Data from: indirect costs of reproduction and the tradeoff between offspring size and number: a framework illustrated by fitness costs and benefits of ovarian fluid

Theory describing evolution of offspring size often assumes that the
production cost per unit volume is the same for small and large offspring.
However, this may not be true if indirect costs of reproduction (e.g.,
material and energetic costs of supporting offspring development) scale
disproportionately with offspring size. Here we show how direct and
indirect costs of reproduction can be explicitly modeled within the
Smith-Fretwell framework and how observations of size-number relationships
can thus be used to evaluate indirect costs. We applied this analysis to
measures of egg volume and fecundity for over 300 individuals of a coastal
fish species and found that the tradeoff was much stronger than the
expected inverse (fecundity scaled with volume-1.843). Larger offspring
were thus more expensive to produce. For our study species, an important
indirect cost was that larger eggs were accompanied by disproportionately
more ovarian fluid. Calorimetry and removal experiments were used to
further measure both the energetic costs and fitness benefits of ovarian
fluid. In addition, we show that indirect costs of reproduction can
intensify size-number tradeoffs in a variety of fishes. Indirect costs of
reproduction can be large and may therefore play an important role in the
evolution of offspring size.