Health Of Wildlife Populations Research Articles

Factors Influencing Salamander Hormones in Created Wetlands

Small isolated wetlands are critical breeding habitat for many amphibians. Many of these wetlands are susceptible to destruction due to lack of regulation. Therefore, it is important to create new suitable habitat that can sustain healthy wildlife populations to mitigate loss. Concentrations of corticosterone, a hormone related to stress, development, and growth, can indicate a populations’ health and habitat quality. The objective of our research was to determine if certain habitat characteristics in created wetlands influence corticosterone levels of Spotted Salamander (Ambystoma maculatum) larvae. From May-June in 2015 and 2016, we sampled Spotted Salamander larvae (n=10/wetland) in 26 wetlands created between 2011 and 2014 in the Monongahela National Forest, West Virginia. We collected environmental data including vegetation cover and water quality at each wetland. Analysis indicates that larval total length and wetland water temperature are positively correlated with corticosterone and the best predictors for corticosterone levels. Vegetation cover also showed a positive correlation with corticosterone levels. Our results can improve understanding of the relationship between physiological conditions and habitat quality and how it can be used to interpret wildlife population health and habitat degradation.

Eimeria spp. infecting quenda (Isoodon obesulus) in the greater Perth region, Western Australia

Parasites of wildlife inhabiting urbanised and peri-urban environments are of interest regarding wildlife population health, and also veterinary public health in the case of parasites that can also infect humans and domestic animals. This study aimed to: identify, and estimate the prevalence of, species of Eimeria parasitic in quenda (Isoodon obesulus) in the greater Perth region, Western Australia; 2) morphologically describe and genetically characterise a novel observed species of Eimeria as E. angustus; and 3) genetically characterise E. kanyana. Eimeria spp. prevalence was 76.1% (95% CI 64.9–84.5%), and four putative species of Eimeria were identified. Eimeria kanyana was identified infecting quenda for the first time, with a prevalence of 54.9% (43.4–66.0%). Eimeria quenda was less prevalent, at 7.0% (3.1–15.5%). The novel species E. angustus was present in 45.1% of sampled quenda (34.0–56.6%). A second novel morphotype of Eimeria was present in 2.8% of sampled quenda (0.9–9.7%). Mixed Eimeria spp. infections were present in 21/71 quenda (29.6%, 95% CI 20.2–41.1%). Molecular phylogenetic analyses of E. kanyana and E. angustus were conducted at the 18S rRNA and mitochondrial cytochrome oxidase loci. At both loci, two isolates identified as E. kanyana grouped in a phylogenetic clade with E. trichosuri. Five isolates identified as the novel E. angustus were most closely related to E. tropidura at the 18S locus. At the COI locus, no sequence data were available for E. tropidura; isolates of E. angustus grouped with E. sciurorum.

Variation in the intensity and prevalence of macroparasites in migratory caribou: a quasi-circumpolar study

Comparative studies across time and geographical regions are useful to improve our understanding of the health of wildlife populations. Our goal was to study parasitism in migratory caribou (Rangifer tarandus (L., 1758)) of North America and Greenland. A total of 1507 caribou were sampled across 12 herds to assess seven of their main helminth and arthropod macroparasites between 1978 and 2010. We sought to determine which factors such as sex, age class, herd size, and season best explained the prevalence and intensity of those parasites. Intensity of warble fly (Hypoderma tarandi (L., 1758)) larvae increased with age for males, whereas the opposite was observed in females. Prevalence of giant liver flukes (Fascioloides magna (Bassi, 1875) Ward, 1917), tapeworm Taenia hydatigena Pallas, 1766, and nose bot fly (Cephenemyia trompe (Modeer, 1786)) larvae was higher in adults than in calves. Prevalence of F. magna and T. hydatigena was higher at high herd size than at lower herd size. Greenland herds had the lowest prevalence of T. hydatigena and of the tapeworm Taenia krabbei Moniez, 1879, a higher intensity of H. tarandi, and a higher prevalence of C. trompe than the other herds. Of the herds from Quebec and Labrador, the Rivière-George herd had a higher prevalence of F. magna than the Rivière-aux-Feuilles herd. Our research provides the first comparative survey of these parasites of caribou across a broad spatial–temporal range.

