Most of these projects are ongoing. Please contact me if you are interested in potential student research opportunities, collaborations, or volunteering to help with our fieldwork.
Physiological Ecology and Conservation of Australian Turtles
Impacts of invasive foxes on turtles
Australian turtles are currently in serious trouble. Some populations of turtles in the Murray River have declined by 90% over the past 30 years. In 2019, we published a paper showing that some populations are essentially gone in South Australia. This project, in collaboration with Ricky Spencer's lab at Western Sydney University, aims to identify the major causes of these declines, and stop them with a novel approach to threatened species conservation. At the same time, we use turtles to answer a range of important questions about reproductive physiology, immunology, and ecology. I am working with Kristen Petrov and Claudia Santori on this project.
Effects of habitat modification on turtle diets and reproduction
The Murray River has been extensively degraded by multiple factors. Its flow has been considerably modified by dams and weirs that store water for human use, produce hydroelectric power, and prevent catastrophic floods. Invasive carp have degraded many remaining habitats, both by consuming aquatic plants and invertebrates, and muddying the water. We are investigating how these degradations impact turtle diets. I am working with Kristen Petrov on this project. We published our first paper on this system in 2018.
Impacts of diet differences on turtle physiology
Ultimately, my goal is to determine whether the diet differences we have identified above reduce growth and/or reproductive success of turtles. As the food available to turtles decreases, their ability to produce new tissues, either in the form of their own growth or the production of offspring, should also be reduced. We are studying these questions by comparing turtle body condition and reproductive variables among wetlands where we suspect turtles are eating different foods.
Gene flow among turtle populations
In addition to studying the effects of wetland degradation on turtle diets, we are also studying how dams and other modifications of the Murray-Darling Catchment have affected the ability of turtles to migrate and interbreed across populations. We are sampling DNA from turtles across the entire catchment, and, in collaboration with Prof Arthur Georges, we will use these data to test whether dams or other human obstacles occur as "breaks" in the genes of turtles on either side. The sampling effort has also provided an opportunity to make rapid assessments of turtle populations across the catchment. We will use these data to determine the basic extents of the ranges of each species, as well as identify potential "trouble spots". Trouble spots could include locations where turtles are rare, appear malnourished, or where turtle populations have large numbers of old adults or young juveniles. Populations with only old adults could be places where fox depredation has eliminated turtle recruitment, while populations with predominately juveniles could be places that have undergone recent die-offs of adults, for a variety of reasons.
Physiology and Evolution of Reproduction
Mechanisms of reproductive allocation
How animals provide their offspring with sufficient nutrients to complete embryonic development is a major focus of my research. Mechanisms of reproductive allocation mechanisms are the "nuts and bolts" processes that provide nutrients to developing embryos. They determine what nutrients embryos receive, and how much. These factors have major consequences for embryos, because embryos rely on their parents for all of their nutrition. They simply can't make or acquire the carbohydrates, protein, fats, and minerals they need to develop on their own. There is also potential that parents might adjust their allocation mechanisms depending on the environments they live in, which could change the size of offspring they produce, or the number, or even have later consequences for offspring growth and performance, like running speed. Most vertebrates rely primarily on yolk for reproductive allocation (lecithotrophy). Yolk is produced in the liver and then deposited in eggs prior to ovulation. Some vertebrates, including all eutherian mammals, some reptiles, and some fishes, sharks, and rays, rely partially or entirely on a placenta to provide nutrients to embryos during development (placentotrophy). My research aims to understand how these allocation mechanisms work, at physiological and molecular levels, and whether they can be tuned to maximize reproductive success given different environmental conditions that parents experience.
Evolution of live birth and the placenta
Live birth (viviparity) has evolved more than 140 times in vertebrate animals, including many times in fish, sharks, rays, and amphibians, over 100 times alone in lizards and snakes, and at least once in mammals. In each case known so far, a placenta of some form has also evolved along with viviparity. A placenta is simply a region of contact between maternal and embryonic tissues that allows transport of materials between mother and offspring, usually oxygen, carbon dioxide, water, and some nutrients. A major focus of my research with Steve Beaupre, Mike Thompson, Matt Brandley, Camilla Whittington, Oliver Griffith, and Dan Blackburn has been to understand how live birth and the placenta have evolved in these animals. We primarily work with lizards and snakes because the sheer number of independent origins of viviparity in these animals make them the best group in which to look for shared mechanisms and reasons for its evolution. A 2015 issue of Journal of Experimental Zoology, B was dedicated solely to discussing hypotheses about how viviparity evolves.
Complex placentae that transport large amounts of nutrients (ie, placentotrophy) have evolved in some fish, sharks, rays, lizards, and once in mammals. Many of my recent projects have studied the function and evolution of the placenta in Australian skinks. Two genera of Australian skinks, Niveoscincus and Pseudemoia, have independently evolved complex placentae. Our research has focused on understanding the molecular mechanisms of nutrient transport across these placentae, and also on the environmental conditions that maximize developmental success in placental lizards. The repeated independent origins of both viviparity and the placenta across many types of animals also make them excellent models for understanding how complex organs evolve. I am also currently studying this question in sharks with Alice Buddle, Camilla Whittington, and Colin Simpfendorfer.
