Comment: Fluctuating asymmetry and sexual size dimorphism

Abstract

Fluctuating asymmetry (FA) is defined as a small random deviation from otherwise perfect bilaterally symmetrical traits (Van Valen 1962; Palmer and Strobeck 1986). Bortolotti and Gabrielson (1995) presented two hypotheses as to why the degree of FA should vary between, the sexes of the American kestrel (Falco spawerius). First, as female kestrels are larger than males (Snyder and Wiley 1976), they require more food to grow. Assuming that parents are under food limitation, females should receive inadequate food to grow and thus should show higher levels of FA than males. Second, because of their large size, females may be dominant over males, and thus may be able to monopolize food. In this situation, males should receive inadequate food and thus should exhibit higher levels of FA than females. Bortolotti and Gabrielson did not find support for either hypothesis, as the degrees of FA did not differ significantly between the sexes. For evaluating their hypotheses, Bortolotti and Gabrielson obtained and measured kestrel skeletons from different Canadian museums. They obtained rather small samples from seven general locations (n I 3, except n I 25 for Ontario and Alberta, and five specimens from unknown locations). The fact that specimens were obtained from populations with unknown histories, coupled with a wrong assumption about the lack of a genetic basis for FA, has made Bortolotti and Gabrielson's analysis hard to interpret. Bortolotti and Gabrielson (1995, p. 141) stated that FA is almost entirely caused by environmental stress and that genetic differences among subjects have no effect on such a phenomenon. Three types of genetic stress can affect FA: loss of heterozygosity, hybridization, and an influx of new mutants through natural selection (Leary and Allendorf 1989). There is evidence for all these three genetic processes affecting FA (e.g., Soul6 1979; Leamy 1984; Leary et al. 1984). Although there is some debate regarding the genetic basis of FA, it cannot be fully excluded (Clarke 1994, 1995). Therefore, Bortolotti and Gabrielson cannot rule out genetic differences among the populations from which their speci-