Characterizing predictors of survival in the American pika (Ochotona princeps)

The measurement of stress hormone (glucocorticoid [GC]) concentration is increasingly used to assess the health of wildlife populations. However, for many species, we do not have a good understanding of the range of GC concentrations that might indicate a compromised individual. A temporary increase in GC concentration can prompt the adoption of behavior or activities to promote individual survival. However, chronic GC elevation results in deleterious effects on health that can reduce survival. In order to use GC concentration as a metric of individual fitness for a given species, it will be necessary to relate individual demographic rates to GC concentration. We related survival in American pikas ( Ochotona princeps ) to 2 different stress metrics, glucocorticoid metabolite (GCM) concentration in fecal samples and GC concentration in plasma samples. Annual survival was analyzed in relation to each of these stress metrics as well as other physiological metrics and habitat characteristics at several sites in the Rocky Mountains. Among the predictors considered, GCM concentration was by far the strongest predictor of annual survival in pikas, and individuals with higher baseline GCM were less likely to survive. Our metric of flea load was also negatively related to annual survival. Given the limited time and resources that characterize many wildlife conservation projects, it is important to establish which endocrine metrics are the most informative for a species. American pikas have been identified as a sentinel species for detecting effects of climate change, and several correlational studies have projected range contraction for the species. Our results suggest that more mechanistic projections might be possible given further study of the relationship between GCM and climate. Our approach contributes to a better understanding of factors affecting survival in this species and provides a basis for further research relating individual stress response and survival to environmental change.

Inferring social structure and its drivers from refuge use in the desert tortoise, a relatively solitary species

For several species, refuges (such as burrows, dens, roosts, nests) are an essential resource for protection from predators and extreme environmental conditions. Refuges also serve as focal sites for social interactions, including mating, courtship, and aggression. Knowledge of refuge use patterns can therefore provide information about social structure, mating, and foraging success, as well as the robustness and health of wildlife populations, especially for species considered to be relatively solitary. In this study, we construct networks of burrow use to infer social associations in a threatened wildlife species typically considered solitary—the desert tortoise. We show that tortoise social networks are significantly different than null networks of random associations, and have moderate spatial constraints. We next use statistical models to identify major mechanisms behind individual-level variation in tortoise burrow use, popularity of burrows in desert tortoise habitat, and test for stressor-driven changes in refuge use patterns. We show that seasonal variation has a strong impact on tortoise burrow switching behavior. On the other hand, burrow age and topographical condition influence the number of tortoises visiting a burrow in desert tortoise habitat. Of three major population stressors affecting this species (translocation, drought, disease), translocation alters tortoise burrow switching behavior, with translocated animals visiting fewer unique burrows than residents. In a species that is not social, our study highlights the importance of leveraging refuge use behavior to study the presence of and mechanisms behind non-random social structure and individual-level variation. Our analysis of the impact of stressors on refuge-based social structure further emphasizes the potential of this method to detect environmental or anthropogenic disturbances. Adaptive and social behavior that affects fitness is now being increasingly incorporated in the conservation and management of wildlife species. However, direct observations of social interactions in species considered to be solitary are difficult, and therefore integration of behavior in conservation and management decisions in such species has been infrequent. For such species, we propose quantifying refuge use behavior as it can provide insights towards their (hidden) social structure, establish relevant contact patterns of infectious disease spread, and provide early warning signals of population stressors. Our study highlights this approach in a long-lived and threatened species, the desert tortoise. We provide evidence toward the presence of and identify mechanisms behind the social structure in desert tortoises formed by their burrow use preferences. We also show how individuals burrow use behavior responds to the presence of population stressors.

Geophysiology of Wood Frogs: Landscape Patterns of Prevalence of Disease and Circulating Hormone Concentrations across the Eastern Range.

One of the major challenges for conservation physiologists is to determine how current or future environmental conditions relate to the health of animals at the population level. In this study, we measured prevalence of disease, mean condition of the body, and mean resting levels of corticosterone and testosterone in a total of 28 populations across the years 2011 and 2012, and correlated these measures of health to climatic suitability of habitat, using estimates from a model of the ecological niche of the wood frog's geographic range. Using the core-periphery hypothesis as a theoretical framework, we predicted a higher prevalence and intensity of infection of Batrachochytrium dendrobatidis (Bd) and ranaviruses, two major amphibian pathogens causing disease, and higher resting levels of circulating corticosterone, an indicator of allostatic load incurred from living in marginal habitats. We found that Bd infections were rare (2% of individuals tested), while infections with ranavirus were much more common: ranavirus-infected individuals were found in 92% of ponds tested over the 2 years. Contrary to our predictions, rates of infection with ranaviruses were positively correlated with quality of the habitat with the highest prevalence at the core of the range, and plasma corticosterone concentrations measured when frogs were at rest were not correlated with quality of the habitat, the prevalence of ranavirus, or the intensity of infection. Prevalence and mean viral titers of ranavirus infection were higher in 2012 than in 2011, which coincided with lower levels of circulating corticosterone and testosterone and an extremely early time of breeding due to relatively higher temperatures during the winter. In addition, the odds of having a ranavirus infection increased with decreased body condition, and if animals had an infection, viral titers were positively correlated to levels of circulating testosterone concentration. By resolving these patterns, experiments can be designed to test hypotheses about the mechanisms that produce them, such as whether transmission of the ranavirus and tolerance of the host are greater or whether virulence is lower in populations within core habitats. While there is debate about which metrics serve as the best bioindicators of population health, the findings of this study demonstrate the importance of long-term monitoring of multiple physiological parameters to better understand the dynamic relationship between the environment and the health of wildlife populations over space and time.