The "decision" to reproduce
Animals face a number of challenges when making the "decision" to reproduce. In order to maximize the number of offspring that survive to reproduce themselves, this decision should be based on cues that communicate three critical variables: suitability of the environment for both parental and offspring survival, physiological ability of the parents to reproduce successfully, and likelihood of encountering potential mates that are also ready to reproduce. This "decision" is not a mental process as we would normally consider it, but can be thought of as the end result of a complex process of sensory and physiological yes or no questions: Is the environment suitable (yes/no)? Am I "ready" to reproduce (yes/no)? Can I find a suitable mate (yes/no)? An example of environmental suitability could be something as simple as, "what time of year is it?" This question would be answered by sensory mechanisms that use cues like day length (long or short) or environmental temperature (hot or cold). Many animals' breeding seasons coincide with spring and/or summer to ensure that parents can find enough food to support their breeding activity, and also to ensure that offspring can find enough food to enable their own survival. Thus, the cues to stimulate the decision-making process to reproduce could start with something as simple as, "Is day length increasing and/or long?" These kinds of questions are easy to model, but we know surprisingly little about how animal sensory and physiological mechanisms detect these cues and communicate them to the reproductive system to stimulate reproduction. My research uses physiological and molecular techniques to identify these mechanisms. Ultimately, my aim is to improve our understanding of how animals' cue detection mechanisms will respond to changes in their environment, which are likely to alter the cues they use to make their reproductive decisions.
Capital and income breeding
My interests in both reproductive allocation and the decision to reproduce arose out of an important part of life history theory called the "capital-income breeding dichotomy". Capital breeders are animals that base the decision to reproduce on their amount of stored fat and protein reserves (capital), while income breeders are animals that base the decision to reproduce on how much food is available (income). This dichotomy sets up the idea that capital breeders eat over long periods (a year or more) to store up massive nutrient reserves before allocating most of them to reproduction in a sort of "explosive" event. Some capital breeders might even stop eating when they reproduce and so rely entirely on their stored reserves. In contrast, income breeders allocate nutrients to reproduction on a semi-continuous basis as they eat and digest those nutrients. These definitions have implications for how well animals can survive in environments with varying food abundance: capital breeders might delay reproduction when food is rare in order to "save up" resources until they have enough to produce a large number of large offspring; income breeders might continue to reproduce even when food is rare, but they might be more likely to reduce the size or number of offspring they produce. I have studied this question using food manipulations to force animals to "adjust" their reproductive allocation strategies, and using stable isotopes to "trace" nutrients from mother to offspring.
Effects of Coal Fly Ash on Vertebrates
The 2008 Kingston, Tennessee coal fly ash spill
In the USA, about 40% of all electricity is produced by coal-combusting power plants. When coal is burned for power, one of the major waste products is fly ash, which contains a number of heavy metals and metalloids that can have biological effects on animals. Many power plants store fly ash in wet slurries in ponds. In 2008, the slurry pond at the TVA power plant in Kingston, Tennessee failed, and over 4 million cubic meters of fly ash slurry were released into Watts Bar Lake. TVA, along with the US EPA and other government agencies, instantly began a remediation and recovery program to deal with the spill. I worked with Bill Hopkins at Virginia Tech to lead a field study to investigate the effect of the spill on turtles in Watts Barr Reservoir. Bill, David Steen, Cathy Jachowski, Brian Jackson, and I are currently finishing a project examining the effects of the spill on heavy metal bioaccumulation in turtles on a landscape scale.
Maternal transfer of ash-derived contaminants
A major focus of our research at the Kingston site was how female animals transport ash-derived contaminants to their offspring. A consequence of the chemical processes of reproductive allocation mechanisms is that contaminants like mercury, selenium, strontium, and others can be integrated into or replace specific nutrients that embryos receive from their mother. For example, strontium is chemically very similar to calcium, and can replace it in eggshells and bone. If a female animal has high concentrations of strontium, some strontium will replace calcium in the reproductive allocation mechanisms that normally provide calcium to embryos. At Kingston, Michelle Beck and I studied how female turtles and Tree Swallows maternally transfer ash-derived contaminants to their offspring. All of these animals are oviparous and lay shelled, yolk-filled eggs, but the patterns of egg production differ between turtles and birds, and we made comparisons between them to determine whether embryonic turtles or birds might be more vulnerable to the effects of ash-derived contaminants as a result of maternal transfer.
Effects of ash-derived contaminants on turtle embryos
In addition to studying how maternal transfer exposes embryos to contaminants during development, we also tested whether contaminants in yolk affected development in turtles. This work was led by David Steen, and focused on determining whether embryo growth and hatching success were impacted by contamination, and also whether hatchling locomotor performance was affected.