There are more data on preference for individuals of the same sex among rams. The first description of a same-sex preference in rams was based on a total of eight subjects, four of which copulated with ewes with the intensity that rams usually show, while the other four showed little interest. All animals were then tested in an enclosure where a sexually receptive ewe and a ram were tethered to opposite sides. It turned out that the four copulating rams approached the ewes much more than the rams, while the reverse was the case for the four rams that did not copulate with the ewes in the preceding test. It was concluded that rams displaying low or no sexual activity with ewes do so because they prefer rams (Zenchak et al., 1981).
All the subjects in this study had been raised in all male groups. An interesting observation is that the ram-preferring rams had engaged in significantly more sexual activities with other rams than those preferring ewes (Zenchak and Anderson, 1980). This observation may be more important than it might appear to be at this moment.
Several later studies have confirmed that some rams exclusively mount other rams even when receptive ewes are available. Data from a total of 584 rams show that the proportion exclusively mounting other males was 9.5%. About 12% did not show any sexual behavior at all, while 55.6% exclusively mounted ewes. The remaining 22% copulated both with rams and ewes (Roselli et al., 2004b). It is not surprising that several attempts to find the cause of the preference for mounting other males rather than females have been made. One quite evident possible cause is hormonal alterations. In fact, it has been reported that rams exclusively mounting other rams have lower plasma concentrations of estradiol, estrone and testosterone (Resko et al., 1996). The differences do not seem to be particularly reliable, since a subsequent study failed to replicate these observations. Males exclusively mounting other males were no different from males exclusively mounting females (Pinckard et al., 2000). In view of these results, it seems very unlikely that some alteration in gonadal hormones is the cause for the habit of mounting other rams rather than ewes. Another finding is a lower number of estrogen receptors in the amygdala of rams mounting other rams. The amount of estrogen receptors in areas that might be of some importance for sexual behavior, like the medial preop-tic area and hypothalamic regions was similar in male-mounting and in female-mounting rams. Nevertheless, it was suggested that the lower number of estrogen receptors in the amygdala might cause these rams to mount males and not females (Perkins et al., 1995). This suggestion is bizarre in many ways. The amygdala has only a marginal influence on copulatory activity in rodent males, as far as can be judged from lesion studies (see Hull et al., 2002). The effects of lesions are usually limited to a temporary enhancement of mount latencies and some prolongation of ejaculation latency. Supposing that estrogen receptors in the amygdala somehow modify nervous activity, these modifications could hardly be more dramatic than those produced by destruction of the tissue.
There are more reasons than those exposed above for not believing that the amygdala controls copulatory behavior and that estrogen receptors within it could influence preference for one sex over another. When observing the behavior of rams mounting other rams, it is usually found that they perform a larger number of mounts per test than those mounting females (e.g. Pinckard et al., 2000). The reason is probably that the rams mounting females perform a larger number of ejaculations than those mounting males. Please remember that each ejaculation is also followed by a period of inactivity in rams. Anyway, there is nothing to suggest that the sexual behavior is less intense in male-mounting than in female-mounting rams. Assuming that the amygdala indeed is of some importance for copulatory behavior, then any functionally relevant alteration of the activity of amygdala neurons should manifest itself in a change in the intensity of this behavior. This is not the case. Furthermore, the many manipulations of the amygdala that have been performed in male rodents have never given the slightest indication of changes in sexual preferences. Finally, there are data showing that estrogens act in the preoptic area when stimulating sexual behaviors in rams (Parrott and Baldwin, 1984). There is a long way from the preoptic area to the amygdala. The possibility that alterations in the amount of estrogen receptors in the amygdala should cause a ram to mount other rams instead of ewes is remote indeed.
