Meeting Abstract
Sexual size dimorphism is a widespread phenomenon in animals. In most cases, in birds and mammals, males are larger than females. However, in several species the reverse is true: females are instead the larger sex (female-biased size dimorphism). One hypothesis for the occurrence of small males, relative to females, is the male agility hypothesis which posits that selection for vigorous, agile courtship performances drives the evolution of small male size. Males of several hummingbird species perform seemingly agile flight behaviors during courtship. To test the aerial agility hypothesis we compiled size data from the literature along with courtship-display and hovering wingbeat frequencies for male and female hummingbirds. This allowed for analyses of scaling patterns that illustrate how selection for courtship performance coincides with small male size and elevated male hovering wingbeat frequency. Hummingbird hovering wingbeat frequency scales negatively with body size. For mass, the allometric exponent of -0.597 compares to -0.269 in other birds (Rayner, 1988). For wing length, the allometric exponent of -1.25 compares to -1.03 in small passerines (Greenewalt, 1975). Furthermore, within the Mellisugini clade, some males utilize particularly high hovering wingbeat frequencies for their sizes, and males of these species tend to be smaller than their female counterparts. Small male size and elevated male hovering wingbeat frequencies in this clade have evolved due to selection on males for exaggerated wingbeat-frequency courtship displays.
