I am an assistant professor of wildlife biology Supervisors: Susannah S. French, Stephen J. Mullin, John C. Maerz, J. Whitfield Gibbons, John D. Willson, Edmund D. Brodie, and Alan H. Savitzky Address: 5305 Old Main Hill Utah State University Department of Biology Logan UT 84322
Wild snake diets are difficult to study using traditional methods, but stable isotopes offer seve... more Wild snake diets are difficult to study using traditional methods, but stable isotopes offer several advantages, including integrating dietary information over time, providing data from individuals that have not fed recently, and avoiding bias towards slowly-digesting prey items. We used stable isotope signatures of carbon and nitrogen from scale tissue, red blood cells, and blood plasma to assess the diet of wild plains hog-nosed snakes (Heterodon nasicus) in Illinois. We developed Bayesian mixing models which, taken together, predicted that H. nasicus shifted from a juvenile diet predominantly (31–63%) composed of six-lined racerunners (Aspidoscelis sexlineatus) and their eggs to an adult diet predominantly (44–56%) composed of eggs of the aquatic turtles Chrysemys picta and Chelydra serpentina, with a contribution from toads (Anaxyrus sp.; 6–27%) during their adolescent years. These results agreed with sparse data from gut contents. Combining traditional and isotopic techniques for studying the diets of wild snakes can increase the utility of both types of data.
Herpetologists are becoming increasingly aware of the importance of hematological parameters for ... more Herpetologists are becoming increasingly aware of the importance of hematological parameters for evaluating the welfare of their study animals. In particular, differential counts of white blood cells (from blood smears) are now commonly performed on amphibians, although interpretation of such counts relies on some knowledge of the normal range for the species in question, and for many species this range is not known. In this study we examined blood smears from 79 cricket frogs (Acris c. crepitans) from two ponds in northeastern Georgia to establish reference ranges of white blood cell differentials for this species and to explore possible differences in relative white blood cell numbers between sexes, ponds, and body sizes. For comparison, we also compiled values of many other amphibians from published and unpublished records. We found that lymphocytes made up the majority (68.3%) of white blood cells in cricket frogs, followed by neutrophils (22.4%). There was no difference in relative cell numbers of any type between sexes, nor was there an association with body size. Relative numbers of certain cell types (lymphocytes and neutrophils) varied between ponds. In general the values for cricket frogs resembled those of other amphibians, with high numbers of lymphocytes and neutrophils and low numbers of eosinophils, basophils and monocytes. However, in assembling the records for other species we discovered that all Ambystomatid salamanders examined thus far have markedly elevated values of eosinophils compared to other amphibian species. Moreover, their eosinophil counts may be the highest of all vertebrates, and clearly warrant further study to determine their significance.
Convergent evolution of tetrodotoxin (TTX) resistance, at both the phenotypic and genetic levels,... more Convergent evolution of tetrodotoxin (TTX) resistance, at both the phenotypic and genetic levels, characterizes coevolutionary arms races between amphibians and their snake predators around the world, and reveals remarkable predictability in the process of adaptation. Here we examine the repeatability of the evolution of TTX resistance in an undescribed predator–prey relationship between TTX-bearing Eastern Newts (Notophthalmus viridescens) and Eastern Hog-nosed Snakes (Heterodon platirhinos). We found that that local newts contain levels of TTX dangerous enough to dissuade most predators, and that Eastern Hog-nosed Snakes within newt range are highly resistant to TTX. In fact, these populations of Eastern Hog-nosed Snakes are so resistant to TTX that the potential for current reciprocal selection might be limited. Unlike all other cases of TTX resistance in vertebrates, H. platirhinos lacks the adaptive amino acid substitutions in the skeletal muscle sodium channel that reduce TTX binding, suggesting that physiological resistance in Eastern Hog-nosed Snakes is conferred by an alternate genetic mechanism. Thus, phenotypic convergence in this case is not due to parallel molecular evolution, indicating that there may be more than one way for this adaptation to arise, even among closely related species.
