The core scientific goal of this project was to develop, parameterize, and test assumptions of a new mathematical approach to modeling temperature dependent parasitic infections in cold-blooded animals. A key challenge with describing the temperature dependence of parasitism is that the host and the parasite are likely to each have their own independent responses to temperature, and it is difficult to tease apart these processes. For example, if a host becomes more infected at cooler temperatures, it is unclear whether this is due to higher parasite infectivity at cool temperatures or due to lower host resistance. We sought to disentangle these processes by developing mathematical models to describe thermal mismatches between parasite infectivity and host resistance, using models of thermal performance derived from metabolic theory (MT; Figure 1). Working with a pandemic chytrid fungus that infects frogs and salamanders, we found that this approach worked remarkably well to describe individual infection dynamics in at least two frog species. Work is ongoing to test how well this approach works in additional host species.

We also sought to determine how well our MT-based thermal mismatch models can be scaled up to predict transmission dynamics in host populations. In a population-level transmission experiment conducted in heated outdoor mesocosms (Figure 2), we found similar temperature dependence of infection for groups of African clawed frogs relative to infections in indivudually isolated frogs. However, overall infection intensities were much higher than in individual-level experiments. These higher infection levels were predicted by an individual-based model of transmission dynamics whose parameter values were derived from individual infection experiments. These results suggest that MT-based thermal mismatch models may be capable of successfully predict unexpected outcomes when scaling from individual infections to population level transmission dynamics.

This project also yielded evidence of interesting patterns in how thermal acclimation affected host thermal limits, metabolic rates, and resistance to infection, as well as evidence that the infective stage of the amphibian chytrid pathogen has either constitutive responses to temperature (i.e., no thermal acclimation effects) or fully acclimates to a new temperature within minutes of a temperature shift. This is consistent with MT-based predictions of rapid acclimation times in microorganisms, due to mass scaling relationships in which smaller organisms tend to have faster metabolic rates. Other experiments revealed that a “cooler is better” thermal acclimation effect on the metabolic rate of a model salamander species might be partially caused by a thermal mismatch between digestive efficiency and metabolic mass loss. This thermal mismatch appears to have led to lower energy reserves and correspondingly lower metabolic rates in warm-acclimated animals. Consistent with this hypothesis, a follow up experiment revealed similar patterns of gene expression in cold-acclimated axolotls following a period of fasting, compared to warm-acclimated axolotls that were given extra food.

As part of the educational component of this project, we designed and implemented multiple versions of a Thermal Metabolism Lab Activity to introduce students to the biological, chemical, and physical reasons why warm- and cold-blooded organisms respond to temperature in different ways. We implemented one version of this activity for the Project Upward Bound (PUB) Summer Academy, a program that prepares high school students for college. Over the course of four summers (not including pandemic years), at least 120 PUB students worked collaboratively to develop and test hypotheses about the temperature dependence of frog metabolism, human responses to cold exposure, the temperature dependence of chemical reactions, and the energetic costs of maintaining a high body temperature (Figure 3). We also designed and implemented online and in person versions of these activities for use in the introductory lab course for our undergraduate Biology majors (BIO 1201), reaching more than 1200 students in years 2-6 of the project.

As another educational component to this project, PI Raffel developed and implemented an intensive summer teaching workshop for graduate students, based on the FIRST IV program (Faculty Institutes for the Reform of Science Teaching) and aimed at introducing them to evidence-based teaching practices including constructive alignment, universal design for learning, and active learning pedagogies. This program is open to all graduate students at Oakland University and so far has awarded certificates of completion to 186 graduate students.

This project also supported two postdoctoral scholars and at least 50 students to conduct independent research with the Raffel Lab. These students included four PhD students (two successfully defended dissertations and two nearly complete), four completed Masters theses, and 42 undergraduate research assistants including eight who completed Honors College theses.

Last Modified: 12/20/2023
Modified by: Thomas Raffel