Natural and human-caused stressors to marine mammals can cause both acute and chronic stress.
Chronic stress is particularly worrisome because it can lead to disease, reduced reproduction, and reduced survival. Stress has long been known to affect the health of humans, and it affects the health of other animals, as well. Marine mammals experience stress from natural factors like disease, lack of food, and predation, but also from human-caused factors such as noise, pollution, and injury due to fisheries interactions.
The Biologic and Bioacoustic Research program partners with the U.S. Navy and multiple universities to investigate baseline stress markers in marine mammals. Because the markers of stress have important functions in the day-to-day life of marine mammals, the Biologic and Bioacoustic Research team strives to understand how these markers change by season, time of day, sex, age, and reproductive status. Combined with other investigations intended to characterize the “stress response” and how it is reflected in the expression of the stress hormones, these studies move us toward being able to monitor wild marine mammals to determine whether human-caused stress contributes to declines in the health and reproduction of marine mammal populations.
Measuring Stress in Marine Mammals
Knowing the stress that a population of marine mammals is under can provide information on how well the population will be performing in the future, potentially with the addition of new stressors. One means of measuring stress is to look at hormones that are typically associated with stress. These include hormones like cortisol, adrenaline, and the thyroid hormones. In addition, there are a number of other biochemical markers of stress that have the potential to inform scientists of the “stress load” that an individual or population is under.
Stress markers vary in concentration and in the time course over which they change depending on the matrices (blood, feces, hair, urine) in which they are measured. Stress markers can be measured in blood, urine, saliva, feces, skin, and blubber, but each of these matrices will contain different levels of the stress marker and will change at different rates in response to changing stress conditions. Unfortunately, little is known about stress markers in marine mammals and even baseline information on the most common of stress hormones is lacking.
The Biologic and Bioacoustic Research team currently has projects investigating natural variations in stress hormones and sex hormones in bottlenose dolphins and elephant seals. These species serve as surrogates for other species and allow techniques for sampling animals in the wild to be improved. In addition, the team has recently conducted a number of “stress test” studies to determine how stress hormones respond to acute stressors.
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