to the fertile window, time to pregnancy can also be improved through increasing the frequency of unscheduled intercourse. These findings need corroboration in large preconception time to pregnancy studies.

Baculum shape and paternity success in house mice: evidence for genital coevolution

Sexual selection is believed to be responsible for the rapid divergence of male genitalia, which is a widely observed phenomenon across different taxa. Among mammals, the stimulatory role of male genitalia and female 'sensory perception' has been suggested to explain these evolutionary patterns. Recent research on house mice has shown that baculum (penis bone) shape can respond to experimentally imposed sexual selection. Here, we explore the adaptive value of baculum shape by performing two experiments that examine the effects of male and female genitalia on male reproductive success. Thus, we selected house mice (Mus musculus domesticus) from families characterized by extremes in baculum shape (relative width) and examined paternity success in both non-competitive (monogamous) and competitive (polyandrous) contexts. Our analyses revealed that the relative baculum shape of competing males influenced competitive paternity success, but that this effect was dependent on the breeding value for baculum shape of the family from which females were derived. Our data provide novel insight into the potential mechanisms underlying the evolution of the house mouse baculum and lend support to the stimulatory hypothesis for the coevolution of male and female genitalia. This article is part of the theme issue 'Fifty years of sperm competition'.

An experimental test of the ovulatory homolog model of female orgasm

The ovulatory homolog model of female orgasm posits that the neuro-endocrine mechanisms underlying female orgasm evolved from and are homologous to the mechanisms mediating copulation-induced ovulation in some mammals. This model predicts that pharmacological agents that affect human orgasm, such as fluoxetine, should also affect ovulation in animals with copulation-induced ovulation, such as rabbits. We tested this prediction by treating rabbits with daily doses of fluoxetine for 2 wk and found that fluoxetine treatment reduces the number of ovulations postcopulation by 30%. In a second experiment we tested whether this result was mediated by an effect on the brain or via peripheral serotonin functions. We treated animals with fluoxetine and induced ovulation with a single injection of human chorionic gonadotropin. In this experiment ovulation rate was nominally reduced by only 8%, which is statistically not significant. We conclude that the effect of fluoxetine on copulation-induced ovulation rate supports the ovulatory homolog model of female orgasm, suggesting that female orgasm has very deep evolutionary roots among the early eutherian mammals.

Spandrels and trait delimitation: No such thing as "architectural constraint"

Forty years ago, Gould and Lewontin used the metaphor of a building's "spandrels" to highlight that organismal traits could be the inevitable consequence of organismal construction, with no alternative configurations possible. Because adaptation by natural selection requires variation, regarding a trait incapable of variation as an adaptation could be a serious error. Gould and Lewontin's exhortation spurred biologists' efforts to investigate biases and limitations in development in their studies of adaptation, a major methodological advance. But in terms of the metaphor itself, over the past 40 years there are virtually no examples of "spandrels" in the primary literature. Moreover, multiple serious confusions in the metaphor have been identified and clarified, for example, that the "spandrels" of San Marco are pendentives, and pendentives are perfect examples of adaptation. I look back over the sparse empirical fruits of the "spandrels" metaphor, and ask what the clarifications of the past 40 years mean for biological theory and practice. I conclude that if there is anything to be rescued from the clarified spandrels metaphor, it is not "constraint" at all. Instead, it is the still-unresolved issue of trait delimitation, which is how to parse organisms into subsets that are tractable and biologically appropriate for study.

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.

What is orgasm? A model of sexual trance and climax via rhythmic entrainment

Orgasm is one of the most intense pleasures attainable to an organism, yet its underlying mechanisms remain poorly understood. On the basis of existing literatures, this article introduces a novel mechanistic model of sexual stimulation and orgasm. In doing so, it characterizes the neurophenomenology of sexual trance and climax, describes parallels in dynamics between orgasms and seizures, speculates on possible evolutionary origins of sex differences in orgasmic responding, and proposes avenues for future experimentation. Here, a model is introduced wherein sexual stimulation induces entrainment of coupling mechanical and neuronal oscillatory systems, thus creating synchronized functional networks within which multiple positive feedback processes intersect synergistically to contribute to sexual experience. These processes generate states of deepening sensory absorption and trance, potentially culminating in climax if critical thresholds are surpassed. The centrality of rhythmic stimulation (and its modulation by salience) for surpassing these thresholds suggests ways in which differential orgasmic responding between individuals-or with different partners-may serve as a mechanism for ensuring adaptive mate choice. Because the production of rhythmic stimulation combines honest indicators of fitness with cues relating to potential for investment, differential orgasmic response may serve to influence the probability of continued sexual encounters with specific mates.