Male copulatory behavior

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Here I will present a description of the behavior. The neural and endocrine mechanisms involved are the subjects of a later chapter. Male copulatory behavior is a sequence of stereotyped motor patterns ending in ejaculation, the expulsion of seminal fluid. Some species may make several ejaculations in a row. During the time when I worked with rabbits, our champion once made 17 ejaculations during a 30-minute test. He had received no treatment whatsoever. It was his spontaneous level of sexual activity he had shown for us. Rats are less active and normally become unresponsive to a female after 5-10 ejaculations. They normally require several hours to achieve that. The human is still less active. In fact, most men stop copulatory activity after the first ejaculation. Occasionally, a second or even a third ejaculation may be achieved before abandoning copulatory activity, but that seems to be quite unusual. Although there may be a substantial variability between species with regard to the number of ejaculations that are normally achieved during a sexual encounter, the basic behavior pattern is rather constant. We will, therefore, concentrate on copula-tory behaviors leading up to the first ejaculation. As always, the rat will be used as our model animal. This is not only because I have some kind of emotional attachment to rats, but also because the rat is by far the best known animal. The principles that have been derived from studies of male rats are applicable to most other species, so there is no waste of time and effort to focus on rats.

The male rat's copulatory behavior starts with a mount. A sexually experienced male almost always mounts the female from behind. He places his forequarters on the female's back and presses his abdominal/preputial region against her rump. This physical contact usually activates a rhythmic anterior-posterior movement of the male's pelvic region, copulatory thrusting. The motor pattern of pelvic thrusting has been carefully described. The frequency of the anterior-posterior movement is around 18 Hz and the duration of thrusting is about 400 ms (Beyer et al., 1982; Moralí and Beyer, 1992). It appears that the generation of this highly stereotyped movement is independent of testicular hormones. Male rats will continue to display exactly the same thrusting pattern as long after castration as they will continue to mount (Beyer et al., 1981). The stimulus triggering pelvic thrusting is unknown, but there are some data suggesting that thrusting is a tactile reflex activated by mechanical stimulation of the preputial region (Contreras and Ágmo, 1993). This is a region surrounding the base of the penis. Anesthesia of the preputial region makes it difficult for the male to initiate pelvic thrusting. Tactile stimulation of other parts of the perineal and inguinal regions may also activate thrusting (Maillard and Edwards, 1990), but this is less clear. Nevertheless, it is quite remarkable that we still ignore the stimulus control of pelvic thrusting, the basic element in male rat copulatory behavior.

Mounting is associated with enhanced intracavernous pressure. Telemetric recordings have shown an increase of around 35 mmHg during mounting (Giuliano et al., 1994). This rather modest increase clearly suggests that penile erection is far from complete, but it should be sufficient for making the penis protrude from the preputial sheath. The duration of the pressure increase was found to be around 0.5 seconds, which means that erection lasts longer than the mount itself. It is, in fact, relatively common to see a moderately erect penis during the male's dismount. A more recent study offered quantitatively different results (Bernabé et al., 1999). In this study, peak intracavernous pressure during mount was reported to be almost 300 mmHg and the duration of enhanced pressure was over 3 seconds.

Some mounts will end in penile insertion. If mounting is associated with erection and appropriate activity in the penile striated muscles, then it is possible for the penis to penetrate into the vagina. Intracavernous pressure is always larger during vaginal penetration, called an intromission among the rat professionals, than during mount. It reaches a level of about 60mmHg according to old data (Giuliano et al., 1994), suggesting that erection is more complete. However, this is not enough for a successful intromission. The male also needs to contract the ischiocavernosus muscles in order to achieve the necessary forward orientation and elevation of the penis. Extirpation of these muscles virtually eliminates a male's capacity for intromission (Sachs, 1982) and electromyographic recordings in copula have established that enhanced activity in the ischiocavernosus precedes vaginal penetration (Holmes et al., 1991). A more recent study of intracavernous pressure (Bernabé et al., 1999) reported that peak pressure during intromission was above 700mmHg and the duration of the pressure increase was, just as in the case of a mount, around 3 seconds. The mean systemic blood pressure in a rat is around 130mmHg. An intracav-ernous pressure above that cannot be caused by vascular mechanisms alone. This means that the pressures recorded during both mount and intromission in the Bernabé et al. (1999) study must be caused by some additional mechanism. It was proposed that contractions of the ischiocavernosus muscles contribute to the high intracavernous pressure. This probably means that these muscles are important not only for the orientation of the penis, but also that they enhance penile rigidity. Therefore, they contribute to successful intromission in two ways.

