Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Biology

Supervisor

MacDougall-Shackleton, Scott A.

Abstract

Organisms regularly adjust their physiology to respond to predictable seasonal or environmental variation. However, annual cycle transitions could be disrupted by contaminants or stressors. For example, methylmercury and stress exposure can independently disrupt birds’ neural and endocrine systems, energy balance, metabolism, or behaviour, all necessary for seasonal transitions. Although, the effects of combined exposure to stressors and methylmercury (MeHg), and how long they last after exposure ends, are poorly understood. The objective of my PhD was to evaluate the impact of MeHg exposure on songbirds’ physiology and its potential carry-over effects on seasonal transitions. I exposed song sparrows (Melospiza melodia) to environmentally relevant doses of MeHg in combination, or not, with unpredictable food stress. I observed birds’ physiological changes throughout two seasonal transitions: summer to fall and winter to spring. In Chapter 2, I demonstrated that MeHg is sequestered within feathers at the time of feather growth, making feathers an appropriate tool for bird monitoring under the condition that moult pattern is well characterised in the monitored species. In Chapter 3, I found that unpredictable food stress increased body condition but decreased basal metabolic rates, while MeHg exposure increased moult duration and feather mass/length ratio in fall. In Chapter 4, MeHg and stress differently affected nocturnal fall migratory activity and the combined treatment group had increased fecal corticosterone metabolites post-exposure; both measures were positively correlated. In contrast, in Chapter 5, MeHg exposure had no detectable effect on winter to spring changes in brain GnRH cells, testosterone levels or testis size. Thus, spring reproductive onset might not be affected by MeHg in birds. However, MeHg exposure did affect cloacal protuberance, fecal corticosterone metabolites and brain neurogenesis, suggesting that exposed birds’ mating success could be reduced later on. Overall, my thesis main findings were: i) except for corticosterone concentrations food stress did not exacerbate the effects of MeHg exposure, and ii) effects of MeHg on moult, migratory behaviour and secondary sexual signals may be a potential cause of concern for populations. My research highlights the importance of studying contaminant effects over multiple seasons and post-exposure periods when assessing risk for wildlife.

Summary for Lay Audience

Organisms adjust their physiology with seasonal or habitat changes. However, contaminants or stressors may reduce how well they perform these changes. It is not well known how combined exposure to stressors and methyl-mercury affect an organism’s physiology. And how long the effect last after the exposure ends remains unknown.

My PhD aimed to assess if stress exacerbates methyl-mercury effects on songbirds’ physiology and their consequence on seasonal transitions. I exposed song sparrows to unpredictable food stress and/or doses of methyl-mercury. Doses of methyl-mercury were similar to that found in insects in polluted areas. Bird's stress was induced by randomly preventing them to access their food, simulating an acute food shortage in natural habitats. I observed the birds’ physiological changes throughout two seasonal transitions: summer to fall and winter to spring.

I demonstrated that feathers sequester methyl-mercury during their growth, making feathers a useful tool for bird monitoring programs. Furthermore, methyl-mercury and food stress affected the birds through different mechanisms: food stress increased birds’ body condition and decreased their metabolic rates, while methyl-mercury prolonged birds’ feather moult and decreasing feather mass/length ratio. When combined, methyl-mercury and stress increased levels of the hormone corticosterone but only after exposure. Corticosterone was also correlated with migratory behaviour.

During the winter to spring seasonal transitions, methyl-mercury exposure did not affect the bird’s reproductive onset. However, methyl-mercury increased corticosterone levels, reduced the development of cloacal protuberance, and the number of new neurons in the brain. This suggests that methyl-mercury exposure could hinder bird’s mating success later on.

In summary, except for corticosterone levels, food stress did not exacerbate the effect of methyl-mercury exposure. But methyl-mercury’s effects on moult, migratory behaviour and other components used for mate selection could affect populations over the long-term. In conclusion, it is important to study contaminant post-exposure effects over multiple seasons if we want to better assess risk for wildlife.

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