Parasites and wildlife in a changing world

Wildlife and their parasites are everywhere. Other than those important in human or domestic animal health, understanding of these host-parasite systems is limited, especially their roles in wildlife population health. Learning more provides opportunities to explore infection ecology, and the use of parasites as sentinels and probes for environmental change.

Traffic noise causes physiological stress and impairs breeding migration behaviour in frogs.

Human-generated noise has profoundly changed natural soundscapes in aquatic and terrestrial ecosystems, imposing novel pressures on ecological processes. Despite interest in identifying the ecological consequences of these altered soundscapes, little is known about the sublethal impacts on wildlife population health and individual fitness. We present evidence that noise induces a physiological stress response in an amphibian and impairs mate attraction in the natural environment. Traffic noise increased levels of a stress-relevant glucocorticoid hormone (corticosterone) in female wood frogs (Lithobates sylvaticus) and impaired female travel towards a male breeding chorus in the field, providing insight into the sublethal consequences of acoustic habitat loss. Given that prolonged elevated levels of corticosterone can have deleterious consequences on survival and reproduction and that impaired mate attraction can impact population persistence, our results suggest a novel pathway by which human activities may be imposing population-level impacts on globally declining amphibians.

Cortisol profiles in sockeye salmon: Sample bias and baseline values at migration, maturation, spawning, and senescence

Accurate and controlled methods to measure physiological stress are crucial to effectively monitor and assess the health of wildlife populations and evaluate resilience to external stressors. Glucocorticoids, particularly cortisol, are frequently used to measure stress in fish. While measurements of cortisol concentrations provide a powerful indicator of physiological stress, there are important considerations in accurately measuring and interpreting results. We assessed methods to capture and sample wild populations of salmonids and evaluated potential biases from sampling disturbance. We present results of a stress series and suggest approaches to mitigate bias associated with sampling disturbance. Studies on physiological stress in salmonids often focus on particular life stages (e.g. outward migration to marine waters, return migration to freshwater systems), or processes (e.g. fisheries interactions, spawning success), characterized by dramatic physiological challenges related to the developmental stage of the fish and the external environment. Such pressures influence baseline cortisol levels and complicate efforts to interpret the effects of additional external stressors. We present a profile for naturally occurring shifts in cortisol levels at migration, reproductive maturation, spawning, and senescence. This profile provides a crucial baseline for use as reference in evaluating physiological stress in Pacific salmon during crucial life stages. Our findings provide guidance for sampling wild salmonids and highlight the need for caution in interpreting cortisol in the context of physical challenges and physiological developments relevant to their complex life history.

The impact of management practices and past demographic history on the genetic diversity of red deer (Cervus elaphus): an assessment of population and individual fitness

This work was supported by Plan Nacional AGL2010-20730 and FAU2008-00004 grants from Ministerio de Economia y Competitividad, the EU FP7 Grant ANTIGONE (project number 278976), and Harmonised Approaches in monitoring wildlife Population Health, And Ecology and Abundance (APHAEA,project number 219235_FP7_ERA-NET_EMIDA). J.Queiros was supported by a PhD grant (SFRH/BD/73732/2010) funded by the Fundacao para a Ciencia e Tecnologia of the Ministerio da Ciencia, Tecnologia e Ensino Superior of the Portuguese government.

