New publication: non-invasive methods to monitor stress physiology in amphibians

Global climate change is negatively impacting global biodiversity and ectothermic vertebrates, with amphibians being the most imperiled vertebrate taxa. Increased mean global atmospheric temperatures, high rates of habitat degradation, and exposure to infectious diseases, such as chytridiomycosis, have contributed to population declines and extinctions of rare and endangered amphibian species. Field-based monitoring of physiological endocrine traits can help determine the sub-lethal effects of environmental stressors and provide early alerts when populations are chronically stressed. Recent advances in amphibian stress endocrinology include the development and use of non-invasive methods to quantify the glucocorticoid, or stress biomarker, corticosterone. Non-invasive methods, such as urinary, skin and buccal swabs, and water-borne hormone monitoring methods (suited for terrestrial and aquatic dwelling species), are available to quantify baseline and short-term physiological stress responses of amphibians under field settings. In this review, we illustrate, by using two case studies of aquatic and terrestrial amphibian species, the applications of non-invasive corticosterone monitoring methods to advance the ecological knowledge and conservation of imperiled amphibian species, discuss the limitations of these methods, and provide future directions for the use of non-invasive hormone monitoring methods. We highlight the use of non-invasive field endocrinology methods to monitor the impacts of environmental stressors on the physiology of amphibians, which can be applied to advance ecological research, conservation, and management of imperiled species.

Narayan EJ, Forsburg ZR, Davis DR, Gabor CR. 2019. Non-invasive methods for measuring and monitoring stress physiology in imperiled amphibians. Frontiers in Ecology and Evolution 7:431. [PDF]


New publication: ranavirus in southeast Oklahoma

Davis.etal.2019b_Table1Several infectious diseases are threatening amphibian species worldwide and have resulted in mass-mortality events across the globe. An emerging group of viral pathogens (ranaviruses) are documented to cause die-offs in amphibian populations worldwide, including in several regions of the U.S. Unfortunately, large gaps remain in our understanding of the distribution of this systemic pathogen in the U.S., including within the state of Oklahoma. To address this gap in our understanding, we carried out surveys of this infectious pathogen across 14 sites in seven southeastern Oklahoma counties in spring 2015, screening 17 amphibian species from this region. Using liver and tail tissue samples collected from individual amphibians, we screened for the presence and infection load of ranavirus. Of the 390 samples, 84 (21.5%) tested positive for ranavirus, with infection prevalence varying among species surveyed. Notably, the family Bufonidae had no samples that tested positive for ranavirus, whereas the remaining families had an infection prevalence ranging from 14–50%. Despite an overall infection prevalence of 21.5%, we detected no clinical signs of ranavirosis and all sampled individuals appeared outwardly healthy. These results provide data on the geographic and host distribution of ranavirus in southeastern Oklahoma, as well as the first documented cases of the pathogen in three species of anurans: Gastrophryne carolinensis (Eastern Narrow-mouthed Toad), G. olivacea (Western Narrow-mouthed Toad), and Pseudacris fouquettei (Cajun Chorus Frog). With widespread ranavirus infection, there is potential for transmission from abundant, widespread species to more vulnerable, state-threatened amphibians.

Davis DR, Farkas JK, Kruisselbrink TR*, Watters JL, Ellsworth ED, Kerby JL, Siler CD. 2019. Prevalence and distribution of ranavirus in amphibians from southeastern Oklahoma, USA. Herpetological Conservation and Biology 14:360–369. [PDF]


New publication: agricultural contaminants affect stress and pathogens in salamanders

Declines in amphibians are a global problem, influenced by complex local factors. While many factors contribute to these declines, much attention has been focused on the roles of contaminants and pathogens. Throughout eastern South Dakota, row-crop farming has contributed to habitat degradation for many amphibians, often through increases in environmental contaminants. For two years we visited four wetlands (two reference wetlands, two agricultural wetlands) to measure water-borne corticosterone (CORT) release rates and ranavirus in larval Western Tiger Salamander (Ambystoma mavortium). We found that both water-borne CORT release rates and ranavirus infection load were greater in larval salamanders from agricultural wetlands compared to reference wetlands. We also found that water-borne CORT release rates were greater in ranavirus-infected individuals compared to uninfected individuals and that water-borne CORT is positively correlated with ranavirus infection load. Though the causal relationships among contaminants, CORT, and ranavirus infection are difficult to determine, chronically elevated CORT is known to be immunosuppressive and may result in high infection loads. This study further describes the negative effects of crop production on amphibian health, provides the first evidence of ranavirus in South Dakota, and supports the use of water-borne CORT as a biomarker of amphibian population health in row-crop landscapes.

