Long-lived Reptiles Research Articles

Culex erraticus (Diptera: Culicidae) utilizes gopher tortoise burrows for overwintering in North Central Florida.

Mosquito-borne diseases represent a significant threat to human and animal health in the United States. Several viruses, including West Nile, Saint Louis encephalitis, and Eastern equine encephalitis are endemic. In humans, the disease is typically detected during the summer months, but not during the winter months. The ability of these viruses to reemerge year after year is still not fully understood, but typically involves persistence in a reservoir host or vector during periods of low transmission. Mosquito species are known to overwinter at different life stages (adults, larvae, or eggs) in manufactured or natural sites. Gopher tortoise burrows are known to serve as refuge for many vertebrate and invertebrate species in pine savannas. In this study, we surveyed the interior of gopher tortoise burrows for overwintering mosquitoes. We identified 4 species (Anopheles crucians s.l., Culex erraticus, Mansonia dyari, and Uranotaenia sapphirina). Cx. erraticus was the most abundant, and its presence and abundance increased in winter months, implying that this species utilized gopher tortoise burrows for overwintering. Bloodfed Cx. erraticus and An. crucians s.l. females were detected. While An. crucians s.l. fed exclusively on the white-tailed deer, Cx. erraticus had a more diverse host range but fed primarily on the gopher tortoise. Tortoises and other long-lived reptiles like the American alligator have been shown to sustain high viremia following West Nile virus (WNV) and Eastern equine encephalitis virus (EEEV) infection and therefore could play a role in the maintenance of these viruses. In addition, Cx. erraticus is naturally infected with WNV and is a known bridge vector for EEEV. As such, these overwintering sites may play a role in perpetuating over-winter arboviral activity in Florida.

Assessing spatial distribution, genetic variants, and virulence of pathogen Mycoplasma agassizii in threatened Mojave desert tortoises.

Mojave desert tortoises (Gopherus agassizii), a threatened species under the US Endangered Species Act, are long-lived reptiles that experience a chronic respiratory disease. The virulence of primary etiologic agent, Mycoplasma agassizii, remains poorly understood, but it exhibits temporal and geographic variability in causing disease outbreaks in host tortoises. Multiple attempts to culture and characterize the diversity of M. agassizii have had minimal success, even though this opportunistic pathogen chronically persists in nearly every population of Mojave desert tortoises. The current geographic range and the molecular mechanisms of virulence of the type-strain, PS6T, are unknown, and the bacterium is thought to have low-to-moderate virulence. We designed a quantitative polymerase chain reaction (qPCR) targeting three putative virulence genes annotated on the PS6T genome as exo-α-sialidases, enzymes which facilitate growth in many bacterial pathogens. We tested 140 M. agassizii-positive DNA samples collected from 2010 to 2012 across the range of Mojave desert tortoises. We found evidence of multiple-strain infections within hosts. We also found the prevalence of these sialidase-encoding genes to be highest in tortoise populations surrounding southern Nevada, the area from which PS6T was originally isolated. We found a general pattern of loss or reduced presence of sialidase among strains, even within a single host. However, in samples that were positive for any of the putative sialidase genes, one particular gene (528), was positively associated with bacterial loads of M. agassizii and may act as a growth factor for the bacterium. Our results suggest three evolutionary patterns: (1) high levels of variation, possibly due to neutral changes and chronic persistence, (2) a trade-off between moderate virulence and transmission, and (3) selection against virulence in environmental conditions known to be physiologically stressful to the host. Our approach of quantifying genetic variation via qPCR represents a useful model of studying host-pathogen dynamics.

Growth and Survival of Wild and Head-Started Blanding's Turtles (Emydoidea blandingii)

