|SREL Reprint #1151|
Constraint on Egg Size: A Challenge
to Optimal Egg Size Theory?
D. Congdon and J. Whitfield Gibbons
Some freshwater turtles appear unable to produce
eggs large enough to achieve the balance between size and number of eggs
predicted by optimal egg size theory. We
present evidence that pelvic girdle structure constrains egg size and thus
offspring size in females of smaller-bodied species (Chrysemys picta)
and Deirochelys reticularia).
The constraint is demonstrated by the correspondence of slopes of the
increase of the pelvic aperture and egg width with increasing body size.
This constraint appears to be relaxed in a larger-bodies species (Pseudemys
scripta), in which the increase in pelvic aperture relative to body size
is greater than the increase in egg width.
This type of structural constraint on a reproductive trait should not
occur unless there is strong selection on pelvic architecture for other
functions, such as locomotion, support, and limb retraction, that prevent
expansion of the pelvic aperture. Although
other explanations may exist for other groups of organisms that appear to vary
egg size, the large variation in egg size associated with body size in some
species of turtles can be reconciled with optimal egg size theory only if a
pelvic constraint model is accepted.
SREL Reprint #1151
Congdon, J.D. and J.W. Gibbons. 1987. Morphological constraint on egg size: a challenge to optimal egg size theory? Proceedings of the National Academy of Science 84:4125-4147.