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Secondary sexual characteristics, and the extent to which they are expressed, can convey information about the signaller. The blue scrotum and red penis of male vervet monkeys (Chlorocebus pygerythrus) make them a good species in which to examine inter- and intramale variation in signal expression. We quantified genital hue and luminance of male vervets at Lake Nabugabo, Uganda from standardized photos of male genitalia taken in May to June 2016, January to March 2019, and April to June 2019 to examine how dominance rank, fecal androgens (fARMs), fecal glucocorticoids (fGCMs), and parasitism related to achromatic (i.e., luminance) and chromatic (i.e., hue) aspects of scrotal and penile coloration, as well as how genital color related to sociosexual behaviors. We examined 182 photoshoots, 214 fecal samples for hormone analyses, and 152 for parasite analyses. Linear models indicate that genital color is linked to male dominance rank; high-ranking males had a more luminant (i.e., brighter) scrotum and a redder penis. Within males, color characteristics remained relatively stable over the short-term and changed moderately over the long-term. The direction of change was inconsistent for all color characteristics except scrotal luminance, which increased in all males over the long-term. Males with a darker penis received more mating presentations, while higher-ranking males received more mating refusals than low-ranking males, suggesting that females pay attention to penile color. We did not find support for any parasite or hormone mediation of color, and while there was a correlation between fGCM and fARMs, this was positive rather than negative as predicted by the stress-linked immunocompetence handicap hypothesis. Overall, our results indicate that the production of genital color may serve as an intra- and/or intersexual signal of male dominance rank and age in vervets., (© 2024 The Author(s). American Journal of Primatology published by Wiley Periodicals LLC.)

Asymmetric genitalia and lateralized mating behaviors occur in several taxa, yet whether asymmetric morphology in one sex correlates or coevolves with lateralized mating behavior in the other sex remains largely unexplored. While lateralized mating behaviors are taxonomically widespread, among mammals they are only known in the harbor porpoise (Phocoena phocoena). Males attempt copulation by approaching a female exclusively on her left side. To understand if this unusual lateralized behavior may have coevolved with genital morphology, we quantified the shape of female and male harbor porpoise reproductive tracts using 2D geometric morphometrics and 3D models of the vaginal lumen and inflated distal penis. We found that the vaginas varied individually in shape and that the vaginas demonstrated both significant directional and fluctuating asymmetry. This asymmetry resulted from complex 3D spirals and vaginal folds with deep recesses, which may curtail the depth or direction of penile penetration and/or semen movement. The asymmetric shapes of the vaginal lumen and penis tip were both left-canted with similar angular bends that mirrored one another and correspond with the left lateral mating approach. We suggest that the reproductive anatomy of both sexes and their lateral mating behavior coevolved.

The cave-dwelling psocid tribe Sensitibillini ( Afrotrogla , Neotrogla and Sensitibilla ) is of special morphological and evolutionary interest because of its possession of reversed copulatory organs: i.e. females of Afrotrogla and Neotrogla have a penis-like organ. The female penis structure is highly variable among taxa, as is the case of the male penis in animals with normal copulatory organs. Here, we present the first molecular phylogeny of Sensitibillini and analyse the evolutionary pattern of their genitalia. Afrotrogla and Neotrogla did not form a monophyletic clade, and their female penis structures are significantly different, suggesting two independent origins of the female penis within Sensitibillini. In Neotrogla , the species that has a simple female penis is embedded among species that have an elaborate penis, and detailed structures of the female penis elaborations are in exact agreement among species, suggesting a secondary simplification of the female penis. A correlated evolutionary pattern between male and female genitalia was also detected. This coevolution of genitalia may suggest that sexual conflict or cryptic 'male' choice drove the diversity of the female penis, as is the case of male penile diversity in animals with conventional genitalia., (© 2018 The Author(s).)

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., (© 2016 The Authors.)

Most barnacles are simultaneous hermaphrodites, but dwarf males are also found attached to hermaphrodites in several species. This biologically rare phenomenon of the coexistence of males and hermaphrodites is termed androdioecy. To test whether the hermaphrodite and male sexes are fixed or plastic in the androdioecious pedunculate barnacle Octolasmis warwickii, we conducted a series of 22-day-long transplanting experiments to evaluate the effects of a) the original site (attached to the conspecific vs. attached directly to the substrate) and b) the transplanting site (conspecific-attached vs. substrate-attached). Penis length (as an index of male function), the presence or absence of egg mass (female function), and growth rate were investigated. As with natural dwarf males, individuals that were transplanted onto conspecifics developed longer penises than did those that were transplanted onto the substrate. The original site of attachment also affected penis length. However, no significant effects of the original site or the transplanting were detected in egg-laying activities, as only one experimental individual laid eggs. Individuals that were transplanted onto conspecifics grew less than those that were attached to the substrate. These results indicate that individual sexual expression is affected by the environment in O. warwickii., (© 2016 Marine Biological Laboratory.)

