Intromission Duration Research Articles

Baculum shape complexity correlates to metrics of post-copulatory sexual selection in Musteloidea.

The penis bone, or baculum, is present in many orders of mammals, although its function is still relatively unknown, mainly due to the challenges with studying the baculum in vivo. Suggested functions include increasing vaginal friction, prolonging intromissionand inducing ovulation. Since it is difficult to study baculum function directly, functional morphology can give important insights. Shape complexity techniques, in particular, are likely to offer a useful metric of baculum morphology, especially since finding homologous landmarks on such a structure is challenging. This study focuses on measuring baculum shape complexity in the Musteloidea-a large superfamily spanning a range of body sizes with well-developed, qualitatively diverse bacula. We compared two shape complexity metrics-alpha shapes and ariaDNE and conducted analyses over a range of six different coefficients, or bandwidths, in 32 species of Musteloidea. Overall, we found that shape complexity, especially at the baculum distal tip, is associated with intromission duration using both metrics. These complexities can include hooks, bifurcationsand other additional projections. In addition, alpha shapes complexity was also associated with relative testes mass. These results suggest that post-copulatory mechanisms of sexual selection are probably driving the evolution of more complex-shaped bacula tips in Musteloidea and are likely to be especially involved in increasing intromission duration during copulation.

Short-term water deprivation has widespread effects on mating behaviour in a harvestman

Individual variation in resource acquisition prior to mating can influence the expression of sexually selected traits and mating dynamics. One important resource that has the potential to affect mating is water; however, little is known about whether variation in individual hydration status impacts mating behaviour. Here, we investigate the effects of short-term water deprivation on mating behaviour in the harvestman Leiobunum vittatum, commonly known as daddy longlegs. These harvestmen follow stereotyped stages of mating during which males attempt to clasp females in a ‘mating embrace’, then attempt to copulate, and may guard females after copulation; females are able to resist male attempts at each stage to end the interaction. During these interactions, males pass fluid nuptial gifts to females for their consumption. We tested how short-term water deprivation affects mating dynamics in harvestmen using a fully factorial, single-choice mating trials between males and females that were deprived or not deprived of water for 4 h. Water deprivation affected behaviour at multiple stages of mating. Females deprived of water were less likely to resist male advances but also less likely to be embraced by males. Most dramatically, the duration of intromission was greatly reduced in trials where both sexes were water-deprived. Together, these results suggest that even short periods of water deprivation impact mating-related behaviour of both sexes and may be an important but neglected factor in sexual selection.

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

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

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.

Do Males Bite Females' Antennae to Coerce Copulation or to Continue Mate Guarding in Oiceoptoma subrufum (Coleoptera: Silphinae)?

In several species, males frequently immobilize females during copulation. In some species, female immobilization enables males to copulate with unwilling females, while in others, female immobilization prolongs postcopulatory guarding. Male carrion beetles often bite and pull hard on one of the female's antennae during copulatory mounting. Previous descriptive studies have hypothesized that antenna biting is important for postcopulatory guarding in Silphinae. Here, we observed the mating behavior of Oiceoptoma subrufum, to understand the roles of antenna-biting in the initiation and termination of copulation. We compared the success and duration of intromission and pre- and postcopulatory mounting duration between males that did and did not bite female antennae during copulatory mounting. The success and duration of intromission and precopulatory mounting duration were unaffected by antenna biting. However, antenna-biting males mounted females for longer after intromission compared to non-biting males. These results indicate that antenna biting contributes to postcopulatory guarding behavior, not coercive copulation, in O. subrufum.