Another explanation that has been put forward for the habit of mounting rams instead of ewes is a reduced activity of aromatase in the preoptic area (Resko et al., 1996). This observation was made in the rams that were also found to have reduced serum concentrations of testosterone, estrone and estradiol, a finding that was not replicated in later studies, as mentioned. Perhaps the reduced activity of aromatase had some relationship to the reduced availability of substrate, or perhaps the observation will turn out as ephemeral as the other data reported in the same paper. However, for the moment, we can imagine that the observation is replicable. It makes some sense, because there is good evidence telling us that sheep belong to those mammals in which aromatization, as well as 5a-reduction of testosterone, is necessary for the activation of sexual behaviors (e.g. Parrott, 1978; Crichton et al., 1991). Thus, a reduced activity of a crucial enzyme at a brain site crucial for male sexual behaviors may reasonably affect those behaviors. Another question is whether it also may affect the sex towards which those behaviors are directed.
There is not any foundation for believing that altered hormonal activity in the preoptic area of an adult animal would change its preference for a particular sex. All kinds of manipulations, like lesion, electrical stimulation, hormone implantations, or implantations of hormone antagonists, have been performed in the pre-optic area of male rats (see Chapter 6 for a summary of some of the relevant studies). Sexual behaviors may be reduced or disappear altogether, or they may be enhanced above the level shown by unmanipulated rats. However, in no case have these manipulations led to a preference for sexual interaction with other males or to a more intense approach of males than females. What can be imagined, though, is that the lower aromatase activity found in adulthood was present also during fetal development and brain differentiation. In rams, the sexual differentiation of the brain occurs well before birth. A deficient aromatase activity during that period could, eventually, lead to a lack of brain differentiation in the male direction. Below I will mention rat data showing that this may be the case. However, when pregnant ewes were treated with an aromatase inhibitor during most of the critical period of pregnancy, and their male offspring were tested for sexual preferences, it turned out that the incidence of same-sex preference was not different from that of controls (Roselli et al., 2002). The dose of the aromatase inhibitor employed in this study reduced the activity of the enzyme by 85% (Roselli et al., 2003). This reduction was much larger than that observed in the adult rams studied earlier. Thus, there is no support for the notion that reduced estrogen formation during fetal development is the cause of same-sex preferences in adult rams.
In a last effort to find some convincing cause for same-sex preferences in rams, brains were cut and a small region called the ovine sexually dimorphic nucleus of the medial preoptic area-anterior hypothalamus was described. The size of this nucleus, expressed either as length or volume, was larger in male rams exclusively mounting females than in rams exclusively mounting males. The latter had a size of this nucleus similar to females. Similar differences were found with regard to the number of cells (Roselli et al., 2004a). These data contrast to another study, where cell sizes in several brain regions were determined in rams displaying a 'normal' level of sexual activity with ewes, in low-performing rams, and in rams exclusively mounting other rams. A comparison of cell sizes with habitual statistical procedures failed to detect any group difference. However, discriminant analysis revealed that low-performing subjects differed from 'normal' subjects with regard to cell sizes in the medial amygdala, preoptic area, bed nucleus of the stria terminalis and the ventro-medial nucleus of the hypothalamus. There was no difference between males mounting other males and 'normal' males (Alexander et al., 2001b). Furthermore, the expression of fos in the preoptic area and bed nucleus of the stria terminalis in response to exposure to a receptive female or another male does not differ between rams mounting females and rams mounting males (Alexander et al., 2001a). In view of the conflicting data reported, no conclusion can be drawn. In any case, and as mentioned in the preceding paragraph, there is no reason to believe that small differences in the cell numbers of the preoptic area can account for the differences in choice of sex of the partner. If this were the case, then small lesions of the preop-tic area should modify partner preference. Such an effect of incomplete preoptic lesion has never been reported and, in view of the many studies in which such lesions have been made, it appears extremely unlikely that it exists.
This review of the search for a cause for the habit of some rams exclusively to mount other rams should have convinced us that there is no convincing explanation for that habit as yet. Hormone concentrations in blood are not different from rams exclusively mounting females, the report of reduced number of estrogen receptors in the amygdala needs to be replicated and a credible rationale for attributing the difference in sexual partner preference to this potential fact needs to be proposed, the aromatase deficiency hypothesis has been disproved by experimental data, and the preoptic cell size/count differences are unreliable and, if confirmed, unable to explain the choice of sexual partner. I would like to suggest that the search for a cause undertaken so far has followed a most unfortunate path. Rather than searching for observable differences in the endocrine, enzymatic and nervous systems, a simple behavioral explanation can be proposed. It can be subjected to and founded upon several kinds of experimental tests in living animals, rather than being based on post hoc descriptions like cell counts in dead brains or estimations of enzyme activities in brain homogenates.