Blanding's Turtles (International Union for Conservation of Nature [IUCN] Endangered) are long-lived reptiles with delayed sexual maturity. Anthropogenic landscape changes have increased threats to juvenile turtles, resulting in unnaturally low recruitment. Head-starting has become a popular conservation strategy that aims to increase juvenile recruitment by avoiding the increased predation of the vulnerable nest and hatchling age class. However, there is still debate about whether or not it is an effective management tool. Assessments of head-starting are becoming more prevalent, but long-term studies are needed to critically evaluate the success of such interventions. In particular, information is needed on how head-starts fare compared to wild-hatched turtles. The Lake County Forest Preserve District (LCFPD) in northeastern Illinois initiated a long-term capture–mark–recapture project in 2004. As of 2018, 127 wild-hatched juvenile turtles had been captured (59 of which had been captured in multiple years) and 148 adult turtles had been captured (116 of which had been recaptured in multiple years). Since 2010, LCFPD has released 491 headstarted turtles during the year following hatching, 138 of which have been recaptured during successive years. We used von Bertalanffy growth analysis to compare growth trajectories and Cormack-Jolly-Seber modeling techniques to compare survival rates of wild-hatched and head-started turtles. At release, head-started turtles were about the size of two-year-old wild-hatched turtles and grew in parallel to their wild-hatched counterparts. The top-ranked survival models demonstrated that survival increased with age for both wild-hatched (71–98%) and head-started turtles (63–90%), with overlapping confidence intervals. These results suggest that head-started juveniles perform similarly to like-aged wild-hatched juveniles despite head-starts having attained greater body size. We estimated adult survival to be 95% with an environmental variance of 0.0011 and stable or positive population growth (k). Although the success of head-starting cannot be fully assessed until turtles are recruited into the adult population and successfully reproduce, patterns of head-start growth and survival provide positive intermediate measures of success. Our estimation of juvenile and adult survival, along with other demographic information from this population, will provide for more accurate population projections that will aid in evaluating conservation strategies for this population and potentially for Blanding's Turtles elsewhere.

Gut microbiome-immune system interaction in reptiles.

Reptiles are ectothermic amniotes in a world dominated by endotherms. Reptiles originated more than 300million years ago and they often dwell in polluted environments which may expose them to pathogenic micro-organisms, radiation and/or heavy metals. Reptiles also possess greater longevity and may live much longer than similar-sized land mammals, for example, turtles, tortoises, crocodiles and tuatara are long-lived reptiles living up to 100years or more. Many recent studies have emphasized the pivotal role of the gut microbiome on its host; thus, we postulated that reptilian gut microbiome and/or its metabolites and the interplay with their robust immune system may contribute to their longevity and overall hardiness. Herein, we discuss the composition of the reptilian gut microbiome, immune system-gut microbiome cross-talk, antimicrobial peptides, reptilian resistance to infectious diseases and cancer, ageing, as well the current knowledge of the genome and epigenome of these remarkable species. Preliminary studies have demonstrated that microbial gut flora of reptiles such as crocodiles, tortoises, water monitor lizard and python exhibit remarkable anticancer and antibacterial properties, as well as comprise novel gut bacterial metabolites and antimicrobial peptides. The underlying mechanisms between the gut microbiome and the immune system may hold clues to developing new therapies overall for health, and possible extrapolation to exploit the ancient defence systems of reptiles for Homo sapiens benefit.

A full life cycle Dynamic Energy Budget (DEB) model for the green sea turtle (Chelonia mydas) fitted to data on embryonic development

The complex life history of sea turtles presents challenges for researchers. These slow-growing, long-lived reptiles occupy several habitats throughout their life cycle, including oceanic environments where they are difficult to study. Consequently, much research on sea turtle biology has focussed on the nesting environment. Yet, to effectively manage sea turtle populations, the documentation of processes such as growth, maturation and reproductive output is critical to understanding their population dynamics. Dynamic Energy Budget (DEB) theory provides a mechanistic framework for investigating such processes. Here we developed a full life cycle DEB model for a green turtle (Chelonia mydas) population from Western Australia, where the model was based primarily on data collected on the embryonic life stage. The model provided a good fit to empirical data, including the limited data on juvenile and adult growth, and suggested that it is practical to investigate full life cycle energetics based on embryonic data that are comparatively easy to collect. Maternal effects on embryonic development and patterns of energy allocation throughout the life cycle were inferred from the DEB model. This is the first DEB model developed for C. mydas, and its potential applications include demonstrating how maturation, maintenance and reproduction of wild individuals depend on food availability and temperature. Such applications are especially valuable for understanding future impacts of climate change, and how best to manage this iconic species that plays a key role in marine ecosystems across the globe.

Life histories and conservation of long-lived reptiles, an illustration with the American crocodile (Crocodylus acutus).