Seeking a Sex‐Specific Coolidge Effect in a Simultaneous Hermaphrodite

AbstractIn polygamous mating systems, a capability to discriminate against familiar mates may be beneficial to both sexes. Polyandrous females, for instance, may enhance the odds of finding sires with optimal genetic compatibility or high genetic quality by mating with multiple different males; polygynous males, in addition, may more efficiently invest their limited ejaculates across multiple (rather than single) females. The Coolidge effect facilitates this kind of mate discrimination, as sexual motivation declines across consecutive copulations with a familiar partner but resurrects with a novel mate. In simultaneous hermaphrodites, we expect the Coolidge effect to show sex role‐dependent patterns and vary with previous sex‐specific mating activity. Using the promiscuous hermaphroditic sea slug Chelidonura sandrana, we investigated (1) whether sexual motivation indeed declines when repeatedly exposed to familiar partners, (2) whether the Coolidge effect occurs in a sex‐specific manner and (3) whether ejaculation is strategic with respect to partner familiarity. We found neither mating latency, nor penis intromission duration, mating propensity or the frequency of sex role alternations to vary significantly with treatments. Furthermore, slugs did not donate larger ejaculates to novel than to familiar partners. Partner novelty thus elicited no detectable response in sexual motivation or mating effort in C. sandrana. We suggest that the sea slugs' promiscuous mating system in often large mating aggregations makes mate discrimination based on novelty obsolete in comparison with more relevant criteria such as partner body size or mating history.

Female Fitness Optimum at Intermediate Mating Rates under Traumatic Mating

Traumatic mating behaviors often bear signatures of sexual conflict and are then typically considered a male strategy to circumvent female choice mechanisms. In an extravagant mating ritual, the hermaphroditic sea slug Siphopteron quadrispinosum pierces the integument of their mating partners with a syringe-like penile stylet that injects prostate fluids. Traumatic injection is followed by the insertion of a spiny penis into the partner’s gonopore to transfer sperm. Despite traumatic mating, field mating rates exceed those required for female fertilization insurance, possibly because costs imposed on females are balanced by direct or indirect benefits of multiple sperm receipt. To test this idea, we exposed animals to a relevant range of mating opportunity regimes and assessed the effects on mating behavior and proxies of female fitness. We find penis intromission duration to decrease with mating rates, and a female fecundity maximum at intermediate mating rates. The latter finding indicates that benefits beyond fertilization insurance can make higher mating rates also beneficial from a female perspective in this traumatically mating species.

The Chelidonura tsurugensis–sandrana (Gastropoda: Cephalaspidea) species complex: do reproductive decisions maintain colour polymorphism?

Cephalaspid gastropod molluscs are increasingly being utilized in studies of tradeoffs associated with conflicts in simultaneous hermaphroditic reproduction. Chelidonura sandrana exhibits a wide range of colour forms, from black to yellow-spotted or marked with white. One of these colour morphs was described as Chelidonura tsurugensis Baba & Abe, 1959, while others have been accepted as natural variation within C. sandrana Rudman, 1973. Previous work has mostly assumed (1) that colour forms represent the same species and (2) that reproductive behaviour does not differ among colour forms. Mating behaviour between three co-occurring colour forms (simplified to ‘black’, ‘yellow’ and ‘white’) of the Chelidonura tsurugensis–sandrana species complex was investigated to test the above assumptions. Individuals of each colour form were crossed with individuals of the same form, and then with each of the two other colour forms, and with additional species as controls. Mating frequency, duration and an individual’s role of sperm donor or receiver were recorded. The results show that there was no evidence for positive assortative mating with respect to colour form. Instead, at least one colour form was shown to mate significantly more often, and for longer, with individuals of colour forms other than its own. There was no evidence for trading of intromissions or intromission duration among colour forms. Although all three colour forms readily mate and behave like a single biological species, the questionable identity of C. tsurugensis prevents synonymization of C. sandrana with this species. Conservatively, we suggest that until a systematic revision of Chelidonura is carried out, the application of the name C. tsurugensis should be restricted to Japan, and elsewhere the yellow-spotted colour form should be referred to as C. sandrana.