Behaviorally, a vaginal penetration starts with the male in the same position he assumes in any mount and pelvic thrusting is initiated. Suddenly the male makes a strong forward thrust, which immediately is followed by rapid backwards withdrawal. Pelvic thrusting has a shorter duration when a mount ends in vaginal penetration. This is due to a rapid cessation of thrusting immediately upon penile insertion. Intravaginal thrusting never occurs in rats during intromission and the time the penis remains in the vagina is very short, a couple of milliseconds. The stimulus control of vaginal penetration is not better known than that of pelvic thrusting, although it is generally believed that tactile stimulation from the glans penis is essential. The forward thrust causing vaginal penetration is supposed to be activated when the penis makes contact with the vaginal orifice. This hypothesis is strengthened by the fact that experimental manipulations reducing sensory feedback from the penis, for example section of the dorsal penile nerve or the application of local anesthetics to the penile epithelium, abolish intromission without reducing mounting or affecting the thrusting pattern (Larsson and Sodersten, 1973; Contreras and Ágmo, 1993). One question that has not been solved is how the male can distinguish penile contact with the vaginal orifice from contacts with the surrounding fur. Temperature or texture may be critical. Although not of direct relevance for the rat issue discussed here, I cannot refrain from mentioning that observations from rabbits would suggest that temperature rather than texture is critical for the forward thrust.

Reproductive physiologists now and then want to collect semen and when they want to collect semen from rabbits they employ a hand-held artificial vagina. This object is nothing exotic. It is simply a glass tube about 15 cm long, with open ends. Inside the tube there is a condom with the top fixed to a rubber stopper at the bottom of the tube. The condom's opening is turned inside out around the tube's opening and fixed with a rubber band. Before use, the rubber stopper at the bottom is removed and the tube is filled with water. Then, the semen collector puts a rabbit fur over her right arm, takes the artificial vagina in the right hand and introduces the ensemble in the rabbit's cage. He will immediately mount and the collector can feel the rabbit penis stroking her hand and the artificial vagina with the typical thrusting frequency of rabbits, about 12 Hz (reviewed in Morali and Beyer, 1992). The water temperature is the critical thing in all this process. If it is too cold, the rabbit will never insert his penis into the artificial vagina. If the temperature is around 39°C, he will do it immediately when his penis touches it. During semen collection the texture of the vaginal opening is always kept constant but temperature can accidentally vary. Thus, texture cannot be the factor determining if there will be intromission or not. These extremely informal rabbit data may perhaps not be generalized to rats, but if I were forced to make a guess as to the critical stimulus provoking the forward trust leading to vaginal penetration, I would place my bet on temperature. The glabrous skin around the vaginal opening is certainly warmer than the surrounding fur.

After several intromissions, the rat will ejaculate. The number of intromissions required for activating ejaculation is fairly variable. Rats ejaculating after less than four intromissions are rare and those requiring more than 20 are equally rare. The usual number is between 6 and 10. Ejaculation starts exactly like a mount. Then it is transformed into an intromission, which does not seem to end. In fact, the vaginal penetration coupled with ejaculation may last a second or more. During ejaculation the male appears to embrace the female's anterior back with his forelegs. Rhythmic abdominal contractions are also observed. In contrast to intromission, ejaculation is associated with intravaginal thrusting. The frequency of the intravaginal thrusting is much lower than that of the thrusting preceding penetration (Beyer et al., 1982). Finally, the male opens his arms and slowly rises on his hind legs. At that moment, the female runs away. Following the ejaculation the male will rest for several minutes.

The intracavernous pressure during ejaculation is above that observed during intromission, around 100mmHg according to old data and almost 800mmHg according to more recent data. This very high pressure is probably caused by contraction of another of the striated penile muscles, the bulbospongiosus. In rats, contraction of this muscle is important for the proper placement of semen. Extirpation of the bulbospongiosus muscles leads to reduced fertility (Sachs, 1982).

