Sexual Stimulation and Sexual Selection
Sexual selection, that is, selection arising out of competition for mates, is widely invoked to explain the evolution of extravagant displays. However, sexual selection theory has a hard time explaining the evolution of certain types of elaborate traits, such as the bright plumages and displays of monogamous, monomorphic birds. Darwin was exercised by this difficulty. He suggested that sexual selection in monogamous species results from variation in mate quality rather than in mate number and that females acquire bright plumage incidentally by "transference" from the male. Writing in The American Naturalist in 1938, Julian Huxley offered an alternative to sexual selection. In a paper titled "Darwin's Theory of Sexual Selection and the Data Subsumed by It, in the Light of Recent Research," he argued that displays are often releasers that stimulate appropriate types and timing of behavior in mates. These ideas came out of his studies of grebes and loons, in which both males and females gain showy plumages for the breeding season and use these plumages in mutual displays between the sexes. Huxley's ideas were extremely influential for 30 years but have been gradually relegated to a footnote in history as Darwin's views have become widely accepted. We highlight Huxley's paper here because it appears to us to be an important complement to Darwin's ideas rather than an alternative. Huxley's paper was essentially a long argument that drew on case studies from his own work as well as that of others to make two main points of great relevance to modern studies. First, he noted that many displays continue long after pair formation and reproduction, so are unlikely to be primarily linked to mate choice; he termed these "epigamic" displays. Second, he noted that displays to attract females "all esthetic" displays) are unlikely to be used exclusively in long distance communication to attract females but may also function in sexual stimulation and in competition for mates and resources. Huxley's first and major point emphasized the role of mating displays after mate choice has taken place. Once individuals are paired, displays can trigger and synchronize a variety of reproductive behaviors, such as nest building, territory defense, and copulation. In species without strong pair bonds, ineffective displays may result in the female seeking extra pair copulations or leaving the male entirely and therefore are subsumed under the general rubric of sexual selection. But mate stimulation behaviors also includewhatHuxley termed "strengthening the pair bond," that is, priming the pair to cooperate in raising the young. He believed this was amajor function of themutual displays in the grebes and loons he studied. Huxley's paper played an important role in shaping ethological research over the next 30 years as the field became focused on how display traits function to release behaviors in mates. For example, Nikolaas Tinbergen regularly quoted Huxley's paper in his studies of mutual interactions between the sexes during courtship. In 1972, an edited book titled Sexual Selection and the Descent of Man, 1871 1971 celebrated the centenary of the publication of Darwin's tome on sexual selection (B. G. Campbell, 1972, Heineman, London) This book contained five contributions on nonhuman animals; all of them referenced Huxley's paper, and some relied heavily on its ideas. However, the main legacy of Campbell's book was an upsurge in interest in mate choice rather than mate stimulation. Twenty years on, in 1994, Malte Andersson published his nowclassic book Sexual Selection (Princeton University Press, Princeton, NJ) Criticizing Huxley for introducing obscure terminology and confounding natural, group, and sexual selection, Andersson stated, "Surprisingly, Huxley's confusing views for decades came to be regarded as superior to Darwin's clear insights." No one could make that claim now. Since Andersson, the literature has become full of papers on sexual selection, regularly referring to Darwin. While we may disagree with Huxley's statement that "female choice is not the common phenomenon postu* E mail: pricet@uchicago.edu.
The American Naturalist