Sexual Dimorphism and Reproduction in the Grass Lizard Takydromus septentrionalis

Sexual dimorphism and reproduction in a population of Takydromus septentrionalis on Xiushan Island of Zhoushan Islands group were investigated during the breeding seasons of 1989 – 1992. Most males and females, approximately two years after hatchling, reached sexual maturity at 54 and 57 mm SVL, respectively. Adult male and female T. septentrionalis were similar in SVL but quite different in the head size characteristics, males having larger heads than females of similar SVL except near the size of sexual maturity. Mating success in males was apparently not related to increased SVL. Males can copulate with females in several hours before or after oviposition. The duration of intromission averaged 227 min. Females can lay 1 – 3 clutches with 2 – 6 eggs each per season. Clutch size or clutch mass was significantly correlated with SVL and was not correlated with the clutch size. Relative clutch weight (RCW) averaged 0.149. Oviposition frequency averaged 16.7 days.

Testing for adaptive explanations of bimodal genital insertion duration in the stalk-eyed seed bug

Sexual selection leads to the evolution of various traits that increase male fitness under male–male competition including sperm competition. In many taxa, bimodal distributions of copulation and/or intromission duration are known to be the result of adaptation to sperm competition risk avoidance. However, the stalk-eyed seed bug, Chauliops fallax Scott (Heteroptera: Malcidae), exhibits completely different mating behaviour to other known examples. The male mounts the female and then inserts his aedeagus repeatedly and intermittently until the termination of copulation. Insertion duration exhibited a bimodal distribution with two peaks: a short insertion within 6 min which occurred one to three times without insemination, followed by long insertions of approximately 30 min with repeated insemination. To examine the adaptive significance of the two types of genital insertion, we investigated whether the amount of sperm transferred, female remating receptivity, number of eggs laid, hatchability and female longevity differed between the two insertion types. Sperm transfer mainly occurred during long insertions but very little occurred during short insertions. Similarly, long insertions induced mated females to resist subsequent mating attempts of other males but short insertions did not. Short insertions did not affect the number of eggs laid, hatchability or female longevity. Although our results showed that C. fallax males transferred sperm to females and inhibited female remating with long insertions, we currently cannot explain the adaptive function of short insertions without insemination.

Citalopram (antidepressant) administration causes sexual dysfunction in male mice through RF-amide related peptide in the dorsomedial hypothalamus

Citalopram is the most potent selective serotonin reuptake inhibitor (SSRI) which is used as an antidepressant but causes sexual dysfunction. Whether citalopram induced sexual dysfunction is a result of gonadotropin-releasing hormone (GnRH), kisspeptin or RF-amide related peptide (RFRP) alteration is unknown. In this study, we tested mice for sexual behavior after vehicle (0.9% NaCl) and citalopram treatment (5 mg/kg) daily for 1 day (acute) and 21 or 28 days (chronic). Effects of acute and chronic treatments on neuronal numbers and mRNA expression of GnRH, kisspeptin and RFRP were measured. In addition, RFRP fiber projections to preoptic (POA)-GnRH neurons were analyzed using double-label immunohistochemistry. The expression of 14 different serotonin receptor types mRNA was examined in immunostained laser dissected single RFRP neurons in the dorsomedial hypothalamus (DMH), however only 11 receptors types were identified. Acute citalopram treatment did not affect sexual behavior, whereas, the total duration of intromission was reduced with chronic treatment. There was no effect in the expression of kisspeptin (neuronal numbers and mRNA) in the anteroventral periventricular nucleus and the arcuate nucleus and expression of GnRH (neuronal numbers and mRNA) in the POA after citalopram treatment. However, RFRP neuronal numbers in the DMH and fiber projections to the POA were significantly increased after chronic citalopram treatment, which suggests citalopram induced inhibition of sexual behavior involves the modulation of RFRP through serotonin receptors in the DMH.

Duration of sexual intercourse related to satisfaction: survey of Japanese married couples.