Perfluoroalkyl acids in the Canadian environment: Multi-media assessment of current status and trends

In Canada, perfluoroalkyl acids (PFAAs) have been the focus of several monitoring programs and research and surveillance studies. Here, we integrate recent data and perform a multi-media assessment to examine the current status and ongoing trends of PFAAs in Canada. Concentrations of perfluorooctane sulfonate (PFOS), perfluorooctanoate (PFOA), and other long-chain perfluorocarboxylates (PFCAs) in air, water, sediment, fish, and birds across Canada are generally related to urbanization, with elevated concentrations observed around cities, especially in southern Ontario. PFOS levels in water, fish tissue, and bird eggs were below their respective Draft Federal Environmental Quality Guidelines, suggesting there is low potential for adverse effects to the environment/organisms examined. However, PFOS in fish and bird eggs tended to exceed guidelines for the protection of mammalian and avian consumers, suggesting a potential risk to their wildlife predators, although wildlife population health assessments are needed to determine whether negative impacts are actually occurring. Long-term temporal trends of PFOS in suspended sediment, sediment cores, Lake Trout (Salvelinus namaycush), and Herring Gull (Larus argentatus) eggs collected from Lake Ontario increased consistently from the start of data collection until the 1990s. However, after this time, the trends varied by media, with concentrations stabilizing in Lake Trout and Herring Gull eggs, and decreasing and increasing in suspended sediment and the sediment cores, respectively. For PFCAs, concentrations in suspended sediment, sediment cores, and Herring Gulls generally increased from the start of data collection until present and concentrations in Lake Trout increased until the late 1990s and subsequently stabilized. A multimedia comparison of PFAA profiles provided evidence that unexpected patterns in biota of some of the lakes were due to unique source patterns rather than internal lake processes. High concentrations of PFAAs in the leachate and air of landfill sites, in the wastewater influent/effluent, biosolids, and air at wastewater treatment plants, and in indoor air and dust highlight the waste sector and current-use products (used primarily indoors) as ongoing sources of PFAAs to the Canadian environment. The results of this study demonstrate the utility of integrating data from different media. Simultaneous evaluation of spatial and temporal trends in multiple media allows inferences that would be impossible with data on only one medium. As such, more co-ordination among monitoring sites for different media is suggested for future sampling, especially at the northern sites. We emphasize the importance of continued monitoring of multiple-media for determining future responses of environmental PFAA concentrations to voluntary and regulatory actions.

Associations between perfluoroalkyl compounds and immune and clinical chemistry parameters in highly exposed bottlenose dolphins (Tursiops truncatus)

Perfluoroalkyl compounds (PFCs) are ubiquitous, persistent chemical contaminants found in the environment, wildlife, and humans. Despite the widespread occurrence of PFCs, little is known about the impact these contaminants have on the health of wildlife populations. The authors investigated the relationship between PFCs (including ∑perfluorocarboxylates, ∑perfluoroalkyl sulfonates, perfluorooctane sulfonate, perfluorooctanoic acid, and perfluorodecanoic acid) and the clinocopathologic and immune parameters in a highly exposed population (n = 79) of Atlantic bottlenose dolphins (mean ∑PFCs = 1970 ng/ml; range 574-8670 ng/ml) sampled from 2003 to 2005 near Charleston, South Carolina, USA. Age-adjusted linear regression models showed statistically significant positive associations between exposure to one or more of the PFC totals and/or individual analytes and the following immunological parameters: absolute numbers of CD2+ T cells, CD4+ helper T cells, CD19+ immature B cells, CD21+ mature B cells, CD2/CD21 ratio, MHCII+ cells, B cell proliferation, serum IgG1, granulocytic, and monocytic phagocytosis. Several PFC analyte groups were also positively associated with serum alanine aminotransferase, gamma-glutamyltransferase, creatinine, phosphorus, amylase, and anion gap and negatively associated with cholesterol levels, creatinine phosphokinase, eosinophils, and monocytes. Based on these relationships, the authors suggest that the PFC concentrations found in Charleston dolphins may have effects on immune, hematopoietic, kidney, and liver function. The results contribute to the emerging data on PFC health effects in this first study to describe associations between PFCs and health parameters in dolphins.