Davis DR, Ferguson KJ*, Schwarz MS, Kerby JL. 2019. Effects of agricultural pollutants on stress hormones and viral infection in larval salamanders. Wetlands. doi: 10.1007/s13157-019-01207-1


New publication: chytridiomycosis seasonality in Oklahoma

Chytridiomycosis, an infectious disease caused by the fungus Batrachochytrium dendrobatidis (chytrid or Bd), has not been well studied in Oklahoma. This is of particular concern regarding the connection between seasonality and chytrid infection. To further investigate this connection, chytrid prevalence and infection load were quantified within amphibians in central Oklahoma from March to October, across two sites in Oklahoma Co. and two sites in Cleveland Co. The results show a trend between seasonality and chytrid, with spring and fall showing higher prevalence and summer showing lower prevalence, which coincides closely with the preferred chytrid growth temperatures. Additionally, periods of high rainfall in May 2015 are linked to increased chytrid prevalence, as has been suggested by other research. Additionally, species exhibiting high chytrid prevalence follow the results of previous studies: Blanchard’s Cricket Frog (Acris blanchardi), American Bullfrog (Rana catesbeiana), and Southern Leopard Frog (Rana sphenocephala).

Watters JL, McMillin SL*, Marhanka EC*, Davis DR, Farkas JK, Kerby JL, Siler CD. 2019. Seasonality in Batrachochytrium dendrobatidis detection in amphibians in central Oklahoma. Journal of Zoo and Wildlife Medicine 50:492–497. [PDF]


New publication: habitat influences Spotted Salamander stress hormones

Spotted Salamanders (Ambystoma maculatum) require vernal pools for breeding habitat. Limited protection and preservation of vernal pools makes suitable habitat creation important. Differences in corticosterone levels, a hormone associated with growth, development, and stress in amphibians, could indicate population health and habitat quality. Our objective was to determine if habitat characteristics in created vernal pools influence corticosterone levels of Spotted Salamander larvae. In May and June of 2015 and 2016, we sampled water-borne corticosterone levels of larval Spotted Salamanders in 34 created vernal pools constructed 1–5 years earlier. Using multiple regression, we determined the best model predicting corticosterone levels included larval total length, pool-water temperature, year sampled, and pool diameter. Pool-water pH, depth, and age; percent cover; and predator presence were not significant predictors. Annual variation in corticosterone levels and habitat characteristics, and positive associations with water temperature and salamander body size highlighted the importance of controlling for external influences. The negative association between pool diameter and corticosterone indicated that larvae in larger pools (up to 12.75-m maximum diameter) were less stressed and potentially healthier. These results indicate that pool diameter contributes to habitat quality and may be important when constructing vernal pools for Spotted Salamanders.

Millikin AR, Woodley SK, Davis DR, Anderson JT. 2019. Habitat characteristics in created vernal pools impact spotted salamander water-borne corticosterone levels. Wetlands 39:803–814. [PDF]


New publications: two new records for Texas

Two new, noteworthy records for Texas were published in the March 2019 issue of Herpetological Review. The first was a western range extension of the Lesser Siren, Siren intermedia (though there is some taxonomic confusion surrounding this species identification), from the Rio Grande watershed in Kinney County. The second represents a new species of reptile for the state of Texas. I collected several specimens of Sri Lankan House Geckos (Hemidactylus aff. parvimaculatus) in east Texas. This species has not been previously reported from Texas and appears to expanding its range along the I-10 corridor from New Orleans and suspect that it will continue to be discovered in new localities.

Davis DR, Ruppert KM, Kline RJ. 2019. Geographic distribution: USA, Texas, Kinney Co.: Siren intermedia (Lesser Siren). Herpetological Review 50:95–96. [PDF]

Davis DR, LaDuc TJ. 2019. Geographic distribution: USA, Texas, Chambers Co., Orange Co.: Hemidactylus aff. parvimaculatus (Sri Lankan House Gecko). Herpetological Review 50:102. [PDF]


New publication: effects of glyphosate and captivity on bacterial microbiota in False Map Turtles