Successful species conservation is dependent on adequate estimates of population dynamics, but age-specific demographics are generally lacking for many long-lived iteroparous species such as large reptiles. Accurate demographic information allows estimation of population growth rate, as well as projection of future population sizes and quantitative analyses of fitness trade-offs involved in the evolution of life-history strategies. Here, a long-term capture-recapture study was conducted from 1978 to 2014 on the American crocodile (Crocodylus acutus) in southern Florida. Over the study period, 7,427 hatchlings were marked and 380 individuals were recaptured for as many as 25 years. We estimated survival to be strongly age dependent with hatchlings having the lowest survival rates (16%) but increasing to nearly 90% at adulthood based on mark-recapture models. More than 5% of the female population were predicted to be reproductive by age 8 years; the age-specific proportion of reproductive females steadily increased until age 18 when more than 95% of females were predicted to be reproductive. Population growth rate, estimated from a Leslie-Lefkovitch stage-class model, showed a positive annual growth rate of 4% over the study period. Using a prospective sensitivity analysis, we revealed that the adult stage, as expected, was the most critical stage for population growth rate; however, the survival of younger crocodiles before they became reproductive also had a surprisingly high elasticity. We found that variation in age-specific fecundity has very limited impact on population growth rate in American crocodiles. We used a comparative approach to show that the original life-history strategy of American crocodiles is actually shared by other large, long-lived reptiles: while adult survival rates always have a large impact on population growth, this decreases with declining increasing growth rates, in favour of a higher elasticity of the juvenile stage. Crocodiles, as a long-lived and highly fecund species, deviate from the usual association of life histories of "slow" species. Current management practices are focused on nests and hatchling survival; however, protection efforts that extend to juvenile crocodiles would be most effective for conservation of the species, especially in an ever-developing landscape.

Phenotypic plasticity of nest timing in a post-glacial landscape: how do reptiles adapt to seasonal time constraints?

Life histories evolve in response to constraints on the time available for growth and development. Nesting date and its plasticity in response to spring temperature may therefore be important components of fitness in oviparous ectotherms near their northern range limit, as reproducing early provides more time for embryos to complete development before winter. We used data collected over several decades to compare air temperature and nest date plasticity in populations of painted turtles and snapping turtles from a relatively warm environment (southeastern Michigan) near the southern extent of the last glacial maximum to a relatively cool environment (central Ontario) near the northern extent of post-glacial recolonization. For painted turtles, population-level differences in reaction norm elevation for two phenological traits were consistent with adaptation to time constraints, but no differences in reaction norm slopes were observed. For snapping turtle populations, the difference in reaction norm elevation for a single phenological trait was in the opposite direction of what was expected under adaptation to time constraints, and no difference in reaction norm slope was observed. Finally, among-individual variation in individual plasticity for nesting date was detected only in the northern population of snapping turtles, suggesting that reaction norms are less canalized in this northern population. Overall, we observed evidence of phenological adaptation, and possibly maladaptation, to time constraints in long-lived reptiles. Where present, (mal)adaptation occurred by virtue of differences in reaction norm elevation, not reaction norm slope. Glacial history, generation time, and genetic constraint may all play an important role in the evolution of phenological timing and its plasticity in long-lived reptiles.

Temperature-independent, seasonal fluctuations in immune function of the Mojave Desert Tortoise (Gopherusagassizii)

As long-lived reptiles, Mojave Desert Tortoises (Gopherus agassizii (Cooper, 1861)) are expected to make substantial energetic investments in immune defense. This species also has many adaptations to living in an arid environment characterized by seasonal extremes in temperature and resource availability. By housing G. agassizii at a controlled, constant ambient temperature, we quantified predominantly temperature-independent, seasonal fluctuations in innate immune function and circulating leukocytes in a reptile. We found a decrease in bacteriocidal activity of the blood plasma in winter, with reduced function lasting into the spring. Lymphocyte numbers were elevated in fall and winter, whereas eosinophil numbers increased in summer. Thus, properties of the immune system were up- or down-regulated in different directions across the seasons. We found a much higher level of variation of leukocyte profiles among individuals than has previously been reported for other chelonians. Heterophil:lymphocyte ratios (indicative of chronic glucocorticoid levels) were not associated with any measure of immune function, and thus glucocorticoid does not seem to mediate the observed seasonal changes. We propose a new hypothesis to explain seasonal changes in immune function, based on seasonal resource limitation in the Mojave Desert, including the availability of dietary protein, energy, and opportunities for thermal regulation.

Growth rates of black caiman (Melanosuchus niger) and spectacled caiman (Caiman crocodilus) from two different Amazonian flooded habitats

Rates of growth and survival in wild populations are affected by the physical environment, biotic interactions, and density-dependent processes, such as growth and fecundity. However, the relative importance of these factors in long-lived reptiles is poorly understood. We analyzed growth rates of Melanosuchus niger and Caiman crocodilus coexisting in two areas of the Brazilian Amazon with very different environmental characteristics. Growth rates of Caiman crocodilus at the two sites were similar, but M. niger grew more slowly in the area with higher productivity and higher density of caimans. Growth rates of the same species from other sites and of the temperate-zone Alligator mississippiensis indicate large differences among sites, but little evidence that these differences are primarily due to differences in productivity or temperature. Demographic models used to estimate sustained yields from caiman harvests should take into account the likely importance of density-dependent growth.