Evaluation of hair cortisol concentration as a biomarker of long‐term stress in free‐ranging polar bears

AbstractLong‐term physiological stress in individual animals may be an important mechanism linking ecological change with impaired wildlife population health. In the Southern Hudson Bay (SH) subpopulation of polar bears (Ursus maritimus), increasing stress associated with climate warming may be related to declining body condition. Accordingly, the development of tools to assess long‐term stress in this species may prove invaluable for conservation efforts in this threatened population. The measurement of hair cortisol concentration (HCC) has shown promise as a potential biomarker of long‐term stress in free‐ranging bears. However, to serve as a useful management tool, factors influencing HCC in polar bears must be identified and then revealed to establish linkages between environmental conditions and the fitness of individual animals. We determined HCC (median = 0.48 pg/mg [range = 0.16–2.26 pg/mg]) in 185 polar bears captured in southern Hudson Bay from 2007 to 2009. HCC was influenced by sex, family group status, and capture period but not by body region or hair type. Using models developed through a combination of hypothesis testing and information theory, we also determined that HCC was negatively associated with growth indices (length, mass, and body condition index) linked to fitness in polar bears. Additional research will be required across several polar bear populations to establish the utility of HCC as a tool for polar bear conservation. © 2012 The Wildlife Society.

Targeted surveillance for Usutu virus in British birds (2005–2011)

Veterinary RecordVolume 172, Issue 1 p. 17-17 Research Targeted surveillance for Usutu virus in British birds (2005–2011) D. L. Horton MA, VetMB, MSc, PhD, MRCVS, Corresponding Author D. L. Horton...

Arctic parasitology: why should we care?

The significant impact on human and animal health from parasitic infections in tropical regions is well known, but parasites of medical and veterinary importance are also found in the Arctic. Subsistence hunting and inadequate food inspection can expose people of the Arctic to foodborne parasites. Parasitic infections can influence the health of wildlife populations and thereby food security. The low ecological diversity that characterizes the Arctic imparts vulnerability. In addition, parasitic invasions and altered transmission of endemic parasites are evident and anticipated to continue under current climate changes, manifesting as pathogen range expansion, host switching, and/or disease emergence or reduction. However, Arctic ecosystems can provide useful models for understanding climate-induced shifts in host-parasite ecology in other regions.

Hair cortisol concentration as a noninvasive measure of long-term stress in free-ranging grizzly bears (Ursus arctos): considerations with implications for other wildlife

Human-caused landscape change negatively affects the sustainability of many wildlife populations. In Alberta, Canada, grizzly bears ( Ursus arctos L., 1758) live in one of the most populated and heavily exploited landscapes in which the species survives. Long-term physiological stress in individual animals may be the predominant mechanism linking landscape change with impaired wildlife population health. Hair cortisol concentration has been validated as a biomarker of long-term stress in humans and domestic animals; however, limited work has examined factors that may affect its measurement or interpretation. We have measured cortisol in as few as five guard hairs of a grizzly bear and have identified factors influencing hair cortisol concentration in this species. Hair cortisol varies with hair type, body region, and capture method. It is not influenced by colour, age, sex, environmental exposure (18 days), or prolonged laboratory storage (>1 year) and does not vary along the length of the hair shaft. Recommendations for prudent use of hair cortisol analysis in grizzly bears are discussed with implications for the development of hair cortisol concentration as a tool to monitor long-term stress in other wildlife.

Developing a non-invasive indicator of pinniped health: Neonate behavior and growth in California sea lions (Zalophus californianus)

Assessing the health of wildlife populations is critical to achieving conservation goals; however, proper assessments can be complicated when study sites are difficult to reach or when focal species are sensitive to human disturbance. Condition and growth of offspring may indicate population health, but obtaining such data generally relies on invasive techniques. Here, we examine the extent to which noninvasive observations of neonate behaviors could serve as a proxy for traditional approaches to estimating neonate body condition and growth of a wild pinniped, the California sea lion (Zalophus californianus), in the Gulf of California, Mexico. Generalized linear models and multiple linear regressions were used to examine the effect of sex, breeding island, year, and neonate behaviors (e.g., nursing, active) on body condition and growth rates. We found a strong correlation between individual growth rates and behaviors of male neonates. Males engaged in proportionally more active behaviors had lower growth rates (β = –0.0005), whereas males engaged in proportionally more nursing events had higher growth rates (β = 0.002). There was no relationship between neonate behavior and condition for either sex, nor between behavior and growth rates of female pups. These results provide insight into the possibility of using behavior as an indicator of individual status of males that could facilitate assessments of population status and illustrate the importance of using alternative approaches to measure animal condition for species conservation.

