A Note on Statistical Power

Abstract

Short Communications and Commentaries 199 supposition is contradicted by recent theoretical models that show intersexual selection can favor elaborate ornaments in monogamous species, where both sexes provide parental care, and even in sexually mono-morphic species (Kirkpatrick et al. 1990), and by many examples of monogamous species with brighter plumage and more elaborate ornaments than curas-sows (e.g. tropicbirds, Phaethon; egrets, Egretta; parrots , Platycercus and Trichoglossus; some auks, Aethia; puffins, Fratercula spp.; bee-eaters, Merops; sunbirds, Nectarinia; kingfishers, Tanysiptera; jays, Calocitta and Cyanocorax; and tyrant flycatchers, Muscivora). Elaborate traits expressed in males and females may be the result of mutual sexual selection related to variation in mating success of both sexes (Kirkpatrick et al. 1990). Thus, the logic of good-genes, runaway, and direct-benefits models may apply to ornaments of mo-nogamous nonlekking species, but understanding of which model best explains evolution of extravagant traits will depend on carefully designed field experiments on a variety of species with different mating systems. Eventually we may find that all three models may work in nature, perhaps even simultaneously. Finally, data indicating a lack of correlation of cu-rassow ornaments with parasite prevalence is not consistent with good-genes models of sexual selection. It is more consistent with Buchholz's (1991) depiction of the runaway model, or with the idea that knob ornaments are arbitrary with respect to viability. Successful evaluation of these sexual-selection models awaits derivation of testable mutually exclusive predictions , and on field studies that experimentally measure active mating preferences and intrasexual competition, while controlling for confounding factors such as age. LITERATURE CITED BUCHHOLZ, R. 1991. Older males have bigger knobs: Correlates of ornamentation in two species of curassow. Auk 108:153-160.. 1991. The evolution of mating preferences and the paradox of the lek. Nature 350:33-38. READ, A. F. 1990. Parasites and the evolution of host sexual behaviour. Pages 117-157 in Parasitism and host behaviour (C. Jones' (1992) thought-provoking commentary points to some statistical issues and intricacies of sexual-selection theory not discussed in my original paper. However, I believe that some of the "pitfalls" he describes are moot in the empirical realm. In the major thrust of his commentary, Jones wrongly contends that the predictions of the "good-genes" and "runaway" models for the evolution of ornaments are not exclusive. He hypothesizes that runaway traits may become good indicators of the bearer's fitness as they become more burdensome. This scenario is not unreasonable. Nevertheless, once