Understanding how environmental factors influence various aspects of freshwater turtle health remains an important yet understudied topic within the context of individual–environment interactions. This is particularly true of hostassociated bacterial microbiota, which are being increasingly recognized as a significant and understudied topic in the context of individual turtle health. While this area of work has expanded in certain areas, research efforts remain limited with regard to host–microbiota interactions in the context of habitat contaminants. Specifically, the commonly used herbicide, glyphosate, is of interest due to its massive worldwide use and known effects on various organisms. Effects of captivity on host-associated microbial community structure also remain largely unknown in various nonmodel organisms. To address these unknown effects of Roundup® and captivity on host-associated microbiomes, we examined the effects of low-level Roundup® exposure and captivity on the cloacal microbiota of the False Map Turtle, Graptemys pseudogeographica. We determined the effect of glyphosate by taking cloacal swabs pre- (0 h) and postexposure (72 h) and examined microbial community beta- and alpha-diversity through 16S rRNA gene high-throughput sequencing. The results of this study indicate that low-level, short-term glyphosate exposure does not significantly alter the microbiota structure of G. pseudogeographica. However, there was a significant decrease in microbial community beta-diversity over time, confirming a trend that has been observed to a limited extent in other non-model organisms when put in laboratory conditions. These results are useful in understanding the baseline cloaca microbial community structure of G. pseudogeographica, as well as the implications and limitations of laboratory-based microbiota studies. Furthermore, this work suggests that low-level and short-term glyphosate exposure does not have a significant effect on the cloacal microbial community structure in wild-caught G. pseudogeographica.

Madison JD, Austin SD*, Davis DR, Kerby JL. 2018. Bacterial microbiota response in Graptemys pseudogeographica to captivity and Roundup® exposure. Copeia 106:580–588. [PDF]


Lined Snake surveys completed

Earlier this year I was awarded funding from South Dakota Game, Fish and Parks to survey for the state-endangered Lined Snake (Tropidoclonion lineatum) along the James River in southeastern South Dakota (see earlier post). As part of this survey work, two trips were planned: one trip in the spring corresponding with the the first days of warm temperature and one trip in the fall, corresponding with the final warm days of the year before snakes enter hibernacula. I recently completed the fall survey trip and despite most overcast days, I was able to overlap with an unusually warm day (3 October 2018; 30°C/86°F), and found eight Lined Snakes; unfortunately, all were dead on the road. These specimens were collected as voucher specimens and will be used as reference material and provide tissue samples from this population of Lined Snakes in Hutchinson County to determine if there is gene flow between this population and populations along the Big Sioux River. These eight Lined Snakes, combined with the eight individuals I found during the spring survey period (6 live, 2 dead) confirm an established population of Lined Snakes in this region along the James River. Due to the lack of detection of Lined Snakes outside of this locality, this may be a small, isolated population rather than a large, widespread population along the lower James River. 

New publication: co-infection of Bd and RV in Oklahoma amphibians

Global amphibian decline continues to be a great concern despite our increased understanding of the causes behind the observed patterns of the decline, such as habitat modification and infectious diseases. Although there is a large body of literature on the topic of amphibian infectious diseases, pathogen prevalence and distribution among entire communities of species in many regions remain poorly understood. In addition to these geographic gaps in our understanding, past work has focused largely on individual pathogens, either Batrachochytrium dendrobatidis (Bd) or ranavirus (RV), rather than dual infection rates among host species. We sampled for prevalence and infection load of both pathogens in 514 amphibians across 16 total sites in northeastern Oklahoma. Amphibians were caught by hand, net, or seine; they were swabbed to screen for Bd; and liver tissue samples were collected to screen for RV. Overall results of quantitative PCR assays showed that 7% of screened individuals were infected with RV only, 37% were infected with Bd only, and 9% were infected with both pathogens simultaneously. We also documented disease presence in several rare amphibian species that are currently being  monitored as species of concern due to their small population sizes in Oklahoma. This study synthesizes a growing body of research regarding infectious diseases among amphibian communities in the central United States.

Watters JL, Davis DR, Yuri T, Siler CD. 2018. Concurrent infection of Batrachochytrium dendrobatidis and ranavirus among amphibians from northeastern Oklahoma, USA. Journal of Aquatic Animal Health 30:291–301. [PDF]


New Position: Associate Research Scientist at UTRGV

Last week I started in a new position as an Associate Research Scientist at the University of Texas Rio Grande Valley. Here, I am working to better understand factors associated with the distribution of the state-threatened Black-spotted Newt (Notophthalmus meridionalis), sirens, and Rio Grande Cooters (Pseudemys gorzugi) in south Texas. I’m excited to get settled into this position and help increase our understanding of these unique species in the Rio Grande Valley.

When not doing fieldwork, I will be based out of the newly acquired Port Isabel Research Station in Port Isabel, TX.

As part of this position, please note my updated contact information:

Address: 100 Marine Lab Drive, South Padre Island, TX 78597
Email: drew.davis (at)