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

Coevolution of male and female genitalia in waterfowl has been hypothesized to occur through sexual conflict. This hypothesis raises questions about the functional morphology of the waterfowl penis and the mechanics of copulation in waterfowl, which are poorly understood. We used high-speed video of phallus eversion and histology to describe for the first time the functional morphology of the avian penis. Eversion of the 20 cm muscovy duck penis is explosive, taking an average of 0.36 s, and achieving a maximum velocity of 1.6 m s(-1). The collagen matrix of the penis is very thin and not arranged in an axial-orthogonal array, resulting in a penis that is flexible when erect. To test the hypothesis that female genital novelties make intromission difficult during forced copulations, we investigated penile eversion into glass tubes that presented different mechanical challenges to eversion. Eversion occurred successfully in a straight tube and a counterclockwise spiral tube that matched the chirality of the waterfowl penis, but eversion was significantly less successful into glass tubes with a clockwise spiral or a 135 degrees bend, which mimicked female vaginal geometry. Our results support the hypothesis that duck vaginal complexity functions to exclude the penis during forced copulations, and coevolved with the waterfowl penis via antagonistic sexual conflict.

The expression of masculine sexual behavior (MSB) in male hamsters is optimally stimulated by aromatizable androgens like androstenedione (AD) and testosterone (T), while the non-aromatizable androgen, 5alpha-dihydrotestosterone (DHT), exerting potent androgenic peripheral effects, only in high doses maintains MSB after castration. No data exist on the ability of these androgens to restore long intromissions after castration. In this study, AD, T, and DHT were administered to four-week gonadectomized, sexually experienced male hamsters, for three weeks, in doses of 25 microg/day or up to 1000 microg/day to compare their potency in restoring MSB, penile size, and penile spines growth. Plasma levels of these steroids and the metabolites estrone and estradiol, were determined at the end of the treatment period. Gonadectomy completely suppressed MSB and induced a regression of penile spines. AD was more potent than T in restoring MSB, ejaculatory behavior being displayed by most castrated subjects with a lower dose of AD (50 microg/day) than of T (300 microg/day), and long intromissions being shown by all AD-treated castrated hamsters but only by 20% of T-treated ones, when doses of 1000 microg/day were given. DHT did not stimulate any copulatory response. The three androgens, even at the lowest dose, partially stimulated penis and penile epithelium growth, DHT showing the highest potency. Treatment of castrated hamsters with AD (50 microg/day), restored steroid levels to similar values as those of intact animals. These results show that AD and T restored MSB even with a partial stimulation of penile spines growth, AD being more potent than T. In contrast, DHT did not restore MSB in the hamster in spite of its peripheral androgenic potency.

In most mammalian species, reduced androgen availability is associated with marked reductions in male sexuality; conversely, androgen replacement in castrated males restores sex behavior within a few weeks. Testosterone (T) pulse duration, amplitude, frequency, and inter-pulse interval may be as important as total amount of hormone in determining target tissue responsiveness. We remain ignorant of the number and duration of daily T pulses necessary and sufficient to sustain male mating behavior. An in-dwelling infusion system was employed to vary T-pulse frequencies and durations. Daily 4 h infusions of aqueous T (100 microg/0.064 ml) and twice daily 4 h pulses of T (each 50 microg/0.064 ml) were sufficient to maintain ejaculatory behavior of sexually experienced castrated hamsters for 11 weeks post-castration; castrated hamsters infused with vehicle ceased to display the ejaculatory pattern 3 weeks after gonadectomy. Circulating T concentrations of hormone-infused hamsters declined markedly 7 h after the termination of each infusion. These results establish that male sex behavior can be sustained with infusions of relatively low T concentrations for 4 h/day and suggests that the basal concentrations of T sustained by the gonad during inter-pulse intervals may not be necessary for maintenance of sex behavior. 4 h T infusions were sufficient to maintain penile and seminal vesicles weights, but not ventral prostate weights or flank gland dimensions; the threshold for maintaining male sex behavior is lower than that for some androgen-dependent peripheral structures. Development of effective androgen replacement regimens that sustain sex behavior in castrated animals may be useful in the design of androgen replacement therapy for hypogonadal men.

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., (Copyright 2006 Wiley-Liss, Inc.)

A comparative evolutionary psychological perspective predicts that species that recurrently faced similar adaptive problems may have evolved similar psychological mechanisms to solve these problems. Sperm competition provides an arena in which to assess the heuristic value of such a comparative evolutionary perspective. The sperm competition that results from female infidelity and polyandry presents a similar class of adaptive problems for individuals across many species. The authors first describe mechanisms of sperm competition in insects and in birds. They suggest that the adaptive problems and evolved solutions in these species provide insight into human anatomy, physiology, psychology, and behavior. The authors then review recent theoretical and empirical arguments for the existence of sperm competition in humans and discuss proposed adaptations in humans that have analogs in insects or birds. The authors conclude by highlighting the heuristic value of a comparative evolutionary psychological approach in this field., (Copyright 2006 APA, all rights reserved.)

