Turtle Size Research Articles

Do turtles follow the rules? Latitudinal gradients in species richness, body size, and geographic range area of the world's turtles.

Understanding how and why biodiversity is structured across the globe has been central to ecology, evolution, and biogeography even before those disciplines took their modern forms. Three global-scale patterns in particular have been the focus of research and debate for decades: latitudinal gradients in species richness (richness decreases with increasing latitude), body size (body size increases with increasing latitude in endotherms; Bergmann's rule), and geographic range size (range size increases with increasing latitude; Rapoport's rule). Despite decades of study, the generality and robustness of these trends have been debated, as have their underlying causes. Here we investigate latitudinal gradients in species richness, body size, and range size in the world's turtles (Testudines), and add more evidence that these rules do not seem to apply across all taxa. We show that turtle diversity actually peaks at 25° north, a highly unusual global pattern. Turtles also fail to follow Bergmann's Rule, and may show the converse (larger at lower latitudes), though trends are weak. Turtles also show a complex relationship between latitude and range size that does not directly follow Rapoport's rule. Body size and geographic range size are significantly correlated, and multiple abiotic and biotic variables help explain the relationships between latitude and species diversity, body size, and range size. Although we show that turtles do not strictly follow some classic biogeographical rules, we also call for further in-depth research to investigate potential causal mechanisms for these atypical patterns.

Hooking locations in sea turtles incidentally captured by artisanal longline fisheries in the Eastern Pacific Ocean

Bycatch by longline fisheries, especially by artisanal small-scale fisheries, is one of the main conservation problems for some sea turtle populations around the world. Since 2004, a network of professionals under the “Eastern Pacific Regional Sea Turtle Bycatch Program” have been working with artisanal longline fishers in the Eastern Pacific Ocean (EPO) to reduce sea turtle bycatch and related mortality. Trials assessing circle hooks of different sizes and shapes, and different baits, have been conducted to determine the effectiveness in the reduction of sea turtle bycatch and changes in hooking location. In this paper, information from 1823 olive ridley sea turtles incidentally captured in the EPO were analyzed to assess how hook type (J, tuna hooks or circle hooks), hook size, bait type (squid or fish), turtle size and target species (tunas, sharks or mahi-mahi) affect hooking location on sea turtles. This were modeled with a Classification and Regression Tree using hooking location as a multinomial variable response (for 6 categories of hooking locations); and also as a binomial response (swallowed vs. non-swallowed) using a Generalized Linear Mixed Model (GLMM). Hook type and size, plus bait type, were the most important factors affecting hooking location, while turtle size and target species did not have any significant effect. J-hooks and tuna hooks had a much greater probability of being swallowed than circle hooks. In addition, as the hook size increased, the likelihood of swallowing it decreased. The use of fish bait in combination with larger circle hooks tended to produce higher proportions of external hookings. An increase in external or lower mandible hookings is preferred since these locations are assumed to be less dangerous for the animal's post-release survival, and because hooks and attached gear are easier to remove by well-trained fishermen.

Comparison of organochlorine pesticides and PCB residues among hawksbill (Eretmochelys imbricata) and green (Chelonia mydas) turtles in the Yucatan Peninsula and their maternal transfer

Organochlorine pesticides and PCB (POPs) concentrations were determined in the blood and eggs of green and hawksbill turtles. We compared concentrations between species, analyzed the relationship between turtle size and the POPs concentrations and the relationship between the concentrations in the blood of the nesting turtles and their eggs. We expected higher concentrations in the hawksbill turtle because of its higher trophic level, but concentrations were not higher in all the cases. Significant differences were found in δ-HCH blood concentrations. Lindane, heptachlor epoxide and PCB 101 concentrations were significantly higher in the hawksbill eggs. The relationship between the size of the turtles and the POP concentrations in the eggs of the hawksbills showed a negative correlation. No correlation was found between the size of the female and concentrations in the blood. In eggs, only the hawksbill turtles exhibited negative correlation in the concentration of mirex and PCB 44 and size.