Large Male Advantage: Phenotypic and Genetic Correlates of Territoriality in Tuatara

In reptiles, phenotypic measures such as body size usually predict a male’s success in territorial interactions. Recent evidence from fish, birds, and mammals has shown that genetic heterozygosity also has a strong influence on competitive ability and territory quality. Here, we provide a comprehensive assessment of the social structure and factors affecting male territory quality and aggressive behavior in a dense population of Tuatara, a long-lived reptile that maintains long-term territories, on Stephens Island, New Zealand. The only significant predictor of female access and competitive ability was male body size, and there was no relationship between male body size or condition and individual genetic heterozygosity. Body size, body condition, and heterozygosity did not predict territory size. Also, heterozygosity, body condition, and territory size had no relationship with the number of females to which a male had access. Large males were more effective at (1) monopolizing areas where females were most dense and (2) guarding females by consistently winning aggressive encounters with other males. Our finding of no relationship between territoriality and heterozygosity probably reflects the genetic background of this large, outbred population or that behavioral attributes or neutral heterozygosity are not appropriate individual fitness correlates for these long-lived reptiles.

Fine‐scale genetic structure of a long‐lived reptile reflects recent habitat modification

Anthropogenic habitat fragmentation--ubiquitous in modern ecosystems--has strong impacts on gene flow and genetic population structure. Reptiles may be particularly susceptible to the effects of fragmentation because of their extreme sensitivity to environmental conditions and limited dispersal. We investigate fine-scale spatial genetic structure, individual relatedness, and sex-biased dispersal in a large population of a long-lived reptile (tuatara, Sphenodon punctatus) on a recently fragmented island. We genotyped individuals from remnant forest, regenerating forest, and grassland pasture sites at seven microsatellite loci and found significant genetic structuring (R(ST)=0.012) across small distances (<500 m). Isolation by distance was not evident, but rather, genetic distance was weakly correlated with habitat similarity. Only individuals in forest fragments were correctly assignable to their site of origin, and individual pairwise relatedness in one fragment was significantly higher than expected. We did not detect sex-biased dispersal, but natural dispersal patterns may be confounded by fragmentation. Assignment tests showed that reforestation appears to have provided refuges for tuatara from disturbed areas. Our results suggest that fine-scale genetic structuring is driven by recent habitat modification and compounded by the sedentary lifestyle of these long-lived reptiles. Extreme longevity, large population size, simple social structure and random dispersal are not strong enough to counteract the genetic structure caused by a sedentary lifestyle. We suspect that fine-scale spatial genetic structuring could occur in any sedentary species with limited dispersal, making them more susceptible to the effects of fragmentation.

Risk of extinction and categories of endangerment: perspectives from long-lived reptiles

Risk of extinction and categories of endangerment: perspectives from long-lived reptiles

Efficacy of Using Scute Annuli to Determine Growth Histories and Age of Gopherus polyphemus in Southern Alabama

Complete demographic studies require an accurate determination of age and age-specific growth rates of individuals in a population. Lifehistory studies of reptiles have shown that individual growth rates influence key traits such as age at maturity and survivorship (Cagle, 1946; Tinkle et al., 1970; Iverson, 1991). Longlived reptiles, such as the gopher tortoise, Gopherus polyphemus, often require 10-20 yr to reach reproductive maturity. The growth interval required for tortoises to attain maturity varies among populations with such factors as climate and habitat quality (Landers et al., 1982; Mushinsky et al., 1994; Aresco, 1998). Life-history traits such as delayed sexual maturity must be considered in the design and implementation of conservation plans for long-lived organisms (Congdon et al., 1993). Because of habitat loss and habitat degradation, gopher tortoise populations have suffered a dramatic decline and are legally protected over much of their range (Landers and Speake, 1980; Diemer, 1986). A reliable method of assessing growth rates and age is necessary to determine population structure and age at maturity and is therefore vital to the management and recovery of gopher tortoise populations. Growth and age in turtles have been studied primarily using either external scute annuli or recapture of marked individuals. Mark-recapture studies of known-age turtles provide the most reliable data on age-specific growth rates. This method requires a long-term capture-recapture program (often at least 20 yr) to follow individuals from the hatchling stage through sexual maturity (Gibbons, 1987; Frazer et al., 1990). However, many chelonians display growth annuli on epidermal scutes that allow an estimation of age and growth rates (Gibbons, 1987; Zug, 1991). To be useful for age determination, it must be demonstrated that growth lines are formed annually. Galbraith and Brooks (1987) cautioned that the accuracy of using scute annuli to estimate age must be established before using the method in a study population. Annulus counting also has been shown to be age-limited in its usefulness (Brooks et al., 1997). Extremely slow growth of older individuals causes growth lines to overlap with one another and appear indistinct (Germano, 1992; Mushinsky et al., 1994). Woodbury and Hardy (1948) reported that number of scute annuli did not correlate well with age of the desert tortoise (G. agassizii). Legler (1960) and Germano (1988) concluded that this discrepancy was caused by the inclusion of old tortoises which were past the age of regular growth. Sergeev (1937) devised a method for estimating a series of previous carapace lengths from measurements of the length of growth lines on the plastral scutes. This method has been modified to estimate annual growth rates in ornate box turtles (Terrapene ornata; Legler, 1960) and desert tortoises (G. agassizii; Germano, 1988, 1994). Landers et al. (1982) developed a similar method based on the width of each growth annulus. These methods may be used to supplement or replace mark-recapture data for calculating growth rates and generating growth