Rams mounting other rams now and then achieve ejaculation. So far as I can understand, ejaculation is associated with anal penetration. I deduce this from a paper where rams mounting other rams achieved a mean of 1.9 ejaculations during three tests of 30 minutes each. For comparison, I mention that rams copulating with ewes achieved a mean of 14.5 ejaculations during similar tests. In that paper we are told that the reason for the rather low number of ejaculations in the male-preferring animals was that '. . . the angle and anatomy of male-oriented rams' preferred sexual partner, the teaser ram, was not conducive to intromission, thus precluding successful ejaculatory attempts ...' (Pinckard et al., 2000, p. 1951). This is extremely interesting and important. Interesting, because it is one of the very rare examples of sodomy in non-human mammals. Important, because it shows that the ram mounting other rams now and then receives the same reward and reinforcement as he could have achieved by mounting and intromitting a female. Those having read the discussion of sexual reward and reinforcement in Chapter 7 will immediately remember that there is abundant data demonstrating that intromission/ejaculation can reinforce learning and produce a state of positive affect, reward. There is no reason to imagine that a ram is so perceptive and has such an exquisite taste that he distinguishes ejaculation in the rectum of a male from ejaculation in a female's vagina. These two events can undoubtedly be considered as equally reinforcing and rewarding. From that point of view, the sex of the partner is uninteresting.
The achievement of ejaculation is less likely when copulating with a ram than when copulating with an ewe. If I abuse the data from the Pinckard et al. (2000) paper to calculate the probability of ejaculation upon mounting, I find that it is about 0.34 for the rams copulating with females. The corresponding value for the rams copulating with other rams is 0.04. From the comment about the difficulty of intromitting the teaser ram in this particular study, it can perhaps be concluded that this is an unusually low figure but, even if we make room for this potential error, we need to accept that the probability of achieving intromission/ejaculation is far lower for rams mounting other rams than for those mounting females. The differential likelihood of reinforcement should make the female preferred over the male. This is exactly what happens in about 55% of rams. Only about 10% persists in the less favorable behavior of copulating exclusively with the male. As mentioned long ago, 22% copulates with both, but presumably they copulate much more frequently with females than with males. We also need to pay some attention to the fact that even the rams considered as exclusively mounting males in fact sire litters almost as frequently as female-preferring rams do (Stellflug et al., 2006). Their reproductive capacity does not seem to suffer much from their engagement in same-sex activities during the test designed for determining sexual preference. This shows that the exclusive mounting of other males is situation-specific, and not an unmodifiable behavioral trait. It could be argued that the difference between male-mounting and female-mounting rams is simply a difference in probability of partner choice. The likelihood of choosing to copulate with a female is lower in what is called male-mounting rams than in female-mounting rams. In some situations, like the sexual preference test, this reduced probability takes the appearance of an exclusive choice of same-sex partner, while in other situations a female may be chosen, as evidenced by the frequent fathering of offspring.