Age and body size of captive hawksbill turtles at the onset of follicular development.

The aim of this study was to record the age and body size of 23 captive female hawksbill turtles at the onset of follicular development. The age, straight carapace length (SCL), and body mass (BM) of the turtles were recorded between 2006 and 2014 at follicular development (determined via ultrasonography) these parameters were 17.7 ± 1.7 years (range: 13-20 years), 77.7 ± 3.3 cm (73.3-83.5 cm), and 61.1 ± 8.0 kg (48.2-76.1 kg), respectively. When the year of the onset of follicular development was designated year 0, the increase in SCL in years -7-0 and 0-3 averaged 2.2 cm and 1.0 cm, respectively. Correspondingly, the increase in BM in years -7-0 and 0-3 averaged 5.0 kg and 2.2 kg, respectively. This is the first study to report the age and body size of captive female hawksbill turtles at the onset of follicular development, which indicates the beginning of sexual maturation. The reduction in growth after follicular development suggests that at the onset of sexual maturation, female hawksbills may utilize energy for follicular development rather than growth.

Direct Evidence for Gradual Ontogenetic Dietary Shift in the Green Turtle,Chelonia mydas

Abstract Here we provide information on the diet of juvenile green turtles from neritic developmental habitats in the South Atlantic that indicates the existence of a gradual dietary shift from a primarily carnivorous to a primarily herbivorous diet. Linear regression showed a relation between the size of turtles (n = 22) and the proportion of vegetal matter in the diet, with smaller animals being predominantly carnivorous. We propose 3 hypotheses that could explain these observations.

Hitchhikers reveal cryptic host behavior: new insights from the association between Planes major and sea turtles in the Pacific Ocean

Studies that incorporate information from habitat-specific ecological interactions (e.g., epibiotic associations) can reveal valuable insights into the cryptic habitat-use patterns and behavior of marine vertebrates. Sea turtles, like other large, highly mobile marine vertebrates, are inherently difficult to study, and such information can inform the implementation of conservation measures. The presence of epipelagic epibionts, such as the flotsam crab Planes major, on sea turtles strongly suggests that neritic turtles have recently occupied epipelagic habitats (upper 200 m in areas with >200 m depth) and that epipelagic turtles spend time at or near the surface. We quantified the effects of turtle species, turtle size, and habitat (neritic or epipelagic) on the frequency of epibiosis (F 0) by P. major on sea turtles in the Pacific Ocean. In neritic habitats, we found that loggerhead (F 0 = 27.6 %) and olive ridley turtles (F 0 = 26.2 %) host crabs frequently across a wide range of body sizes, and green turtles almost never host crabs (F 0 = 0.7 %). These results suggest that loggerheads and olive ridleys display variable/flexible epipelagic-neritic transitions, while green turtles tend to transition unidirectionally at small body sizes. In epipelagic habitats, we found that loggerheads host crabs (F 0 = 92.9 %) more frequently than olive ridleys (F 0 = 50 %) and green turtles (F 0 = 38.5 %). These results suggest that epipelagic loggerheads tend to spend more time at or near the surface than epipelagic olive ridleys and green turtles. Results of this study reveal new insights into habitat-use patterns and behavior of sea turtles and display how epibiont data can supplement data from more advanced technologies to gain a better understanding of the ecology of marine vertebrates during cryptic life stages.

Do roads reduce painted turtle (Chrysemys picta) populations?