Before leaving ejaculation, I will just point out that in rats, as in other mammals, this is a double process, one controlled by the autonomous nervous system and the other by the somatic nervous system. The first process consists of the transport of semen from the epididymis through the vas deferens to the urethra and contraction of the accessory sexual glands, notably the seminal vesicles and the prostate. Physiologists consider this as a process under the control of the sympathetic nervous system and call it emission. Then the seminal liquid must be expelled from the urethra, preferably to the outside. This is achieved by contraction of skeletal muscles, evidently under the control of the somatic nervous system, in a process called ejaculation. Because emission is an unobservable process both for rats and humans while ejaculation is quite notorious, we have a tendency to forget about the preceding emission. This tendency is so strong that we, in vulgar language, call the entire process ejaculation. The somatic contractions enhance intraurethral pressure, causing flow of liquid in the direction of least resistance. Since the vasa deferentia void close to the bladder end of the urethra, the principle of least resistance would direct the seminal liquid to the bladder. Therefore, a sphincter muscle, with the rather poetic name of musculus sphincter vesicae, around the bladder opening has to be contracted at the appropriate moment. Failure to contract the sphincter produces a retrograde ejaculation, which sometimes is a problem in men. This, however, is of no concern for us.

The stimulus triggering ejaculation (here used in the vulgar sense) is certainly mechanical stimulation of penile mechanoreceptors. The amount of such stimulation needed for activation of the autonomous reflex of emission and the subsequent somatic reflex of ejaculation is apparently variable between individuals and within an individual from one day to another. The causes of this variability are unknown, as are the exact mechanisms of the two reflexes. A discussion of these issues does not seem to be warranted here.

The basic unit of male rat copulatory behavior is the mount, with or without intromission/ejaculation. If a mount is not leading to an intromission, most males will shortly perform another mount, and perhaps a third and a fourth. Between these mounts the male will do nothing but groom his genitals or pursue the female. Such a group of mounts has been defined as a mount bout, 'a sequence of mounts (one or more) with or without intromission, uninterrupted by any behavior (other than genital autogrooming) that is not oriented toward the female' (Sachs and Barfield, 1970, p. 360). If a mount leads to intromission, it is frequently but not always the last in a bout. Between mount bouts there is a far longer interval than between each mount within a mount bout. The intermount bout interval is independent of whether the last mount in the preceding bout was a mount with or without intromission. This was elegantly shown in a brilliant paper published many years ago (Sachs and Barfield, 1970). That paper is undoubtedly one of the most important papers that ever has been published in the field of sexual behavior, yet it has been cited only 85 times, at least according to the electronic version of the Science Citation Index at the beginning of 2006. What Ben Sachs and Ron Barfield did was surprisingly simple. Rats copulated either with normal females, with whom they could display intromissions as often as they could or would, or with females with a closed vagina. In this latter case, their copulatory behavior was forcibly limited to mounting. Then they compared the intermount bout intervals between these two conditions. There was no difference. What this simple study shows is that the mount is the basic unit of male rat sexual behavior. Intromission only has a consequence as the interruption of an ongoing mount bout. The implication of this is that there is, somewhere in the brain, a kind of 'mount generator' with an oscillating excitability.

The execution of a mount produces a temporary inhibition of the mount generator and, when this inhibition is large enough, mounting stops. One, two, or three mounts in a row might be necessary for arriving at the level of inhibition that stops mounting. An intromission produces a larger inhibition than a mount, normally making one intromission sufficient to stop mounting. However, it occurs now and then that a rat makes an intromission that is rapidly followed by another mount or intromission. As soon as mounting has stopped, the inhibition starts to disappear and within some time (habitually 30-120 seconds) mounting is reinitiated. Ejaculation causes a more profound inhibition of the mount generator, which requires several minutes for recovery.

The concept of a mount generator may seem hopelessly old fashioned, and perhaps it is. Nevertheless, the basic idea, that male rat copulatory behavior is an orderly series of excitation-inhibition processes, coincides with everything we know about the temporal pattern of the behavior. An important contribution to our understanding of copulatory behavior would be to characterize the neural mechanisms responsible for the oscillating excitability. Regrettably, I do not think that it will ever be done. Any practical application of such knowledge seems remote, to say the least, making the subject uninteresting to funding agencies.

