Turtles of the Order Testudines first evolved in the Triassic Period ca. 220 mya (Thomson et al. 2021) and have retained their distinctive external shell morphology since that time. However, despite the rigid constraints of their evolutionarily conserved, bony shell, they have evolved an amazing diversity of life-history strategies. That diversity includes huge marine species that exceed 2 m in carapace length (CL) and produce up to 12 clutches of up to 106 relatively small eggs every other year on average (Chua and Furtado 1988) to dwarf South African terrestrial tortoises that produce at most only 1 relatively huge egg annually (Loehr et al. 2011). They inhabit marine, estuarine, freshwater, and terrestrial ecosystems from the tropics to cold temperate/low arctic environments, at latitudes to 55° (Emys;TTWG 2021) and 71° (Dermochelys; Eckert et al. 2012), and altitudes between 2700 and 3000 m (Loveridge and Williams 1957) and depths to 1230 m (Eckert et al. 2012). Many species are well known for their delayed maturity and extreme longevity (e.g., Testudo hermanni; Reinke et al. 2022), but others mature early and are short-lived (e.g., Deirochelys reticularia; Congdon et al. 2022). Turtles also exhibit an array of genetic to temperature-dependent sex determination mechanisms far beyond that of any other tetrapod group (Valenzuela and Lance 1994), in 1 case all in the same family (Kinosternidae). Sadly, they also represent the most endangered major vertebrate group, with 51% considered threatened by the IUCN (TTWG 2021), and recent unpublished IUCN assessments indicating that this percentage is an underestimate.

Despite several recent turtle life-history meta-analyses (see below), we still lack a full understanding of their life-history patterns and their evolutionary basis. Huge gaps in the available life-history data are responsible for impeding our progress toward the goals envisioned by Elgar and Heaphy (1989) and Iverson (1992) over 30 yr ago. For example, Myrhvold et al. (2015) compiled life-history data for 274 turtle taxa (266 currently recognized species; 75% of the total recognized), although the data for each species were summarized from multiple studies and frequently estimated from congeners. Nevertheless, the study included clutch size (CS) data for 249 recognized species, egg mass (EM) data for 189, but egg length (EL) and egg width (EW) data for only 22 and 15, respectively. In 2018 Hallmann and Griebler (2018) compiled data for 52 turtle species (15% of all species; with data for each merged from several studies) and included CS data for 49 species and EM data for only 10 species, but reported no EL or EW data.

In an incredible effort, Rachmansah et al. (2020) compiled data for 461 populations of 165 species (46% of the total recognized), including CS data for 164 species and EM data for 140, but no EL or EW data. In 2022 Jorgewich-Cohen et al. (2022) compiled data for 164 species, including CS range (but not means) and EL and EW data for all 164, but no EM data. Finally, Oskyrko et al. (2024) compiled data for 361 “species” (including several synonyms), although data for each species were merged from multiple sources. Their final data set included CS data for 259 recognized species (73% of those recognized), and EL and EW data for only 47 species, and no EM data. Thus, although each of these studies increased the understanding of the diversity of turtle life histories (and their correlates), each data set suffered from incomplete taxonomic coverage, as well as serious gaps in the available life-history data.

We made a systematic collection of life-history and other data for the 357 turtle species of the world (according to TTWG 2021) in order to facilitate future work on the diversity and evolution of turtle life-history patterns. The current version (in spreadsheet format in the Supplemental Material; all supplemental material is available at http://dx.doi.org/10.2744/CCB-1646.1.s1) represents our data collection efforts through mid-2024 and includes data from 2466 populations of 323 species. Despite our efforts to be exhaustive, the data set is still incomplete, with no reproductive data for 34 species (Table 1; ca. 9.5% of recognized species). Thus, 1 important function of this compilation is the identification of the gaps in the knowledge of turtle life-history data, which should stimulate research to fill those voids. For example, our compilation includes data for 94%–100% of the species within 11 turtle families, but only 70% of pelomedusids, 76% of trionychids, and 84% of chelids.

Table 1. List of 34 species for which the data set lacks reproductive data (e.g., age at maturity, clutch size, egg size, clutch mass, relative clutch mass, or clutch frequency).

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To facilitate more complete life-history meta-analyses for turtles, we here share our compilation by family, genus, species, subspecies, and population (when possible), which follows the taxonomy of TTWG 2021. A revision of that Checklist is expected in early 2025, and several recent taxonomic changes have been made, so readers are encouraged to check that revised source (when available) before assuming our taxonomy is fully up to date. Data from individual populations are separated by line to permit the examination of geographic variation within species. For example, the data set lists 68 populations of Chrysemys picta, 54 of Sternotherus odoratus, 50 of Trachemys scripta, and 47 of Chelydra serpentina.