Road mortality is thought to be a leading cause of turtle population decline. However, empirical evidence of the direct negative effects of road mortality on turtle population abundance is lacking. The purpose of this study was to provide a strong test of the prediction that roads reduce turtle population abundance. While controlling for potentially confounding variables, we compared relative abundance of painted turtles (Chrysemys picta) in 20 ponds in Eastern Ontario, 10 as close as possible to high traffic roads (Road sites) and 10 as far as possible from any major roads (No Road sites). There was no significant effect of roads on painted turtle relative abundance. Furthermore, our data do not support other predictions of the road mortality hypothesis; we observed neither a higher relative frequency of males to females at Road sites than at No Road sites, nor a lower average body size of turtles at Road than at No Road sites. We speculate that, although roads can cause substantial adult mortality in turtles, other factors, such as release from predation on adults and/or nests close to roads counter the negative effect of road mortality in some populations. We suggest that road mitigation for painted turtles can be limited to locations where turtles are forced to migrate across high traffic roads due, for example, to destruction of local nesting habitat or seasonal drying of ponds. This conclusion should not be extrapolated to other species of turtles, where road mortality could have a larger population-level effect than on painted turtles.

Do loggerhead turtle (Caretta caretta) eggs vary with alternative foraging tactics?

Quantifying variations in the size and viability of offspring is essential to accurately evaluate fitness differences between alternative life histories. In the present study, the size and nutritional components (water, protein, lipid, carbohydrates, and ash) of loggerhead turtle (Caretta caretta) eggs were compared between small planktivores inhabiting oceanic areas (water depths >200m) and large benthivores inhabiting neritic areas (depths <200m) that sympatrically nest on Yakushima Island, Japan. We aimed to verify whether variations in the diet, which are dependent on the environment, of a large marine reptile influence egg quality. No significant differences were found in the size or component values of eggs between oceanic and neritic foragers classified based on stable isotope ratios in the yolks. The above results imply that foraging tactics adopted by loggerhead turtles do not affect the characteristics of the eggs and hatchlings. Among all 20 nesting females analyzed, egg mass increased significantly with straight carapace length, and the absolute contents for all components (except carbohydrates) significantly increased with egg mass. Interspecies and interpopulation variations in egg characteristics are discussed in relation to adaptations to local environments.

Relation of Minimum Viable Population Size to Biology, Time Frame, and Objective

Shoemaker et al.’s (2013) estimate of the minimum viable population size (MVPS) of the bog turtle (Glyptemys muhlenbergii) is 3 orders of magnitude below population sizes typically viewed as minimally viable (Reed et al. 2003; Trail et al. 2007). Even Flather et al. (2011), who question the utility of MVPS for conservation planning, concede that long-term persistent populations will require thousands of individuals. Rather than conclude their result is incorrect, Shoemaker at al. suggest that MVPS has been overestimated for long-lived species. Although we agree in principle that small populations, especially of long-lived species, have conservation value, we do not agree that Shoemaker et al.’s result provides evidence that small populations are viable. This position is inconsistent with existing empirical evidence of persistence of small populations (Simberloff & Gibbons 2004; Fagan et al. 2005; Fagan & Holmes 2006). We suggest that Shoemaker et al.’s estimate is a gross underestimate, emanating from their definition of viability, which is too narrow, not biologically meaningful, and ignores factors such as demographic and environmental stochasticity, loss of genetic variability, and catastrophes. Their conclusion that populations of long-lived species can be orders of magnitude smaller than currently believed does a disservice to species conservation and raises the longterm discussion in conservation biology about whether we should be focusing on the minimum for management. Shoemaker et al. used too short a time frame, 100 years. Frankham and Brook (2004) and O’Grady et al. (2008) identify the importance of setting the proper time frame for conservation questions and point out that generation length is often the appropriate scale. It can be shown easily that even a strongly declining population can be viable over the short time frame of 10 generations ( 100 years for long-lived species) and that the