The proposal that the ram-mounting males may have learned to prefer other males may be perceived as far-fetched by those who like to explain preferences for individuals of the same sex as an inborn characteristic. I will dedicate a few paragraphs to argue in favor of it. First of all, we need to know that all ram-mounting males known have been raised similarly. At puberty, they were separated from females. From that moment on they spent their time together with other rams. There is nothing special in that. Most sheep breeders use this procedure. The young rams like to mount each other. In fact, mounting is a rather frequent activity. It is not impossible that some of the many mounts performed are associated with anal penetration/ejaculation. Whenever this happens, the mounter is reinforced and rewarded. If this occurs a sufficient number of times, the stimuli present at the moment will become associated to the positive affect produced by ejaculation through classical conditioning. They will become conditioned sexual incentives in the way explained in Chapter 7. The most salient stimuli in the moment are certainly those emitted by the ram which is mounted. Thus, these stimuli will acquire the capacity to activate approach behaviors on future occasions. Moreover, the motor pattern leading to intromission/ejaculation will be reinforced as in any operant learning procedure and their likelihood of repetition will gradually increase. In the preference test, usually performed after months or years of cohabitation with and mounting and intromitting/ejaculating in other rams, the stimuli emitted by the teaser ram will have sexual incentive properties and the subject will approach them, and then mount. At the same time, the ewe will emit unconditioned sexual incentives. The rams exclusively mounting the teaser ram will be those where conditioning to male stimuli has been particularly strong. So strong, in fact, that stimuli emitted by the female have lost their capacity to evoke mounting in the test for sexual preference. Evidently, they have not lost that capacity elsewhere, since the male-mounting males sire offspring. Here it must be observed that even the male-preferring rams do respond to the female with some approach behaviors during the sexual preference tests, although they do not mount her. In other situations, mounting, as mentioned, follows this approach.
Some empirical support for the hypothesis outlined in the preceding paragraph comes from a study where it was shown that rams mounting other rams had engaged in significantly more sex-like behaviors with other rams in the all-male rearing environment than rams mounting females had (Zenchak and Anderson,1980). When I first mentioned that study a couple of pages ago, I promised that we later should discover its importance. We will see it now. It is not too far-fetched to assume that the rams mounting most also are the rams obtaining the largest number of intromissions/ejaculations. As we know, intromission/ejaculation is a reinforcing event, and the larger the number of reinforcing events, the more intense the association between male stimuli and sexual approach and activity becomes. In consequence, these rams should be more likely to mount other rams. Thus, the Zenchak and Anderson (1980) study supports an explanation of the preference for mounting other rams in terms of learning. The support would have been considerably more convincing if data on the frequency of intromission/ejaculation in the rams had been reported rather than the frequency of sex-like behaviors. This term is rather vague. Here we have another example of the unfortunate habit of not reporting complete behavioral data.
A somewhat more recent study failed to find any relationship between home-pen mounting of other males and subsequent preference for males (Price et al., 1988). However, the results are reported as correlations between mounting in the home-pen and the proportion of mounts directed towards the male in the sexual preference test. This is quite different from a comparison of males exclusively mounting other males during the test with males exclusively mounting the female as was done in the Zenchak and Anderson (1980) study, and I do not consider these data as contradicting the arguments exposed in the preceding paragraph. Regrettably, this study also failed to report complete behavioral data. It is almost certain that the crucial variable is the number of ejaculations achieved with other males in the home pen, but no data on this are reported.
It is easy to summarize this rather long discussion of rams. Beyond doubt, a small proportion of rams mount exclusively other rams at a test designed for determining sexual preferences. They even do so at several tests. These male-mounting rams appear to have blood concentrations of gonadal hormones that are not different from those of rams exclusively mounting females. They may have less estrogen receptors in the amygdala, but that proposal is based on a single study. Likewise, they may have less active aromatase in the preoptic area, but again this affirmation is based on a single study. There is one study reporting differences in cell number and size in a small subregion of the preoptic area, while another study failed to find any difference between male-mounting and female-mounting rams. There does not seem to exist any convincing explanation for the preference of other rams in the test for sexual preferences. Furthermore, the male-mounting rams impregnate females at a rate not much different from the female-mounting rams, showing that the preference for other rams is specific to a particular test situation. An explanation for the preference of other rams in some situation can be put forth in terms of learning. Basic to this is that rams mounting other rams occasionally achieve anal penetration (intromission) and ejaculation, events known to be both reinforcing and rewarding. This fact not only provides a basis for learning, it also explains why rams belong to the extremely rare groups of non-human animals where individuals having a sexual preference for their own sex have been described. Because of some anatomical/motor coincidence, rams can achieve intromission while copulating with other rams, exactly as the human male copulating with other males can. The ram can learn to associate stimuli emitted by other rams with sexual reward, and to execute copulatory responses (mounting) in response to these stimuli by classical and operant conditioning, respectively. Some limited, indirect support for this notion exists.
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