Does optimal egg size vary with demographic stage because of a physiological constraint?

Summary 1 Optimal egg size theory predicts that females should produce propagules of a size and number that maximize maternal fitness. However, studies of the allocation of resources to eggs have rarely provided evidence of such optimization. This is presumably because of constraints that limit reproductive allocation. One such example is that of pelvic aperture morphology constraining egg size in turtles. 2 Growing evidence suggests that even this classic example is incomplete. Hormones that regulate both the reproductive cycle and vitellogenic activity in turtles may provide a novel physiological mechanism for control of egg size. 3 This physiological constraint hypothesis was explored by examining the relationship between maternally derived yolk hormones and egg mass in the Painted Turtle (Chrysemys picta, Schneider). 4 Eggs from younger females contained significantly more testosterone in the yolk than did eggs from older females. Younger females laid eggs nearly 20% smaller than those of older females even where body sizes overlapped. 5 Elevated testosterone levels in younger females may thus constrain egg size physiologically, beyond the well-known physical constraints imposed by pelvic aperture morphology. The endocrine system may play an important but previously unrecognized role in the evolution of egg size in this model system.