Histological Lesions in Mink Jaws Are a Highly Sensitive Biomarker of Effect After Exposure to TCDD-Like Chemicals: Field and Literature-Based Confirmations

The mink (Mustela vison) is one of the most sensitive mammals to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-like chemicals. By literature review we established that a histological lesion of the jaw bone of mink, evidenced by squamous epithelial hyperplasia in the gingival tissue that forms nests or cords that infiltrate the periodontal ligament and alveolar bone causing osteolysis of the mandible and maxilla that could lead to squamous cell carcinoma, is the most sensitive known biomarker of effect following exposure of mink to TCDD-like chemicals. Lesions have been observed when total TCDD toxic equivalents (TEQ: dioxins, furans, coplanar polychlorinated biphenyls or PCBs) in liver exceed 40 ng/kg wet weight (ww) or when total PCB exceeds 1698 ng/g ww. This is the second report of histological evidence of this lesion in wild-caught mink, and it is the first report of the lesion being grossly detectable in naturally exposed mink. Some mink living near the south shore of Lake Ontario (exposed to the lake's food web), but not inland mink (not exposed to the lake's food web), accumulate more than 40 ng total TEQ/kg or 1698 ng total PCB/kg in liver. Because of its sensitivity, the jaw lesion biomarker is very useful for assessing the health of wildlife populations exposed to TCDD-like chemicals.

Mountain Sheep: An Environmental Epidemiological Survey of Variation in Metal Exposure and Physiological Biomarkers Following Mine Development

Problem Statement: Anthropogenic activities, such as mining and industrial operations, are a major environmental source of metal exposure for wildlife. Quantitative data regarding biochemical effects of exposure to mineral extraction on mountain sheep, Ovis canadensis, are limited, although their habitats are becoming increasingly affected by expanded resource development. Decisions relating to environmental protection and wildlife conservation should be based on sound scientific understanding of the interaction of released metals with the biota. Approach: Because understanding of the biogeochemistry, food webs, and metals as stressors was limited for arid environments, scientists and managers need to take a monitoring approach that incorporates a longer time scale using resembling designs. We tested the hypothesis that mountain sheep exposed to mining activities would express elevated levels of biomarkers compared with a reference group that was not directly exposed to mining activities. During this study, conducted in the Mojave Desert of California, USA, from 1995-1997, we analyzed concentrations of selected metals in blood of mountain sheep as biomarkers of exposure using a simultaneous treatment-reference design. We included common physiological biomarkers of effect, such as haptoglobin and interleukin-6. These variables were measured in two subpopulations of female mountain sheep inhabiting distinct geographic areas (mined and non-mined) within a single mountain range over 2 years. Results: Although the sample size was small, metal exposure was significantly higher in mountain sheep inhabiting the mined area. Plasma protein levels in these sheep inhabiting the mined area were also significantly higher than for mountain sheep in the non-mined area. Conclusion: Local biomonitoring studies during mining and other developments were important sources of baseline information about the health of wildlife populations, which allowed managers to establish benchmarks related to exposure.

Exposures to Estradiol, Ethinylestradiol and Octylphenol Affect Survival and Growth of Rana pipiens and Rana sylvatica Tadpoles

Endocrine disrupting compounds (EDCs) are often detected in the aquatic environment and can negatively affect the health of wildlife populations. However, little is known about the sensitivity of native amphibians to EDCs. Wood frogs (Rana sylvatica) and Northern leopard frogs (Rana pipiens) were exposed to three estrogenic EDCs: estradiol (E2), ethinylestradiol (EE2), and 4-tert-octylphenol (OP). In addition, R. pipiens were exposed during two developmental stages (Gosner stages 26 and 36) to examine life-stage differences in sensitivity. Tadpoles were exposed for 2 wk to 8 nominal concentrations (0.25 μM–10 μM) of each compound. Individual mortality was recorded during the exposure period, while body weight was measured at the end of 2 wk. LC50 values were calculated, and differences in body weight between vehicle control and exposed groups were assessed. Rank order toxicity of the compounds for both R. pipiens stages and both species was OP > EE2 > E2. Gosner stage 26 tadpoles were more sensitive (LC50: E2 [5.57 μM], EE2 [3.01 μM], OP [1.36 μM]) to all three compounds when compared to stage 36 tadpoles (LC50: E2 [>10 μM], EE2 [4.17 μM], OP [2.80 μM]). Interspecies comparisons revealed R. sylvatica tadpoles (LC50: E2 [2.50 μM], EE2 [1.89 μM], OP [0.74 μM]) as being more sensitive to the three compounds than R. pipiens (LC50: E2 [4.56 μM], EE2 [2.75 μM], OP [1.42 μM]). Xenoestrogen exposure also affected tadpole body weight which may have long-term adverse effects on the rate of metamorphosis. These results provide toxicological data needed for assessing sublethal effects of estrogenic compounds on amphibian development and suggest that environmental levels of OP may pose a serious risk to the health of amphibian populations.