Congdon J D, Gibbons J W
Proc Natl Acad Sci U S A. 1987 Jun;84(12):4145-7. doi: 10.1073/pnas.84.12.4145.
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-bodied 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.
一些淡水龟似乎无法产出足够大的卵，以达到最佳卵大小理论所预测的卵大小与数量之间的平衡。我们提供的证据表明，骨盆结构会限制体型较小物种（彩龟和网纹地图龟）雌性的卵大小，进而限制后代大小。骨盆孔径增加的斜率与卵宽度增加的斜率随体型增大而对应，这证明了这种限制。在体型较大的物种（黄腹彩龟）中，这种限制似乎有所放松，其骨盆孔径相对于体型的增加大于卵宽度的增加。除非对骨盆结构存在针对其他功能（如运动、支撑和肢体回缩）的强烈选择，从而阻止骨盆孔径扩大，否则这种对生殖性状的结构限制不应出现。尽管对于其他似乎卵大小各异的生物群体可能存在其他解释，但只有接受骨盆限制模型，某些龟类物种中与体型相关的卵大小的巨大差异才能与最佳卵大小理论相协调。
