Research Interests

We are interested in the role of the environment and genetics on the ecology, physiology, and development of organisms. In our research, we take a comparative approach to answer questions about how the physiology and life history of organisms allow them to cope with their environment. We study a wide range of taxa from Daphnia to Emu eggs, to examine the influence of environmental stress on the physiology of organisms during their development.

Burggren and Fritsche (1996) have proposed a model by which environmental perturbations may affect the physiology of developing organisms. Initiation of an environmental perturbation may result in a change in the developmental trajectory of the phenotype of an organism. The phenotype of the organism through development may be blown off course from the wildtype phenotype. Do organisms have the ability to correct their trajectory back towards the wild type, or are they destined for a different phenotype? Few studies have examined developmental plasticity and trajectories in response to environmental stress.

Hypoxia may have different effects on development depending on the timing of the hypoxic bout. Hypoxia occurring early in development may have different effects than hypoxia occurring at later stages of development. Similarly, an organism may have a better capacity to deal with environmental stress at different times during development. With a colleague from UNT and Free University in Berlin I am looking at the developmental trajectories of regulation of metabolic rate and heart rate in developing avian eggs in response to hypoxia occurring during the first, second, or third portion of incubation. Hypoxia (15% O₂) during incubation has the largest effect on the metabolism when it occurs during the middle third of incubation (Abstract).

Developing chicken embryos exhibit lasting changes after incubation in hypoxia during the middle third of incubation. In a collaborative effort with Dr. Tonhardt at the Free University in Berlin I have begun to examine the role of 2,3DPG, cAMP, and catecholamines in avian embryos developing under hypoxic conditions. We are examining the changes that occur in these compounds during hypoxic incubation during the middle third of incubation in an effort to help determine why the embryos are most sensitive during the middle third of incubation. Further efforts are being made to examine the development of the embryonic gas exchange organ, the chorionallontosis, during incubation under hypoxic conditions.

Extensive physiological investigations of avian eggs over the last 25 years have produced general models that reliably predict the relations among egg shell morphology, egg shell gas conductance, egg mass and incubation length for many species. However, there are many species whose eggs do not fit the general predictions of these models. Investigation of these species allows avian physiologists to address crucial ecological and evolutionary questions concerning avian egg diversity (Carey 1980). Although we have long known that departures from general predictions are related to environmental factors such as altitude, extremely humid or xeric conditions and long distances from food, little is known about the physiological mechanisms that enable the adaptive departures. Given the current knowledge of the developmental physiology of avian eggs, including a growing understanding of the embryonic cardiovascular system, we are now in a position to address the departures from classic predictions and begin to explain their mechanistic underpinnings.

Our long-term goal is to understand the physiological mechanisms in eggs / embryos and their evolutionary origins that have enabled seagulls to reproduce in desert environments. Seagulls are well-suited for comparative studies that address convergent, divergent and phylogenetic origins of adaptations in their developmental physiology: They are cosmopolitan; adapted to a diversity of ecological conditions; colonial nesters; and their phylogeny is being confirmed using robust molecular techniques (Crochet and Desmarais 2000).

Isla Raza, Mexico - 95% of all Heermann's Gulls nest on this small island in the Sea of Cortez

Ecological significance of egg size variation on hatchling composition

Studies have suggested that two factors may be important in determining the size of hatchlings: 1. the amount energy available to the growing embryo in the yolk or 2) the amount of water in the egg. We are using emu eggs and hatchlings (Dromiceius novaehollandiae) to examine the effect of egg size variation on life history and physiology. The emu makes a good study animal because of the large mass of the eggs. Both wet and dry egg composition of emu eggs tend to increase with egg mass, thus a larger egg produces a larger hatchling. We are also interested in the scaling of metabolic rate with egg mass and hatchling mass in pre-pipped embryos. This year we have a unique opportunity to examine the role of between egg size variation and hatchling sizes. Half of the breeding emus this year are first year breeders that produce 350 g eggs. This is almost half the size of the eggs produced by the second year breeders, about 625 g.

Daphnia lumholtzi has successfully invaded a number of reservoirs in the United States. With a colleague from UNT and Kansas University, I am investigating the effect of various levels of chronic hypoxia on the physiology and ecology of this species. D. lumholtzi produces large spines, and thus we are also looking at the response of the spines on this species in response to hypoxia and turbulence.

Many pharmaceuticals used in the medical field are only partially metabolized by humans before they are excreted into the wastewater. Measurable concentrations of estrogenic drugs from birth control or hormone replacement have been measured in wastewater effluent throughout America (including the City of Denton) and Europe. These pharmaceuticals have the potential to accumulate in our water systems and influence the health of aquatic organisms. We are working in collaboration with Dr. Bryan Brooks from Baylor University to examine the toxicity of pharmaceuticals on freshwater cladocern.

The potential for pharmaceuticals to have toxic effects on the physiology and reproduction in aquatic organisms has only recently been suggested. The goal of this project is to examine both physiological and reproductive endpoints in toxicological studies on aquatic invertebrates. While pharmaceuticals are targeted towards vertebrates, specifically humans, they have the potential to target aquatic invertebrates. To evaluate toxicity of these pharmaceuticals and any synergistic effect on non-target biota, we assess standard ecotoxicological endpoints (reproduction and growth) along with physiological endpoints in chronic and acute exposure studies with Daphnia.