Cracks in the egg theory.

Cracks in the egg theory

There is, scientists say, a perfect size for an egg: notnecessarily that of the grade AA beauties of the local store, but the size that represents a stable balance between the best prospects for offspring survival and the amount of energy the mother must invest in making and caring for her eggs. The "optimal egg size theory'--nested in the natural-selection concept of evolution--predicts that within an individual species producing more than one egg at a time, individuals can alter their reproductive output by changing the number of eggs produced, but not the average egg size.

Some freshwater turtles, however, apparently have a differentidea, say Justin D. Congdon and J. Whitfield Gibbons of the Savannah River Ecology Laboratory in Aiken, S.C. They report in the June PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (Vol.84, No.12) that, unlike lizard species previously studied, the turtles produce variable egg sizes more attuned to the size of the monther. By taking X-rays of captured pregnant turtles, the scientists found that average egg size increased as the body size of adult female turtles increased, reflecting parallel increases in the size of the pelvic opening.

Why two of the smaller turtle species studied have notachieved the "optimum' in egg size is unknown, say Congdon and Gibbons. No relationship between maternal and egg sizes was observed in a larger freshwater turtle species included in the study. Perhaps, the scientists suggest, the structures surrounding, and therefore influencing, the pelvic opening are so important to mobility and other nonreproductive behavior that expansion of the opening to allow larger eggs has not had priority in terms of evolutionary changes for some species.