Inappropriate intromission time causes sexual problems for couples, and therefore it is important for the couple to set treatment targets. We investigated appropriate intromission times by conducting a questionnaire survey of the interval from initiation of insertion to just before ejaculation in Japanese married couples. A questionnaire survey of 300 married couples was conducted by mail. The estimated mean intromission times were 14.5 min (median: 10 min) for male subjects and 13.6 min (median: 10 min) for female subjects. The mean desired intromission time for female subjects was 15.7 min (median: 15 min). Regarding the difference between the actual and desired intromission times, the desired time was longer, the same, and shorter for 43.0%, 38.7%, and 18.3% of all the female subjects. It seems that female subjects may consider a wide variety of intravaginal insertion times to be desirable. Accordingly, married couples need to improve communication regarding the desired duration of intromission and other related issues.

Male postcopulatory reproductive success in the beetle, Diaprepes abbreviatus

For most species that have been studied, considerable variation exists in male postcopulatory reproductive success, including female sperm use, reproductive investment and remating patterns. We investigated postcopulatory sexual selection in the beetle Diaprepes abbreviatus (L.) (Coleoptera: Curculionidae) by examining the correlation of two measures of a male's success (proportion of eggs fertilized (P2) and number of eggs that a female laid after mating with him) with male body size, copulatory behaviour and copula duration. We allowed each female to mate with an irradiated sterile male and a normal male. Relative duration of intromission (i.e. the difference in the intromission durations of the two males) and relative stroking rate, but not the absolute duration and stroking rate, were positively correlated with P2. The number of eggs that a female laid was negatively related to the thrusting rate of her mate. Irradiated males mated for a shorter duration and thrusted at a lower rate than normal males. Irradiated males also fertilized a lower proportion of eggs than normal males, and females were more likely to remate after mating with an irradiated male than after mating with a normal male. These findings suggest that behavioural traits may influence male postcopulatory reproductive success. Furthermore, since variation in male stroking rate and intromission duration are associated with variation in male fertilization success, these traits may have evolved through postcopulatory sexual selection.

Copulation behavior within one-male groups of wild Rhinopithecus roxellana in the Qinling Mountains of China

This study was conducted using focal animal sampling on the west ridge group of the Sichuan snub-nosed monkey (Rhinopithecus roxellana) located in the Zhouzhi National Nature Reserve on the north slope of the Qinling Mountains, from 8 July 2003 to 24 May 2004. The difference in the average frequency of copulations for each focal male for each month was significant (F=3.068, P=0.016, one-way ANOVA test) with the majority of copulations occurring between September and November. Duration of intromission ranged from 2 to 39 s, with a mean of 16.0+/-0.4 s. Females initiated 627 courtship attempts (96.2%), while males only initiated 3.8%. Both adult females (72.8%) and sub-adult females (27.2%) were involved in sexual interference acts. Females who gave birth in 2004 performed more sexual interference acts than would be expected by chance in the reproductive period of 2003 (X2=13.73>X2(0.005,2), df=2, P<0.005). Male response to female interference was equally divided into "solicitor mounting" and "interrupter mounting". The resident males of one-male units were not observed to mount both the solicitor and the interrupter or mount neither following female solicitation interruptions. Three post-conception copulations were also observed in this study. These results suggest a skewed sexual competition, with multiple females competing for a single male, which was shown by courtship attempts and female interference.

Do “sperm trading” simultaneous hermaphrodites always trade sperm?

Sperm trading can be a mechanism to solve the conflict over sex roles in hermaphrodites with copulation, sperm competition, and sperm digestion. If present, sperm donation depends on sperm receipt, resulting in conditional reciprocal inseminations. Conditional reciprocity can involve three traded commodities: penis intromissions on a yes-or-no basis, intromission durations (indicating ejaculate size), or sperm transfer. If present, animals that refuse to donate (cheaters) should be deserted by their partner or receive smaller ejaculates. We tested whether any of these commodities is traded in the simultaneously hermaphroditic sea slug, Chelidonura sandrana. Matings involve a stereotypic single sex role alternation. As a result, reciprocity was more frequent than expected under random mating, supporting trading of penis intromissions. Contrary to the predictions under trading of ejaculate sizes, intromission durations were not balanced between mating partners. To test trading of sperm transfer itself, we interrupted the sperm groove that transports sperm into the penis during copulation. Treated animals (experimental cheaters) could still copulate normally but could not transfer sperm. When paired to a cheater, sham-treated control animals did not respond to the absence of sperm receipt, neither by desertion nor by shortening intromission. We thus reject trading of insemination durations and sperm transfer in C. sandrana. Although trading of intromissions may be present, we discuss why reciprocity in this species may also be a by-product of the mutual willingness of both partners to donate as well as receive sperm, and not an indicator of sperm trading. Copyright 2005.