Is the island rule general? Turtles disagree

AbstractAimThe island rule describes a trend toward dwarfism of large animals and gigantism of small animals on islands. Studies of vertebrates, including reptiles, report conflicting results regarding the generality of this evolutionary pattern. We studied the size evolution of insular turtles at the intra‐ and interspecific levels, as well as the clade level. We examined the evolutionary patterns, the relationship of habitat preferences and insular turtle size and whether the island rule extends to turtles.LocationIslands world‐wide and adjacent continents.MethodsWe collected data from the literature and expanded our database by measuring turtle specimens in museum collections. We used reduced major axis regression to test the island rule at the inter‐ and intraspecific levels, after accounting for phylogenetic effects. In addition, we compared body size frequency distributions of insular versus mainland turtles at the clade level.ResultsOur analyses revealed no support for the island rule in turtles. Intraspecifically, we found no systematic pattern of body size evolution. Interspecifically, large turtle species tend to become larger on islands, in contrast to the island rule predictions. Islands generally harbour larger turtles than mainland regions. This pattern is especially apparent when recently extinct species are considered. We found no impact of habitat preferences on the evolution of body size on islands.Main conclusionsTurtles on islands do not follow the island rule. We suggest that both physiological and ecological factors drive turtle body size evolution on islands, and their tendency for insular gigantism. Large turtles can survive longer periods of water or food shortage on small oceanic islands. Increased intraspecific competition and reduced interspecific competition and predation on islands may favour large body size. A non‐mutually exclusive explanation is that the presence of giant tortoises on islands may reflect founder effects rather than evolution – large turtles are better island colonizers.

An Assessment of Green Turtle (Chelonia mydas) Stocks Along the Texas Coast, with Emphasis on the Lower Laguna Madre

The in-water status of green turtles (Chelonia mydas) in the northwestern Gulf of Mexico has received comparatively little attention in the management and conservation of this species in US waters. We addressed this void via entanglement netting surveys at 3 estuarine areas (Lavaca-Matagorda Bay, the Aransas Bay complex, and the lower Laguna Madre [LLM]) to assess trends in the relative abundance, distribution, and size composition of green turtles in Texas inshore waters during 1991–2010. Overall catch per unit effort (CPUE) in the LLM (1.5 turtles/km-hr) was significantly higher than that observed in the other 2 estuaries and corresponded to general climate patterns and seagrass areal coverage along the Texas coast. Yearly CPUE of green turtles from the LLM exhibited a significant exponential increase from 1991 to 2010, despite variability in capture rate during recent years. Mean straight carapace length (SCL) of green turtles was statistically similar across the 3 study areas and indicative of dominance by juvenile individuals (LLM mean = 42.2 cm SCL). Historical green turtle size data in the LLM also reveal an overall decreasing trend in mean SCL and shift toward an increased percentage of smaller individuals (30–39.9- and 40–49.9-cm SCL size classes). Also, sighting frequency of postpelagic green turtles at the Brazos-Santiago Pass jetties within the LLM during 2009 was roughly 9 times that reported for comparable months during 1992–1993. These results are indicative of enhanced recruitment of neritic green turtles to the northwestern Gulf of Mexico, most likely due to elevated nesting productivity at beaches in Mexico, Florida, the Caribbean, and the western Atlantic. Overall, the lower Texas coast serves as an important developmental foraging habitat for an increasing population of juvenile green turtles in the northwestern Gulf of Mexico.

Bone density in the loggerhead turtle: functional implications for stage specific aquatic habits

AbstractSeveral studies investigated how changes in bone density are related to the evolution of complex buoyancy control systems in aquatic mammals. Very little is known on sea turtles, although this is one of the most ancient tetrapod groups that successfully colonized the marine environments. Here, we investigated for the first time the relationship between bone density and body size in the loggerhead turtle, Caretta caretta, with the aim to elucidate possible functional connections with the species’ aquatic habits. Humeri were extracted from the carcasses of 72 loggerhead turtles ranging in size from 7 to 89 cm (males = 18, females = 44, unknown = 10). Whole bone density was determined by Archimedes’ principle. Sexes exhibited comparable humerus densities (t‐value = 0.49, P &gt; 0.05). Mean humerus density (1.33 g cm−3) was intermediate within the range reported for marine mammals and suggested no extreme specialization towards an either pelagic or benthic lifestyle. Turtle size and humerus density were significantly correlated (Pearson's correlation = 0.638, P &lt; 0.01). Small juveniles had very light bones compared to adults in accordance with their stage specific pelagic diving and foraging behaviour.

