Egg Size vs. Mass Relationships in Turtles
Abstract
For over 40 yrs we compiled egg length, egg width, and egg mass data for turtles in order to estimate the latter from published and unpublished data for the 2 former variables. The resulting data set includes nearly 18,000 eggs for 122 species and includes species-specific graphs (with linear regression equations and correlation coefficients) of egg width vs. egg mass and egg width vs. egg mass, as well as multiple regression equations of egg mass as a function of egg length and egg width (with statistical support). It is our hope that this note will stimulate authors to report egg mass data more rigorously, but also facilitate contemporary studies of reproductive strategies in turtles with more complete and accurate data.
In the mid-1980s, the senior author began compiling data on egg length (EL), egg width (EW), and egg mass (EM) for turtles from his own field work and published papers, in order to be able to estimate the latter from studies with EL and/or EW data, but no EM data. This facilitated his early study of reproductive strategies in turtles, because it allowed mass-standardized measures of reproductive output (e.g., Iverson 1992; Iverson et al. 1993). In the late 1980s Michael A. Ewert joined the effort and added his data, and shortly thereafter Edward O. Moll contributed his unpublished data. Subsequently we reached out to other colleagues who also generously shared their unpublished data (see Acknowledgments).
It was the original intention of Iverson and Ewert to coauthor a paper on egg size variation in turtles, and therein publish this data set. That project was never realized (in part because of Ewert’s untimely death in 2005), although Iverson has continued to add to the data set through 2023. In addition, the equations in this data set contributed to the compilation of a much larger data set of turtle life-history data to be published separately (Iverson, Ennen, and Lovich, unpubl. data, 2024).
Unfortunately, many recent authors continue to neglect recording EM. For example, the recent comprehensive compilation of reptile traits (RepTraits; Oskyrko et al. 2024) included EL and/or EW data for only 47 turtle species but lacked any EM data. That of Jorgewich-Cohen et al. (2022) included 164 species but only reported EL and EW (no EM), and finally that of Hallmann and Griebeler (2018) included only 52 species and reported only EM for 9 (not EL or EW). Hence, because our data set includes nearly 18,000 egg records for 122 species (including 2 subspecies for 3 of those species) that are not available elsewhere, it now seems appropriate to share our egg data publicly (e.g., Figs. 1–3; Supplemental Material).
Citation: Chelonian Conservation and Biology: Celebrating 25 Years as the World’s Turtle and Tortoise Journal 23, 1; 10.2744/CCB-1620.1
Citation: Chelonian Conservation and Biology: Celebrating 25 Years as the World’s Turtle and Tortoise Journal 23, 1; 10.2744/CCB-1620.1
Citation: Chelonian Conservation and Biology: Celebrating 25 Years as the World’s Turtle and Tortoise Journal 23, 1; 10.2744/CCB-1620.1
The data set is organized by family, genus, and species, and includes state or country of origin (unless based on captive data), eggshell type (pliable [leathery, flexible], hard [inflexible when laid, but expandable], or brittle [calcareous and inflexible]; Ewert 1979; Iverson and Ewert 1991), EL and EW in millimeters, EM in grams, and source. Plots of EL vs. EM and EW vs. EM (with least squares regression equations and correlation coefficients) are included along with the multiple regression equations relating EM to EL and EW by species (with statistical support).
It is our hope that the publication of this data set will stimulate authors to report EM data more rigorously, but also facilitate contemporary studies of reproductive strategies in turtles with more complete data, even if estimated.
Relationship between egg width and egg mass for 17,993 eggs from 122 turtle species.
Relationship between egg length and egg mass for 17,993 eggs from 122 species.
Relationship between egg length and egg width for 17,993 eggs from 122 species.
Contributor Notes
Handling Editors: Jeffrey E. Lovich and Jeffrey A. Seminoff
