Sexual Choice among the Animals
( Originally Published 1957 )
Sexual pairing in the animal world sometimes occurs with little or no choice. Thus Brehm, writing of polygamous birds, says of the female cuckoo that she "mates with one male today and another tomorrow, may indeed bestow her affections on several in the course of an hour . . . It is simply out of the question to talk of mating among them at all."
Evidence of choice is equally clear. Darwin, as earlier mentioned, collected some facts on individual preferences and aversions among domesticated animals. Among certain birds the females show, "from unknown causes," intense preferences for certain males. (14) Darwin was interested in the aesthetics of courtship among birds, and in the elaborate maneuvers and displays of the males to "excite, attract or fascinate" the female. He described varieties of vivid coloring and elaborate ornamental details, along with songs and "instrumental" music, drumming, trumpeting, cooing, and so on. There is evidence, he believed, that in response to such lures the females exhibit preferences expressed in definite choice of one of several or many males. That choice really occurs must be inferred, he admits, by analogy with human behavior. "If an inhabitant of another planet were to behold a number of young rustics at a fair courting a pretty girl and quarreling about her, like birds at one of their places of assemblage, he would, by the eagerness of the wooers to please her and to display their finery, infer that she had the power of choice."
Birds appear to be responsive to the colors and sounds associated with the mating season and exhibit strong and sustained likes and dislikes for particular individuals. For this reason Darwin was inclined to believe that these songs and colors are not without purpose, that the matings do not occur at random, and that capacity for choice is present. Birds have acute sensibilities, he thought, and noted in his discussion of the argus pheasant that even highly refined forms of beauty may serve as sexual charms. On the other hand, there is evidence "that some males and females of the same species, inhabiting the same district, do not always please each other, and consequently do not pair." There are also instances of birds belonging to different species which become "absolutely fascinated" with each other though living within their own species. Among mammals Darwin found the aesthetic factor in sexual selection less prominent, but even here are examples of preference and rejection among dogs and horses, and here again, less commonly in the male than the female. He concluded that the latter are "allured or excited by particular males who possess certain characters in a higher degree than other males," though it is uncertain what these characters are.
The studies made by R. M. Yerkes on chimpanzees show that great differences occur in the behavior of different males toward a female. One male, for example, may persistently respond in a negative way, sexually, to a particular female, while the latter is eagerly sought by another male. Such preferences are largely independent of the phases of sexual receptivity in the female. In a like manner the female makes selection, and may repeatedly refuse a given male, the records showing "many instances of such unexpected sexual refusals." The main factor in these rejections seems to be "social incompatibility," Yerkes suggests, with this in turn possibly traceable to such influences as physique, or preference for another. He reports an interesting instance of conflict of motives in the setting of mating behavior:
Wendy, on cycle day 15, when known to be fully receptive and willing to mate with Bokar, definitely rejected Pan, although he evidently expected and desired her to present sexually. On the following day she again refused to go to him. On day 17, Pan, by genital gesture, solicited presentation, and Wendy responded vocally but without at first approaching him. In the fourth minute of the experimental period, the male again solicited her by postural and genital gestures. In response she descended from the netting of the cage wall and flattened out on the floor a few feet in front of him. Accepting her behavior as permission or invitation to mate, Pan started to approach, but before he reached her she sprang up, whirled about and viciously attacked him.
What this meant, according to Yerkes, was that the female, though markedly negative toward the male "socially . . . as mate," was briefly aroused toward him because of her sexual excitability. Then her aversion for him suddenly gained the upper hand and threw her on the defensive. The observer marks the behavior as "peculiarly interesting," since the two animals had lived together for a number of years in "harmonious intimacy." The latter fact suggests the possibility that the aversion of the female was in some way specific to the sexual situation.
Ford and Beach note that "Man is the only animal capable of formulating abstract concepts of beauty, ugliness, or sexual attractiveness, but by their behavior animals of other species sometimes demonstrate an obvious preference for one type of sexual partner as compared with another." They cite evidence of preference among dogs and cattle. With the apes, as among men, certain individuals are much more discriminating in sexual choice than others, and definite preferences for certain females are shown. The likeness to human behavior increases when we learn that "some males seem to develop definite preferences for females with a particular facial appearance."