Impacts of fisheries bycatch on marine turtle populations worldwide: toward conservation and research priorities

Fisheries bycatch is considered the most serious threat globally to long‐lived marine megafauna (e.g., mammals, birds, turtles, elasmobranchs). However, bycatch assessments to date have not evaluated population‐level bycatch impacts across fishing gears. Here, we provide the first global, multi‐gear evaluation of population‐level fisheries bycatch impacts for marine turtles. To compare bycatch impacts of multiple gears within and among marine turtle populations (or regional management units, RMUs), we compiled more than 1,800 records from over 230 sources of reported marine turtle bycatch in longline, net, and trawl fisheries worldwide that were published between 1990–2011. The highest bycatch rates and levels of observed effort for each gear category occurred in the East Pacific, Northwest and Southwest Atlantic, and Mediterranean regions, which were also the regions of highest data availability. Overall, available data were dominated by longline records (nearly 60% of all records), and were non‐uniformly distributed, with significant data gaps around Africa, in the Indian Ocean, and Southeast Asia. We found that bycatch impact scores—which integrate information on bycatch rates, fishing effort, mortality rates, and body sizes (i.e., proxies for reproductive values) of turtles taken as bycatch—as well as mortality rates in particular, were significantly lower in longline fishing gear than in net and trawl fishing gears. Based on bycatch impact scores and RMU‐specific population metrics, we identified the RMUs most and least threatened by bycatch globally, and found wide variation among species, regions, and gears within these classifications. The lack of regional or species‐specific patterns in bycatch impacts across fishing gears suggests that gear types and RMUs in which bycatch has the highest impact depend on spatially‐explicit overlaps of fisheries (e.g., gear characteristics, fishing practices, target species), marine turtle populations (e.g., conservation status, aggregation areas), and underlying habitat features (e.g., oceanographic conditions). Our study provides a blueprint both for prioritizing limited conservation resources toward managing fishing gears and practices with the highest population impacts on sea turtles and for enhancing data collection and reporting efforts.

By-catch frequency and size differentiation in loggerhead turtles as a function of surface longline gear type in the western Mediterranean Sea

Fisheries by-catch is considered to be a major threat to loggerhead turtles in the Mediterranean Sea. Technical differences in both gear configurations (e.g. hook and bait type) and fisheries operations carried out by the Spanish Mediterranean surface longline fleet could have an effect on by-catch rates and size selectivity. The aim of the present study was to test the differences in by-catch per unit effort and body size of loggerhead sea turtles caught using different gear types in the Mediterranean surface longline fishery. Our results suggest that differences in the gear type used have an effect on catch rates and size selectivity. Thus, surface longliners targeting albacore (LLALB) using smaller hooks tend to capture smaller loggerheads but have the highest by-catch per unit of effort (BPUE), whereas other longlines, such as surface longliners targeting bluefin tuna (LLJAP) and traditional surface longliners targeting swordfish (LLHB), using larger hooks tend to select the larger animals; moreover, LLHB had the lowest BPUE. Disproportionate rates of fisheries-induced mortality on certain size/age-classes can differentially affect sea turtle populations, as each sea turtle age-class contributes differently to current and future reproduction. Thus, fisheries management should not only be focusing on preserving the large juvenile and mature turtles, but also on reducing the total by-catch. Thus, we recommend encouraging the use of LLHB versus other surface gears. We suggest that it is very important to take into account the gear type (and its particular catch rates) when making inferences about the impact of longline fisheries on sea turtle populations.

Increased By-Catch Rates in the Gulf of Taranto, Italy, in 20 Years: A Clue About Sea Turtle Population Trends?