Identification of female tuatara in ovulatory condition using plasma sex steroid concentrations

Abstract Female tuatara (Sphenodon punctatus) are long-lived reptiles that do not nest each year. Identification of gravid females (those carrying oviducal eggs) has in the past been achieved only with radiography or laparoscopy. Both techniques require considerable equipment and/or operator skill, and both pose an unquantified degree of risk to tuatara. For these reasons, we sought to develop a blood test for identifying females destined to nest in a given year. We observed that on Stephens Island, New Zealand, those females that nested in a given year had significantly higher plasma concentrations of oestradiol, testosterone, and progesterone at the time of mating-ovulation (February-March) than in those females that did not ovulate and nest that year. Used in conjunction, the criteria of a testosterone concentration ≥2.0 ng/ml or a progesterone concentration ≥5.8 ng/ml in plasma, obtained within 3 h of capture in February–March, can be used to distinguish females destined to nest that year from those that do not nest. These criteria may prove useful for comparing nesting frequencies among years on Stephens Island, for comparing nesting frequencies among different islands, and for monitoring reproduction in captivity. To our knowledge, this is the ftrst time that plasma hormone concentrations have been used to identify nesting females in reptiles that do not nest each year.

Crocodylus johnstoni in the McKinlay Area, N.T. VIII. A Population Simulation Model

A population simulation model was constructed for the McKinlay River population of the freshwater crocodile Crocodylus johnstoni. C, johnstoni are long-lived reptiles (50 y +) which take 9-16 years to reach maturity. As a consequence, the effects of legal hunting before 1963 are still reflected in an unstable population age structure. No quantitative data on the original population densities exist, nor are there data on the densities at which equilibrium can be expected in the future. The model examines the dynamics of a population which is still expanding and does not take into account density-dependent factors which may ultimately limit the population. If the population was undisturbed for 10 years, the model predicted the mean natural rate of population increase would be 1 .5% per annum. However, the population has been disturbed during research activities, and when these disturbances and manipulations were simulated the model predicted a 4% decrease in the population between 1979 and 1983. Independent surveys in both 1979 and 1983 indicated a 5% decrease in the population. This consistency has been interpreted as indicating that the model's predictions are not grossly erroneous. Sensitivity tests were carried out in which most parameters in the model were independently varied by plus or minus their estimated error, while other parameters were held constant. The resultant changes in the estimated population size after 10 years indicated the model was most sensitive to the age-specific mortality estimates. C. johnstoni management has the conservation requirement of maintaining or even enhancing the density of wild populations. Sustained-yield harvesting can theoretically be achieved without compromising this requirement by harvesting eggs and/or hatchlings, and later returning a proportion of the harvest to the population when they are larger and have a greater probability of surviving. When released into the wild, captive-raised C. johnstoni survive as well as wild ones of equivalent sizes. The model was used to simulate egg and hatchling harvests with different collection and return rates, and different ages of returned animals. It was also used to simulate harvests of post-hatchling crocodiles, without a return of captiveraised animals.

Mirex residues in eggs and livers of two long-lived reptiles (Chrysemys scripta and Terrapene carolina) in Mississippi, 1970-1977.

Mirex residues in eggs and livers of two long-lived reptiles (Chrysemys scripta and Terrapene carolina) in Mississippi, 1970-1977.