Visual incentive stimuli

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The widespread conviction that odors are basic for non-human copulatory behaviors has led to a neglect of vision. Furthermore, blinding animals is not a standard procedure in any animal behavior laboratory and it would probably be difficult to convince an ethics committee about the urgency of studying copulatory behavior in blinded animals. It is particularly so, since the limited amount of available data suggest that visual stimuli are unimportant for copulatory behavior in the species where they have been studied. Blind rats show a completely normal behavior (Stone, 1922, 1923; Beach, 1942) and so do blind humans as far as I know.

Visual stimuli are also unimportant for sexual incentive motivation, at least in rats and ferrets. A series of experiments in which male rats were tested in total darkness (infrared lighting together with an infrared sensitive video camera was used for observation) showed that their approach behavior to a receptive female was indistinguishable from that seen when rats are tested in light (Hetta and Meyerson, 1978; Agmo, 2003). In male and female ferrets, approach behavior was evaluated in a situation where the experimental subjects were exposed to either an active individual of the opposite sex behind Plexiglas doors allowing the passage of air (sight, sound and odor condition) or to an anesthetized individual of the opposite sex behind cardboard screens, again allowing the passage of air (odor only condition). The intensity of approach behaviors was similar in the two conditions in both males and females (Kelliher and Baum, 2002). Thus, at least in one rodent and one carnivore species we know that vision is of little or no importance.

In primates, the situation may be quite different. We know that visual sexual incentive stimuli are of paramount importance in the human and there is a small amount of data suggesting that visual stimuli may enhance sexual arousal also in a non-human primate. The human data will be discussed at length in Chapter 4. Here I will briefly mention a couple of studies made in the chacma baboon, Papio ursinos. This species attracted scientists' attention because the females show an unusually large swelling of the genitals and the perineum during estrus. Genital swelling around estrus is known to occur in females of many primate species, but zoologists have suggested that the swelling in the chacma baboon is far larger than that of any other primate species. Because of that, it was quite natural to imagine that this conspicuous visual stimulus might have some effects on sexual arousal in males. In a very creative study, individually caged males were exposed either to estradiol-treated females with full-blown sexual swelling or to ovariectomized females without hormone replacement, and consequently no sexual swelling. The females were kept in cages at some distance from the male and no sexual interaction was allowed. Obviously, the males could see, hear and smell the females at all times. In order to evaluate male sexual arousal, in this study as well as in many others defined as erection and masturbation to ejaculation, the number of seminal spots found below the males' cages was counted every morning. It so happens that the semen from the male chacma baboon coagulates very rapidly after ejaculation. Incidentally, we have already seen this phenomenon in rats. The male chacmas try to collect their own semen with the hands upon ejaculation and then eat it, but a small amount always escapes between the fingers and falls down on the floor underneath the cage. There it forms a visible, characteristic spot. Exposure to a female with swollen genitalia increased the median number of spots from 0, the value obtained during exposure to non-swollen females, to 6. These data are suggestive, but they do not really show that visual stimuli are of any particular importance. The estradiol-treated females could have displayed proceptive behaviors (visual stimuli), they might have vocalized (auditory stimuli) and they certainly smelled (olfactory stimuli) differently from the females not given hormones. In order to determine the critical stimulus modality, the same experiment was repeated, but now a screen was placed between the cages housing the females and the males' cages. The screen prevented the males from receiving visual stimulation from the females, but auditory and olfactory stimuli were not prevented from reaching them. It turned out that the males' response to the estradiol-treated females was no different from that to the untreated females (Bielert, 1982). The rather clear conclusion from this experiment was that visual stimuli are critical for the distant female chacma baboon's capacity to enhance male sexual arousal.

The study described in the preceding paragraph established that visual stimuli are important for distant female-induced sexual arousal in male chacma baboons. The next problem to solve was, logically, the identification of the exact visual stimulus. Female chacmas display proceptive behaviors when exposed to males in periods with swollen genitals. Both phenomena are under the control of estrogens. However, the genital swelling continues to increase with increasing doses of estradiol even after the frequency of proceptive behaviors has arrived at its maximum. This makes it possible to vary the amount of genital swelling while keeping pro-ceptive behaviors at a constant level. Interestingly, results show that the males' response intensity is determined by the degree of genital swelling and unrelated to the display of proceptive behaviors (Bielert and Anderson, 1985). This conclusion was further supported in a very nice study in which ovariectomized females were fitted with a plastic model of fully swollen genitalia. These females efficiently enhanced male sexual arousal, although they did not display any proceptive behaviors at all and had no smell of an estradiol-treated female (Girolami and Bielert, 1987). This splendid series of studies convincingly show that distant visual stimuli can enhance sexual arousal in a primate species. I know of no data from other non-human primates which clearly demonstrate that visual stimuli are efficient sexual incentives. Likewise, I know of no data concerning the role of visual stimuli for the enhancement of sexual arousal in female non-human primates. Nevertheless, the results from the chacma baboon suggest that the human male is not the only primate responding sexually to distant visual stimuli.

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