Parent-offspring conflict and selection on egg size in turtles

The trade-off between offspring size and number can present a conflict between parents and their offspring. Because egg size is constrained by clutch size, the optimal egg size for offspring fitness may not always be equivalent to that which maximizes parental fitness. We evaluated selection on egg size in three turtle species (Apalone mutica, Chelydra serpentina and Chrysemys picta) to determine if optimal egg sizes differ between offspring and their mothers. Although hatching success was generally greater for larger eggs, the strength and form of selection varied. In most cases, the egg size that maximized offspring fitness was greater than that which maximized maternal fitness. Consistent with optimality theory, mean egg sizes in the populations were more similar to the egg sizes that maximized maternal fitness, rather than offspring fitness. These results provide evidence that selection has maximized maternal fitness to achieve an optimal balance between egg size and number.