Surprisingly few birds have penises, but among those that do, the Argentine lake duck (Oxyura vittata) tops the bill - the penis of this small stifftail duck from South America is shaped like a corkscrew and, at almost half a metre long, is the largest of any bird measured so far. Factors responsible for the evolution of this remarkable organ could include runaway selection, whereby drakes with longer penises gain dominance and copulate with more females, or preference by females for drakes with longer and more decorated penises.

Male and female gray short-tailed opossums were gonadectomized (GDX), or treated with the estrogen receptor antagonist tamoxifen citrate (TX), or corn oil (OIL) (control) during the 5th postnatal week, a time period equivalent to the 3rd postnatal week in rats and associated with high levels of circulating gonadal hormones and neural aromatase activity in this marsupial species. In adulthood following gonadectomy (for animals not previously gonadectomized) and replacement therapy with estradiol or testosterone, GDX males showed less male-typical scent marking and had shorter phalluses than OIL and TX males. Following replacement therapy with estradiol, GDX females were more likely to fight with and less likely to mate with stimulus males than TX females; OIL females were intermediate in these measures. Along with previous findings, these results suggest that gonadal hormones act over an extended postnatal period to organize sexually dimorphic behavior and morphology in male gray opossums and may have some effect on the organization of aggressive behavior in females of this species., (Copyright 2001 Academic Press.)

Correlations between mating system and various aspects of genital anatomy suggest a strong influence of sexual selection on genital morphology. We test the generality of the influence by examining whether primate taxa in which there might be enhanced sexual selection (those with multi-male mating systems) possess, as expected, relatively more spinous penises than do taxa with other mating systems. As most prosimians, but few anthropoids (monkeys and apes), possess penile spines, and because the predominant mating systems of the two taxa differ, taxonomic constraints are taken into account. Sexual selection apparently does not act on penile spines in the same manner as on other aspects of genital anatomy: spinosity is not greatest in multi-male taxa of either prosimians or anthropoids. In some taxa, spines might stimulate reproductive readiness and synchrony in situations in which the sexes live apart and do not have other means of communicating reproductive state (dispersed social systems and 'stolen' extra-pair copulations), but problems exist with the hypothesis, as they do with the idea that spines are involved with scent marking. It seems that either penile spines have several functions, or penile spinosity in primates, and other orders, remains to be explained.

This paper deals with the process of mating among captive aye-ayes (Daubentonia madagascariensis). Data on sexual behaviour of 3 wild-caught aye-ayes were collected over a period of 3 years (1986-1989). The group of 2 adult females and a young male was housed at the Vincennes Zoo, Paris. The seasonal poly-oestrous sexual cycle was found to be marked by four clear phases of the females' vulval tumescence and coincided with sexual interest shown by both sexes. The external genitalia of the females are described, as are marking behaviour and sexual behaviour patterns before, during and after copulation. The behaviour and sexual cycles of these 2 females were compared with records from other lemurs.

Male rats were selected for showing or not showing lordosis in response to manual stimulation. They were subsequently tested for mounting behavior with receptive females and for lordosis behavior in response to manual stimulation and to male mounting. Males showing lordosis as intacts displayed this behavior more readily following castration or castration and treatment with estradiol benzoate or testosterone propionate than males which did not show lordosis before castration. No group differences in mounting behavior could be detected under any of the endocrine conditions studied. It is suggested that the neural mechanisms mediated mounting and lordosis are dissociated and that individual differences in the occurrence of lordosis in male rats are due to differences in neural sensitivity to estrogen.

In the first experiment gonadectomized male rats were injected daily with various combinations of vehicle (V), estradiol benzoate (E), dihydrotestosterone (D) and/or the anti-androgen cyproterone acetate (CA). The eight treatment combinations consisted of V, E, D, CA, E + D, E + CA, D + CA and E + D + CA. Only those males treated with both E and D were found to display ejaculations. Concurrent treatment with CA completely blocked ejaculatory behavior and it significantly reduced both mount and intromission frequencies. Examination of peripheral androgen target tissues indicated that following stimulation with D, CA effectively reduced seminal vesicle and penile weights, and penile lengths, but it did not reduce penile spines. In the second experiment gonadectomized female rats were treated with two daily injections of E, E + CA or E + the anti-estrogen CI-628. A second set of gonadectomized females received three daily injections of testosterone (T), T + CA or T + CI-628. On the day after completion of these injections all females received progesterone and were tested for sexual receptivity three hours later. Both E and T treated females were found to display the lordotic posture in response to mounting stimulation while both CI-628 and CA were found to block this behavior. In E treated females, CI-628 and CA were also found to reduce uterine weight. Thus, CA was shown to have anti-estrogenic as well as anti-androgenic properties. These results were discussed in terms of the aromatization and 5alpha reduction theories of testosterone action in sexual behavior.