An experimental test of body volume constraint on female reproductive output.

The body volume constraint hypothesis is a widely accepted notion that explains proximate limitation of female reproductive output, but lacks empirical support. To examine how body volume determines reproductive output, we experimentally manipulated the body volume of female lacertid lizard (Takydromus septentrionalis), to examine whether a reduction in body volume caused a reduction in reproductive output. Clutch size and mass decreased with a reduction in body volume. Reproductive output (residual clutch mass, R(CM)) was negatively related to body volume reduction (BVR) (R(CM)=-0.008BVR+0.225). In contrast, neither egg size nor offspring size was affected by the manipulation. Relative clutch mass was lower in manipulated females than in control females, but the actual physical burden (egg plus pearls) carried by manipulated females was much bigger than that of the control females. A normal clutch of eggs only filled 55% of maternal body volume, whereas manipulated females used 75% of body volume to contain eggs and pearls. Therefore, females are capable of carrying more eggs than a normal clutch. Our observations provide unequivocal empirical support for the maternal body volume constraint hypothesis. In addition, these results suggest that female T. septentrionalis may produce a clutch of eggs that fill the available abdomen space to an optimal level rather than the physical limit.