The hematological response of amphibians to stress and its implications for research, management and conservation
Davis, Andrew Kenneth
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Populations of free-living animals frequently encounter natural and human-driven environmental variation that can affect their physiology and ultimately their long-term viability. In some cases, animals can experience physiological manifestations of such variation long before effects on more traditional population parameters are realized. One component of physiology that is useful for ascertaining how animals cope with such variation is their stress level. Chronically high stress levels can affect the performance of animals by reducing growth rates, long-term survival or increasing disease vulnerability. With this in mind, my dissertation research examines how natural and anthropogenic environmental variation affects stress levels in vertebrates, using amphibians as model subjects. To assess stress levels, investigators traditionally measure the concentration of stress hormones (corticosterone in birds, reptiles and amphibians) in blood plasma of subjects, although this can be logistically difficult because of the rapid response time of hormones and high volume of blood needed for the test. An alternative approach capitalizes on the stress hormone’s effect on the host’s circulating white blood cells, and in fact this cellular approach has advantages over direct hormone sampling because it is not as time sensitive and requires much smaller blood samples. Although this cellular method is gaining in popularity among wildlife researchers interested in quantifying stress, it has encountered some resistance in the scientific literature, partly stemming from a poor understanding of the relationship between stress hormones and blood cells. My thesis aimed to address this information gap. Major findings included: 1) a literature review showing how stress causes changes in numbers of circulating white blood cells such that the ratio of neutrophils to lymphocytes (N:L ratio) rises and that this effect is conserved across all vertebrate taxa, 2) experimental administration of stress hormone to salamanders causes a rise in N:L ratios, 3) bringing wild salamanders into captivity causes increases in N:L ratios, 4) rearing salamanders from eggs in crowded conditions causes high N:L ratios in surviving metamorphs, 5) infection with chytridiomycosis can cause changes in blood cell counts that resemble effects of stress, and 6) that amphibians reared in captivity have similar responses to stress as do wild individuals, which justifies the use of captive-rearing initiatives for amphibian conservation. The collective projects in this thesis should not only serve to establish the hematological approach to measuring stress for ecologists, but also to highlight the importance of considering the physiology of the animal when drawing conclusions from research projects or when making decisions for management and conservation of wildlife.