Abstract Information on sea turtle population trends is fundamental to assess the population status and the effects of conservation measures, and is considered a priority for sea turtle conservation. To provide insights on trends at sea, we compared by-catch data from long-liners fishing in the Gulf of Taranto in 2 periods: 1978–1979 and 1998–2003. A total of 653 loggerhead sea turtles (Caretta caretta) were captured in 2679 fishing sets targeting swordfish and albacore tuna; catch rates were significantly higher in the second period, while average turtle size was smaller. Although possible effects of sampling techniques cannot be excluded, a negative trend in the number of turtles in the study area and period was unlikely.

Gastrointestinal helminth community of loggerhead sea turtle Caretta caretta in the Adriatic Sea

We analysed the intestinal helminth community of 70 loggerhead sea turtles Caretta caretta with a curved carapace length ranging from 25 to 85.4 cm, recovered dead in neritic foraging habitats in the Adriatic Sea in 1995 to 2004. The overall prevalence of infection was high (70.0%), with a mean abundance of 36.8 helminth parasites per turtle. Helminth fauna comprised 5 trematodes (Calycodes anthos, Enodiotrema megachondrus, Orchidasma amphiorchis, Pachypsolus irroratus, Rhytidodes gelatinosus) and 3 nematodes (Sulcascaris sulcata, Anisakis spp., Hysterothylacium sp.), with 6 taxa specific for marine turtles. In terms of infection intensity and parasite abundance, O. amphiorchis was the dominant species (mean intensity: 49.8; mean abundance: 12.8), followed by R. gelatinosus (30.5 and 8.3, respectively) and P. irroratus (23.5 and 7.0, respectively), while larval Anisakis spp. exhibited the highest prevalence (34.3%). The intensity of helminth infection ranged from 1 to 302 (mean: 52.6 ± 69.1) and was not correlated with the size of turtles; this relationship held for all species, except R. gelatinosus (rS = 0.556, p < 0.05). In comparison to other marine habitats, the helminth community of Adriatic loggerheads is characterised by higher species diversity (Shannon-Wiener H' = 1.58) and evenness (E = 0.76), and lower dominance values (Berger-Parker d = 0.35), which can be attributed to the life history and feeding ecology of sea turtles in recruited neritic grounds and the diversity of their benthic prey.

To Eat or Not to Eat? Debris Selectivity by Marine Turtles

Marine debris is a growing problem for wildlife, and has been documented to affect more than 267 species worldwide. We investigated the prevalence of marine debris ingestion in 115 sea turtles stranded in Queensland between 2006–2011, and assessed how the ingestion rates differ between species (Eretmochelys imbricata vs. Chelonia mydas) and by turtle size class (smaller oceanic feeders vs. larger benthic feeders). Concurrently, we conducted 25 beach surveys to estimate the composition of the debris present in the marine environment. Based on this proxy measurement of debris availability, we modeled turtles’ debris preferences (color and type) using a resource selection function, a method traditionally used for habitat and food selection. We found no significant difference in the overall probability of ingesting debris between the two species studied, both of which have similar life histories. Curved carapace length, however, was inversely correlated with the probability of ingesting debris; 54.5% of pelagic sized turtles had ingested debris, whereas only 25% of benthic feeding turtles were found with debris in their gastrointestinal system. Benthic and pelagic sized turtles also exhibited different selectivity ratios for debris ingestion. Benthic phase turtles had a strong selectivity for soft, clear plastic, lending support to the hypothesis that sea turtles ingest debris because it resembles natural prey items such as jellyfish. Pelagic turtles were much less selective in their feeding, though they showed a trend towards selectivity for rubber items such as balloons. Most ingested items were plastic and were positively buoyant. This study highlights the need to address increasing amounts of plastic in the marine environment, and provides evidence for the disproportionate ingestion of balloons by marine turtles.

The role of turtles as coral reef macroherbivores.

