Measured and Predicted Egg Volume of Pseudemys texana with Comments on Turtle Egg Shape

Abstract

The relationship between egg size (length, width, mass), female body size, and clutch size has been investigated in a variety of reptiles (Ferguson et al., 1982; Congdon and Gibbons, 1985; Bjorndal and Carr, 1989; Ford and Seigel, 1989; Deeming and Ferguson, 1990; Iverson and Ewert, 1991). In spite of this attention, little has been written regarding egg shape and volume and those factors that egg shape and volume might reflect. Turtle eggs are described as being spheres, ellipsoids, or bicones. As a general rule, chelonids, chelydrids, trionychids, and larger tortoises produce round, hard-shelled eggs. Most members of other groups produce some form of an elongated, symmetrical egg. (For reviews of turtle egg biology see Ewert, 1979 and Moll, 1979.) Phylogenetic considerations were given to egg shape and hardness by Iverson and Ewert (1991). Elongated eggs provide greater volume without increasing the short-side diameter beyond presumed morphological constraints; e.g., diameter of pelvic canal (Congdon and Gibbons, 1987) or carapace-plastron exit port (Rose and Judd, 1991). Increased egg volume could add significantly to energy and nutrient storage capacity, resulting in a hatchling with greater fitness (Sinervo, 1990). Large egg or hatchling size would be balanced against negative pressures of reduced reproductive output (Smith and Fretwell, 1974) or restrictions imposed on reproductive product exiting the female (Congdon and Gibbons, 1985). It is surprising, then, that so few workers have addressed the issue of egg volume in turtles (Iverson and Ewert, 1991), especially as it might relate to egg shape. Pseudemys texana, a recently elevated taxon (Ward, 1984), is an emydine turtle that inhabits Texas streams, rivers, and ponds. Information regarding its general biology and reproductive characteristics are scarce despite the fact that this turtle might be the most conspicuous vertebrate in many aquatic systems in Texas. Several fish hatchery ponds and the adjacent San Marcos River on the Southwest Texas State University campus, Hays County, are excellent habitats for this species and females frequently are observed constructing nests during spring and early summer months. Eggs available to us provided an opportunity to evaluate egg shape-volume relationships of a common, relatively unknown, aquatic turtle. We used 14 eggs, selected from two clutches that were laid on 21 May 1994. Each egg was washed, dried and frozen. Upon thawing, we measured length (long axis) and width (short axis) to the nearest mm using Mitutoyo digimatic calipers. Mass was determined to the nearest 0.01 gram with an electronic Mettler BasBal scale. All linear measurements were recorded in mil-