We began our compilation with an extensive search of the turtle literature (including each of our own extensive personal libraries; e.g., Lovich and Ennen 2013) and our unpublished data in an attempt to compile as many life-history and associated data as possible, including general location, latitude, habitat, diet, wild or captive study, mean female carapace length (CL in cm), mean gravid female body mass (BM in g), age at maturity (in years), mean clutch size (CS), egg shell type, mean egg size (EL, length, and EW, width, in mm) and mass (EM in g), mean clutch mass (CM in g), relative clutch mass (RCM, clutch mass/gravid body mass in %), annual clutch frequency (CF), estimates of annual relative clutch mass (CF × RCM), longevity, and data source. Although our survey of nonmarine turtles was extensive and rigorous, we acknowledge that our compilation for marine turtles was not as intensive. We also used Google Scholar and Web of Science searches using a combination of binominal or common name and the following terms: egg*, reproductive ecology, reproduction, reproductive biology, clutch size, and reproductive output. We also sought unpublished input from many of our colleagues (see Acknowledgments).

In our data set, diet, habitat and eggshell type were categorized by species. Diet was classified as herbivore, carnivore, or omnivore. This information was initially extracted from general works (e.g., Ernst and Barbour 1989; Bonin et al. 2006; Ernst and Lovich 2009) and cross-checked with the primary literature and input from colleagues. We recognize the difficulty of dividing turtle diet into only these 3 categories, but relied on the source reference for that determination. Habitat data (i.e., semiaquatic, aquatic, and terrestrial) were also compiled relying on the determination made by the data source. Egg type (brittle, hard, or pliable) was initially recorded from Ewert (1979, 1985), Iverson and Ewert (1991), and Bonin et al. (2006), and then expanded and confirmed with the primary literature or input from colleagues.

We included geo-referenced data and data obtained from captive animals, clearly noting the latter to allow their inclusion or removal from future analyses. Turtles are popular in the international pet trade and with private turtle husbandry enthusiasts. As a result, for several species, most of our reproductive knowledge is limited to captive populations, and for some (e.g., Chelodina mccordi and most Cuora spp.), only captive data are known. Hence, we included data from both sources and scored each study as wild or captive. If reproductive data in the literature were described as semicaptive, we scored the data as being captive.

Published papers often refer to body size in turtles by linear measures only (e.g., CL or PL), and many of those measures are not explicitly defined (Iverson and Lewis 2018). Because it is important to be able to size-standardize reproductive variables, measures of body mass (BM) are crucial. Therefore, we calculated exponential equations relating CL to BM based on data in the published literature, our unpublished data, and raw data provided by generous colleagues. We then used those equations to estimate missing body size data in other published papers so that they could be included in our life-history data set. In each such case those estimations are annotated with a comment explaining our actions. This CL-BM equation compilation is also included as a separate spreadsheet with our data set.

Similarly, authors often do not report all 3 measures of egg size (EL, EW, and EM). However, Iverson et al. (2024) recently published a separate egg size data set that provided linear regressions relating EL to EM and EW to EM and multiple regressions relating EL and EW to EM for 122 turtle species (n = 17,993 total eggs). These regressions allowed us to estimate missing egg size data, and in each such case the action is annotated in a comment. We sometimes used egg size regressions from closely related species (with similar egg size, shape, and shell type) to estimate missing variables. These were also annotated with a comment.

Clutch frequency (CF) remains 1 of the most difficult life-history traits to quantify in turtles (e.g., Gibbons 1982). We tabulated 2 measures of CF when possible. The first was the mean clutch frequency reported by the authors (when available). However, in many cases, only a range was reported, and we report that range rather than a mean. In a second column, we categorized CF into 3 bins: usually only 1 clutch per year; usually 2 or 3 clutches per year; and 4 or more clutches annually.

We conducted several quality assurance and quality control measures. For each species, we calculated the mean, minimum, and maximum values for each reproductive trait to identify outliers and errors. We produced numerous bivariate plots for various relationships between traits at the family level and visually identified any outliers, which were rechecked with the original data (and corrected if necessary). These plots included the relationship between CL (presumably straight-line) and the following: BM, age of maturity, CS, EL, EW, EM, CM, and RCM (see Figs. 1–4). Finally, we investigated relationships between the various egg-size traits at the family level. Through these efforts obvious outliers and errors were corrected or removed from the data set, and we incorporated comments explaining our rationale. Data from original publications that we suspected to be erroneous were also quality assured and quality controlled by these same methods and were removed (if deemed necessary), with comment. We are responsible for any overlooked errors.

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For references that provided ranges in life-history traits, we used the median value if the range was narrow and included the full range in a comment for that cell. If the reported range was broad, we left the cell for that trait empty but added a comment with the full range.

It is our hope that colleagues will thoroughly mine this data set (e.g., Iverson 2024). Furthermore, given the recent nearly complete nuclear-DNA-based phylogeny for turtles (Thomson et al. 2021), we look forward to treatments expounding the detailed evolution of life-history traits across and within turtle families.