Sexually dimorphic activation of galanin neurones in the ferret's dorsomedial preoptic area/anterior hypothalamus after mating.

Male ferrets in breeding condition possess three times as many galanin-immunoreactive (IR) neurones as oestrous females in the sexually dimorphic dorsomedial preoptic area/anterior hypothalamus (dmPOA/AH). Using Fos-IR as a marker of activation, we investigated whether mating with intromission differentially activates this sexually dimorphic group of galanin-IR neurones in male and female ferrets. Male ferrets that intromitted had a significantly greater percentage of galanin-IR neurones in the dmPOA/AH that were colabelled with nuclear Fos-IR than oestrous females that received an intromission. Intromissive stimulation augmented Fos-IR in an equal percentage of galanin-IR neurones in both sexes in the medial amygdala (MA) and bed nucleus of the stria terminalis (BNST). Peripheral anosmia induced by bilateral occlusion of males' nares did not reduce the mating-induced activation of galanin-IR neurones in the dmPOA/AH, and there was a significant correlation among individual males between intromission duration and the percentage of dmPOA/AH galanin-IR neurones colabelled with Fos-IR. Exposure of castrated, testosterone propionate-treated male ferrets to either soiled bedding or to volatile odours from oestrous females failed to induce nuclear Fos-IR in galanin-IR neurones located in the dmPOA/AH, BNST or MA, suggesting that the mating-induced activation of galanin-IR forebrain neurones in male ferrets depends more on genital-somatosensory than on olfactory inputs. The observed sex dimorphism in the mating-induced activation of galanin-IR neurones in the dmPOA/AH raises the possibility that these neurones perform a mating-dependent function that occurs only in males.

Constant light and blinding effects on reproduction of male South Indian gerbils.

The effects of exposure to constant light (CL) and blinding on male reproductive behavioral physiology of South Indian gerbils (Tatera indica cuvieri) were assessed. Exposure to CL diminished reproductive efficiency of males with a reduction in the proportion of ejaculating males. This is further evidenced by increase in the number and duration of intromissions and intromission latency. Concomitantly, reproductive organ weight and epididymal sperm count were also reduced. However, in weanlings CL did not induce changes either in their maturational process or their reproductive organs' weight. Blinding of adult reproductively inactive males resulted in a considerable proportion of them exhibiting sexual activity, which is reflected in the seminal vesicle weight and epididymal sperm count. Blinded weanlings showed earlier testicular descent and higher seminal vesicle weight. These studies reveal that blinding (constant darkness) has a stimulatory effect whereas CL is inhibitory in the reproduction of the tropical rodent Tatera indica cuvieri. J. Exp. Zool. 289:59-65, 2001.

Effects of paced mating on c-fos gene expression in the female rat brain.

When estrous female rats control or pace (P) their sexual contacts with males, several neuroendocrine and behavioral responses to mating occur that are not observed or are greatly attenuated after nonpaced mating. The present study examined whether the distribution and amount of FOS immunoreactivity (FOS-IR) induced in brain by mating would be altered in females receiving paced rather than nonpaced mating stimulation. In the first experiment, females received 5 or 15 intromissions during paced mating tests (5P and 15P), 5 or 15 intromissions during nonpaced mating tests (5NP and 15NP), 15 mounts-without-intromission (MO) or remained in their homecages (HC). Selective increases 1 h after paced mating stimulation were observed in the posterodorsal medial amygdala (MePD), where significantly more FOS-IR cells were present in the 5P and 15P groups than in the respective NP groups. The 5P, 5NP and 15NP had significantly more FOS-IR than the HC, MO, and 5NP groups, and the 5P group had levels of FOS-IR which were equivalent to that seen in the 15NP group. In the posteromedial portion of the bed nucleus of the stria terminalis (BNSTpm) and the ventrolateral portion of the ventromedial nucleus of the hypothalamus (VMHvl), paced mating induced significantly greater numbers of FOS-IR cells than did either MO or HC treatments; increases induced by nonpaced mating were not statistically greater than HC controls. No differences between groups were seen in the medial preoptic area (mPOA). In the second experiment, experimentally lengthening the interintromission interval (III) as well as increasing the intromission duration to mimic the characteristics of paced mating, resulted in significant increases in FOS-IR in the MePD but not in the other three brain regions. These results demonstrate that paced mating is more effective in inducing c-fos expression than nonpaced mating, and that the MePD is particularly sensitive to differing characteristics of the mating stimuli received.