ABSTRACT Competition for prey is thought to be important in structuring snake assemblages. Howeve... more ABSTRACT Competition for prey is thought to be important in structuring snake assemblages. However, due in part to the secretive behavior and low detectability of many snake species, this generalization is based on a limited number of studies, most of which focus on a single study site. We examined differences in diet composition, trophic niche overlap, site occupancy and detectability of five sympatric aquatic snake species between two different habitat types in the Southeastern US, replicated at the landscape scale: permanent wetlands with fishes (n = 13) and isolated, often ephemeral wetlands without fishes (n = 10). We collected >3700 prey items from snakes and compared diet composition among snake species to examine niche breadth and overlap, correcting for relative availability of prey captured independently in the same wetlands. We evaluated evidence for competitive exclusion by estimating the probability of co-occupancy for pairs of snake species in each habitat type using occupancy modeling. In wetlands with fishes, niche overlap was low, suggesting resource partitioning. Conversely, in wetlands without fishes, niche overlap was high, with most species feeding on larval or paedomorphic ambystomatid salamanders, but competitive exclusion did not occur. We suggest that high co-occupancy of aquatic snakes in wetlands without fishes despite the apparent lack of resource partitioning is due to a combination of seasonally high abundance of high quality amphibian prey, unique aspects of predator physiology and stochastic abiotic processes that prevent these systems from reaching equilibrium. Our results demonstrate that snake diets can be highly context (e.g. habitat)-specific. Studies should consider other factors in addition to competition for prey when attempting to understand snake population and community dynamics.
Wild snake diets are difficult to study using traditional methods, but stable isotopes offer seve... more Wild snake diets are difficult to study using traditional methods, but stable isotopes offer several advantages, including integrating dietary information over time, providing data from individuals that have not fed recently, and avoiding bias towards slowly-digesting prey items. We used stable isotope signatures of carbon and nitrogen from scale tissue, red blood cells, and blood plasma to assess the diet of wild plains hog-nosed snakes (Heterodon nasicus) in Illinois. We developed Bayesian mixing models which, taken together, predicted that H. nasicus shifted from a juvenile diet predominantly (31–63%) composed of six-lined racerunners (Aspidoscelis sexlineatus) and their eggs to an adult diet predominantly (44–56%) composed of eggs of the aquatic turtles Chrysemys picta and Chelydra serpentina, with a contribution from toads (Anaxyrus sp.; 6–27%) during their adolescent years. These results agreed with sparse data from gut contents. Combining traditional and isotopic techniques for studying the diets of wild snakes can increase the utility of both types of data.
Herpetologists are becoming increasingly aware of the importance of hematological parameters for ... more Herpetologists are becoming increasingly aware of the importance of hematological parameters for evaluating the welfare of their study animals. In particular, differential counts of white blood cells (from blood smears) are now commonly performed on amphibians, although interpretation of such counts relies on some knowledge of the normal range for the species in question, and for many species this range is not known. In this study we examined blood smears from 79 cricket frogs (Acris c. crepitans) from two ponds in northeastern Georgia to establish reference ranges of white blood cell differentials for this species and to explore possible differences in relative white blood cell numbers between sexes, ponds, and body sizes. For comparison, we also compiled values of many other amphibians from published and unpublished records. We found that lymphocytes made up the majority (68.3%) of white blood cells in cricket frogs, followed by neutrophils (22.4%). There was no difference in relative cell numbers of any type between sexes, nor was there an association with body size. Relative numbers of certain cell types (lymphocytes and neutrophils) varied between ponds. In general the values for cricket frogs resembled those of other amphibians, with high numbers of lymphocytes and neutrophils and low numbers of eosinophils, basophils and monocytes. However, in assembling the records for other species we discovered that all Ambystomatid salamanders examined thus far have markedly elevated values of eosinophils compared to other amphibian species. Moreover, their eosinophil counts may be the highest of all vertebrates, and clearly warrant further study to determine their significance.