Herbivory is widely accepted as a vital function on coral reefs. To date, the majority of studies examining herbivory in coral reef environments have focused on the roles of fishes and/or urchins, with relatively few studies considering the potential role of macroherbivores in reef processes. Here, we introduce evidence that highlights the potential role of marine turtles as herbivores on coral reefs. While conducting experimental habitat manipulations to assess the roles of herbivorous reef fishes we observed green turtles (Chelonia mydas) and hawksbill turtles (Eretmochelys imbricata) showing responses that were remarkably similar to those of herbivorous fishes. Reducing the sediment load of the epilithic algal matrix on a coral reef resulted in a forty-fold increase in grazing by green turtles. Hawksbill turtles were also observed to browse transplanted thalli of the macroalga Sargassum swartzii in a coral reef environment. These responses not only show strong parallels to herbivorous reef fishes, but also highlight that marine turtles actively, and intentionally, remove algae from coral reefs. When considering the size and potential historical abundance of marine turtles we suggest that these potentially valuable herbivores may have been lost from many coral reefs before their true importance was understood.

Where Lakes Were Once Rivers: Contrasts of Freshwater Turtle Diets in Dams and Rivers of Southeastern Queensland

We contrasted diets of three turtles (Elseya albagula, Myuchelys latisternum, Emydura krefftii) from free-flowing or impounded rivers in southeastern Queensland, Australia, to evaluate the effects of flow regulation. The turtle species encompassed the herbivorous, carnivorous, and omnivorous feeding guilds, respectively. The study design simultaneously considered ontogenetic dietary shifts and seasonal effects on prey availability. Relative to the river samples, diets for three turtles in impoundments were substantially reduced in prey abundance, species richness, and dietary breadth. Turtles with narrower dietary preferences in free-flowing rivers were more affected by dams than was the dietary generalist. For the most omnivorous turtle, qualitative differences were evident in diets from impoundments and rivers; yet considering its greater dietary breadth, quantitative effects on diet were minor. Diets of turtles in impoundments included fewer subaquatic plants and wind fallen fruits than did the diets of turtles in rivers. For the largely carnivorous turtle, fewer aquatic invertebrates were ingested in impoundments than in riverine habitats, and scavenging behavior was noted mainly in impoundments. Multidimensional scaling of the site characteristics identified dams or weirs with back-ups exceeding 20 km as being relatively similar in impacts on prey diversity. Canonical correspondence analysis identified factors of habitat alteration and turtle size as major determinants of underlying variance in the diets. The results suggest that turtle food webs are altered by river regulation. A general finding that turtle diets in impoundments were depleted of aquatic plants or macro-invertebrates pertains to other turtles of conservation importance.

Widespread reproductive variation in North American turtles: temperature, egg size and optimality

Theory predicts the existence of an optimal offspring size that balances the trade-off between offspring fitness and offspring number. However, in wild populations of many species, egg size can still vary from year to year for unknown reasons. Here, we hypothesize that among-year variation in population mean egg size of freshwater turtles is partly a consequence of their gonadal sensitivity to seasonal temperatures, a physiological mechanism which principally functions to synchronize reproduction with a favorable time of year. As part of this process, among-year variation in seasonal temperatures modifies the extent of egg follicle development, and this may translate into variation in mean egg size among years (both at the individual and population level). To test this hypothesis, we applied an information-theoretic approach to model relationships between mean egg mass and the temperatures experienced during discrete periods of follicular development in wild populations of three turtle species (Chrysemys picta, Chelydra serpentina, Glyptemys insculpta) over 12 consecutive years. Because follicular development occurs in the fall for C. serpentina and G. insculpta, whereas it occurs both in the fall and spring for C. picta, we expected only fall temperatures would explain egg size variation in C. serpentina and G. insculpta, whereas both fall and spring temperatures would correlate with egg size variation in C. picta. These predictions were upheld. We then compared among-year variation in within-female egg and clutch sizes of each species in order to evaluate whether such variation might still be consistent with some tenets of optimal egg size theory. In all three species, we found that clutch sizes vary more than egg sizes in spite of temperature-induced egg size variation, and this pattern of relatively high clutch-size variation matches theoretical predictions. Future work should explore the roles of direct and indirect (i.e., nutritional) influences of temperature on egg size in natural settings.