Sexual behavior of adult male chimpanzees of the Mahale Mountains national park, Tanzania

This study, based on 687 hr of focal observations, aims to describe overall patterns of the sexual behavior of the adult male chimpanzees of the Mahale Mountains, to compare the results with previous reports, and to explain the variations between studies. Genital inspection of cycling females by adult males was eight times as frequent as that of lactating females, and twice as frequent as that of pregnant females. Inspection of the genitals of cycling females increased dramatically 7–10 days before the onset of maximal swelling and gradually decreased as the day of ovulation approached. Adult males likely obtained information on the attractivity of females by inspecting their genitals. Mating was usually initiated by male courtship and followed by pelvic thrusts in a dorsoventral posture, performed on, rather than above, the ground, which continued for 7 s. on average, and was typically followed by female squeaking and darting from the male, or by the male grooming the female. Higher-ranking males mated with females in the peri-ovulatory period more frequently than did lower-ranking males. In particular, two alpha males mated with such females more often than did any other adult males. A male who interfered with a mating pair was dominant over the mating male in other agonistic contexts. The duration of intromission was correlated with neither dominance rank nor age. However, when an adult male declined in rank from alpha in 1991 to third in 1992, he showed a significantly shorter duration of intromission. This indicates that for a particular male, the alpha rank guaranteed longer duration of intromission. Allies of alpha males tended to mate with peri-ovulatory females more frequently than expected from their low dominance ranks. The number of mating partners was not correlated with male dominance rank, but was sometimes negatively correlated with male age. Females were significantly more likely to emit a copulatory squeak when mating with younger, rather than older, adult males. Male dominance rank and the rate of female copulatory squeaking were not correlated. Weaning infants regularly interfered with their mothers' mating. Occasionally, unrelated adolescent males and rarely females pushed themselves in between copulating adults. Female choice was indicated when they performed a “penis erection check” or took the initiative in courtship, or on the other hand showed strong reluctance to mate with particular males. Young adult males more often received erection checks than did prime males, while none of the three old adult males did. Courtship initiated by estrous females was not directed to two of the oldest males, the exception of which was the alpha male. The oldest males, except for the alpha, were consistently avoided by many estrous females, both young and old. In response to female reluctance, males behaved violently, however, this was not effective, because other more dominant males came to rescue the female. Neither courtship nor mating was seen between mature sons and their mothers, nor between brothers and sisters.

Male Gray Short-Tailed Opossums (Monodelphis Domestica) Receive Penile Intromissions When Treated with Estrogen and Progesterone in Adulthood

Following treatment with estradiol and progesterone, gonadectomized male as well as female gray opossums received penile intromissions from intact stimulus males. Intromission was possible in male gray opossums because, like marsupials of both sexes, they possess a single cloaca-like anogenital opening. All subjects that allowed intromission showed anogenital dragging just prior to intromission. While intromission latency was similar in tests involving male and female subjects, total intromission duration was longer in tests involving male subjects than in tests involving female subjects, and sex locks were seen only in tests involving female subjects. These findings are discussed with respect to the potential usefulness of gray opossums for studying the effects of peripheral vs central factors on the display of sex differences in behavior.