Reproductive Allometry in the Common Map Turtle, Graptemys geographica

Abstract

According to optimal offspring size theory, natural selection pressures balance the egg size/clutch size trade-off at a point where increases in maternal body size result in increases in clutch size but not increases in egg size. However, many turtle species show increasing egg size with increasing maternal body size. The anatomical-constraints hypothesis explains this pattern by hypothesizing that smaller females lay smaller-than-optimal eggs because of morphological constraints; larger eggs simply would not pass through the pelvic aperture and caudal gap of the shell. We examined relationships among female body size (measured as plastron length), clutch size and egg size for a population of common map turtles (Graptemys geographica) at Presque Isle State Park in Erie, Pennsylvania. Correlation analyses were conducted using log-transformed data in order to address questions of isometry and allometry. Clutch mass increased isometrically with plastron length. Egg size and clutch size were both significantly negatively allometric in their relationship with plastron length. It appears that larger females split the increased reproductive allocation made possible by increased maternal volume devoted to eggs between increasing both clutch size and egg size, consistent with predictions of the anatomical-constraints hypothesis.