Convergent evolution of tetrodotoxin (TTX) resistance, at both the phenotypic and genetic levels,... more Convergent evolution of tetrodotoxin (TTX) resistance, at both the phenotypic and genetic levels, characterizes coevolutionary arms races between amphibians and their snake predators around the world, and reveals remarkable predictability in the process of adaptation. Here we examine the repeatability of the evolution of TTX resistance in an undescribed predator–prey relationship between TTX-bearing Eastern Newts (Notophthalmus viridescens) and Eastern Hog-nosed Snakes (Heterodon platirhinos). We found that that local newts contain levels of TTX dangerous enough to dissuade most predators, and that Eastern Hog-nosed Snakes within newt range are highly resistant to TTX. In fact, these populations of Eastern Hog-nosed Snakes are so resistant to TTX that the potential for current reciprocal selection might be limited. Unlike all other cases of TTX resistance in vertebrates, H. platirhinos lacks the adaptive amino acid substitutions in the skeletal muscle sodium channel that reduce TTX binding, suggesting that physiological resistance in Eastern Hog-nosed Snakes is conferred by an alternate genetic mechanism. Thus, phenotypic convergence in this case is not due to parallel molecular evolution, indicating that there may be more than one way for this adaptation to arise, even among closely related species.
ABSTRACT Competition for prey is thought to be important in structuring snake assemblages. Howeve... more ABSTRACT Competition for prey is thought to be important in structuring snake assemblages. However, due in part to the secretive behavior and low detectability of many snake species, this generalization is based on a limited number of studies, most of which focus on a single study site. We examined differences in diet composition, trophic niche overlap, site occupancy and detectability of five sympatric aquatic snake species between two different habitat types in the Southeastern US, replicated at the landscape scale: permanent wetlands with fishes (n = 13) and isolated, often ephemeral wetlands without fishes (n = 10). We collected >3700 prey items from snakes and compared diet composition among snake species to examine niche breadth and overlap, correcting for relative availability of prey captured independently in the same wetlands. We evaluated evidence for competitive exclusion by estimating the probability of co-occupancy for pairs of snake species in each habitat type using occupancy modeling. In wetlands with fishes, niche overlap was low, suggesting resource partitioning. Conversely, in wetlands without fishes, niche overlap was high, with most species feeding on larval or paedomorphic ambystomatid salamanders, but competitive exclusion did not occur. We suggest that high co-occupancy of aquatic snakes in wetlands without fishes despite the apparent lack of resource partitioning is due to a combination of seasonally high abundance of high quality amphibian prey, unique aspects of predator physiology and stochastic abiotic processes that prevent these systems from reaching equilibrium. Our results demonstrate that snake diets can be highly context (e.g. habitat)-specific. Studies should consider other factors in addition to competition for prey when attempting to understand snake population and community dynamics.
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advantages, including integrating dietary information over time, providing data from individuals that
have not fed recently, and avoiding bias towards slowly-digesting prey items. We used stable isotope
signatures of carbon and nitrogen from scale tissue, red blood cells, and blood plasma to assess the diet
of wild plains hog-nosed snakes (Heterodon nasicus) in Illinois. We developed Bayesian mixing models
which, taken together, predicted that H. nasicus shifted from a juvenile diet predominantly (31–63%)
composed of six-lined racerunners (Aspidoscelis sexlineatus) and their eggs to an adult diet
predominantly (44–56%) composed of eggs of the aquatic turtles Chrysemys picta and Chelydra
serpentina, with a contribution from toads (Anaxyrus sp.; 6–27%) during their adolescent years. These
results agreed with sparse data from gut contents. Combining traditional and isotopic techniques for
studying the diets of wild snakes can increase the utility of both types of data.
advantages, including integrating dietary information over time, providing data from individuals that
have not fed recently, and avoiding bias towards slowly-digesting prey items. We used stable isotope
signatures of carbon and nitrogen from scale tissue, red blood cells, and blood plasma to assess the diet
of wild plains hog-nosed snakes (Heterodon nasicus) in Illinois. We developed Bayesian mixing models
which, taken together, predicted that H. nasicus shifted from a juvenile diet predominantly (31–63%)
composed of six-lined racerunners (Aspidoscelis sexlineatus) and their eggs to an adult diet
predominantly (44–56%) composed of eggs of the aquatic turtles Chrysemys picta and Chelydra
serpentina, with a contribution from toads (Anaxyrus sp.; 6–27%) during their adolescent years. These
results agreed with sparse data from gut contents. Combining traditional and isotopic techniques for
studying the diets of wild snakes can increase the utility of both types of data.