BODY-SIZE AND FECUNDITY IN THE WATERSTRIDER AQUARIUS-REMIGIS - A TESTOF DARWIN FECUNDITY ADVANTAGE HYPOTHESIS

The general female bias in body size of animals is usually attributed to fecundity selection. While many studies have demonstrated a positiv e relationship between body size and fecundity, the most common interp retation of fecundity selection is that larger females have larger abd omens and can hold more eggs, yet the relationship between abdomen siz e and fecundity has rarely been examined. For the waterstrider, Aquari us remigis, we find a significant relationship between body size and f ecundity and demonstrate that the target of fecundity selection is abd omen size. Thus, larger females have higher fecundities because they h ave larger abdomens and not because of their total size per se. The ra te at which fecundity increases with increasing abdomen size exceeds t hat which would be expected due to a simple volume constraint and sugg ests that other factors, such as increased ability to obtain resources , may contribute to the increase in fecundity with body size. Selectio n intensities estimated from our data indicate that fecundity selectio n could be a significant selective force on both total and abdomen len gths. Previous studies have found that abdomen size increased faster t han body size and thus, larger females had relatively larger abdomens. The relationship of abdomen length and thorax length in A. remigis is hypoallometric and indicates that larger females have relatively smal ler abdomens. We hypothesize that this may reflect conservation of abd omen size in females developing under poor conditions. Finally, while egg size is not directly related to body size, we find a trade-off bet ween egg size and number when female abdomen length is held constant, suggesting that selection on egg size may influence abdomen length onl y indirectly through its effects on fecundity.