Before leaving male sexual behavior, I will briefly mention the procedure normally used for the quantitative description of it. A male and a female are put together in a rather small observation arena and a standard set of behavioral measures is recorded in one way or another. These behavioral measures will be mentioned frequently in this book and it is almost a necessity to know them. They are also part of the basic knowledge you can expect from a reasonably well-cultivated human. First we record the time the male needs to perform the first mount, that is the time that passes between the moment the couple gets together until the first mount. This is the mount latency. Please do not forget that when I talk of a mount it always requires the presence of pelvic thrusting. Then we record the time to the first mount with vaginal penetration, the intromission latency. The time between the first intromission and ejaculation is the ejaculation latency. The duration of the period of inactivity following ejaculation is the post-ejaculatory interval. Normally, the sexual inactivity is considered ended when the male makes his first post-ejaculatory mount with vaginal penetration. Occasionally, the criterion for the end of inactivity is the first post-ejaculatory mount independently of whether it is associated with intromission or not. In addition, the number of mounts without intromission as well as the number with it are counted. It is also common to calculate the proportion of mounts ending in intromission (the number of mounts with intromission divided by the number of mounts with intromission + the number of mounts without intromission). The result is frequently called the intromission ratio, or more vulgarly the hit rate. A more complete description of the male's behavior can be given by calculating the interval between intromissions, that is the ejaculation latency divided by the number of intromissions. Likewise, the copulatory rate may be determined by adding the number of mounts with and without intromission and dividing it by the time from the first mount until ejaculation. Other measures of copulatory behavior have now and then been used. In fact, it is only the scientist's creativity that limits the number of possible descriptors of male copulatory behavior.

How all these measures should be interpreted is a matter of eternal discussion. Some like to imagine that the mount latency somehow represents the intensity of the male's sexual motivation. Others consider that the post-ejaculatory interval is a better indicator of motivation. The number of pre-ejaculatory intromissions may depend on the amount of sensory stimulation required for activation of the reflexes of emission and ejaculation, or on some other factor. The intromission ratio is often interpreted as an indicator of erectile function. If erection is deficient, then few mounts will end in intromission. This is a perfectly logical reasoning, but it is complicated by the fact that a reduced intromission ratio can be a consequence of deficient contraction of the ischiocavernosus muscles. The dramatic consequences of extirpation of these muscles were mentioned above, but I will further reinforce the argument by an additional example. Certain drugs facilitating Y-aminobutyric acid- (GABA) ergic neurotransmission have as a major effect on copulatory behavior a dramatic reduction of the intromission ratio. This reduction is unrelated to deleterious effects on pelvic thrusting. Drug treated animals show a thrusting pattern identical to that of controls (Agmo and Contreras, 1990). On the contrary, electromyographic recordings of activity in the ischiocavernosus muscles showed that they contracted much less in drug treated rats than in controls (Paredes et al., 1993b). At the time, we concluded that the reduced intromission ratio was an immediate result of the deficient ischiocavernosus contraction leading to insufficient elevation of the penis. However, we now know that ischio-cavernosus contraction contributes to intracavernous pressure in a significant way, most likely enhancing penile rigidity. Thus, the deficient contraction of the muscles not only altered the elevation of the penis, a prerequisite for vaginal penetration, but it also affected erection in a deleterious way. The real cause of a diminished intromission ratio could, then, be either deficient elevation of the penis (a direct consequence of drug effects on ischiocavernosus contraction) or a deficient erection (an indirect effect of weakened ischiocavernosus contraction) or a combination of both. It is not easy to figure out how an experiment that offered unequivocal data as to which alternative is the correct one should be designed. The conclusion we can draw from this example is that it is safest to limit our interpretation of the behavioral descriptors to very simple statements of what actually was observed.

