The evolutionary fight between a mother and her offspring

 Apalone mutica     

 Chrysemys picta

Chelydra serpentina
 
According to research done by F. Janzn and D. Warner, it is the mothers fitness that wins in this struggle.  In their article entitled  Parent–offspring conflict and selection on egg size in turtles, they explore the balance between offspring and maternal fitness in relation to egg and clutch size. They began by collecting the eggs of 3 turtle species,  Apalone mutica, Chelydra serpentina, and Chrysemys picta, for a period of two years. Fertile eggs were counted and then weighed, before being placed in incubation until the offspring hatched. A white spot on the egg indicates the formation of extraembryonic membrances and therefore a fertile egg. The eggs were incubated at a temperture representative of their nature habitat. 

The fitness of the mother was determined by proportion of her eggs that hatched. Egg fitness was determined by the successful hatching of an egg. The hatching of the egg from a clutch was also used to determine the relative fitness of the mother. While selection pressure on egg size did vary in both the C. serpentina and C. picta, overall a larger egg size was favored for all three species. Selection on egg size by the reproductive out put of the mothers as well as the form of selection varied between species also. Relative fitness however, was double the amount in C. serpentina and C. picta, than seen in A. mutica. The optimum egg size for mother was lower than the optimum egg size of offspring.  The mean size of successfully hatching eggs in all three species was closer to the optimum egg size for maternal fitness than individual hatchling fitness, they concluded that fitness of the mother was of greater evolutionary importance.  The results found in this study supported the optimal egg size theory.  There must be a balance between how many and how large the eggs are that a mother produces. Without the convince of live birth, mothers must make a large initial investment into their eggs. While a larger egg is likely to produce a larger more successful offspring, the cost to the mother's fitness is much more expensive than producing many smaller eggs.

Article link:

http://onlinelibrary.wiley.com/doi/10.1111/j.1420-9101.2009.01838.x/pdf