Dt. Gwynne et al., THE SEX IN SHORT SUPPLY FOR MATINGS VARIES OVER SMALL SPATIAL SCALES IN A KATYDID (KAWANAPHILA NARTEE, ORTHOPTERA, TETTIGONIIDAE), Behavioral ecology and sociobiology, 42(3), 1998, pp. 157-162
In katydids such as Kawanaphila nartee, a female bias in the operation
al sex ratio (OSR) results in female competition for mates and male ch
oice of mates. Previous work showed that the excess of sexually active
females occurs when food availability is low, in part because less fo
od increases the propensity of females to mate as they forage for the
large edible spermatophores produced by males. In this study with K. n
artee, a pollen-feeding species, we estimate natural variation in numb
ers of sexually active males and females by assessing male calling act
ivity and the propensity of females to respond to experimental calling
males. We found an excess of sexually active males at a site with man
y flowers and an excess of sexually active females at a site with few
flowers about 900 m away. Between-site differences in gut masses of ca
lling males were consistent with the hypothesis that pollen availabili
ty controls OSR. Finally, at a third site where flowers were at first
scarce, we found that the initial excess in sexually active females ch
anged to an excess of sexually active males after a clump of grass-tre
es flowered. The mean gut mass of all sampled males from this site inc
reased after flowering. The large variation in OSR that we document fo
r K. nartee highlights the importance of identifying the appropriate s
patial and temporal scales over which OSRs are measured in studies of
factors controlling sexual selection.