I will not contribute to the never-ending discussion of the meaning of the many descriptors of male copulatory behavior. A minimalist point of view is more in agreement with my personal taste. Thus, I prefer to maintain that they are descriptors with an unknown meaning. The effects of experimental manipulations, either behavioral, physiological, pharmacological or genic can be carefully described and these descriptions can provide us with an understanding of the basic mechanisms involved in copulatory behavior without any need for far-fetched interpretations. The point can be illustrated by a straightforward example. An experimental treatment, for example a drug, leads to reduced mount latency. The minimalist interpretation would be that the drug made the animals initiate mounting faster, which is an interpretation strictly tied to the observational data. A more fanciful conclusion would be that the drug enhanced sexual motivation, and that in turn made the males mount faster. This is certainly a possible explanation, but there are many alternatives that could equally well account for the behavioral effect. The rats may mount faster because their general activity was increased, or their perceptual system identified the partner as a receptive female faster after drug treatment, or the sensitivity of the male's abdominal skin was enhanced making tactile stimulation from the female's back more efficient, and so on. None of these explanations refer to any change in motivation, but to changes in general arousal, or in perceptual and sensory systems. The fact that the mount latency was reduced does not tell us anything about the cause of that reduction and we should not make false acclaims of knowing something that we cannot know. However, we could obviously proceed with all kinds of control experiments ruling out all alternative explanations and, in the end, we could be entitled to state that the reduced mount latency was caused by enhanced sexual motivation. In scientific practice, all these control experiments are almost never performed before launching the conclusion they could justify. This practice, as widespread as unfortunate, has led to innumerable myths concerning the effects of many experimental manipulations on male copulatory behavior and associated phenomena, like sexual motivation. The interested reader can find an extensive analysis of one of these myths in a review published some time ago (Paredes and Agmo, 2004).

With this outcry for modesty in our interpretations we will almost leave male copulatory behavior. Before doing so, however, I must add some comments concerning mammalian species other than rats. Otherwise it may be thought that the account of copulatory behavior given in the preceding paragraphs is of limited interest. This is not the case. One or another form of mounting is extremely common among mammalian males and penile insertion associated with mounting is equally common. Although the exact motor patterns coupled to mounting are very stereotyped within a species, they may differ considerably between species, but there is no reason for believing that the neural and behavioral mechanisms are equally diverse. As will be seen in a later chapter, the neural control of copulatory behavior seems to have been highly conserved. The penile structure may vary and the mechanisms of erection and insertion are not necessarily identical among species. Humans and rodents and many other mammals have penile corpora cavernosa, hence a vascular erection, while still others do not have corpora cavernosa in the penis and, consequently, employ other mechanisms for erection. In rats, the ischiocavernosus muscles are important for penile insertion, while these muscles, although present also in the human, do not contribute, in the slightest, to this act. Instead, the partner's hand may be employed, something never happening among rodents. Some species, like mice and men, normally show intravaginal thrusting, while rats do so only at ejaculation. Hamsters display intravaginal thrusting at some intromissions but not at others. Rats and hamsters need several intromissions before ejaculating, while rabbits, mice and men only need one. The rabbit ejaculates within milliseconds of penile insertion, without any trace of intravaginal thrusting, while mice make a few thrusts and ejaculate within a couple of seconds. Men are usually somewhat slower than mice and perform more extensive intravaginal thrusting. Despite all these apparent differences, the neural and behavioral mechanisms associated with intromission and ejaculation seem to be remarkably similar in different species. I insisted on rats not only because I like them but also because we know much more about rat copu-latory behavior than we know about any other species, as mentioned before. Moreover, a large number of experiments employing rats will be presented in the next few chapters. Most of them would not be adequately understood without some basic knowledge of rat copulatory behavior. I guarantee that we will discover how useful these rat data indeed are for the understanding of human sexual behavior during the last few chapters of this book. Nevertheless, if someone would like to insist on an analysis of copulatory behavior in a wider selection of species I can suggest some first class reading. There are two brilliant reviews that come to mind, one dealing with most male mammals from which there are usable data (Dewsbury, 1972) and one concentrating on primates (Dewsbury and Pierce, 1989). Both reviews are rather old, but any better analysis of male copulatory behavior from the viewpoint of comparative psychology has not been published since.

Now I will turn to females. Not surprisingly, the discussion will have a slight positive bias toward rats.

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