A fascinating new paper has just been published which examines how stress hormones are measured in seals.
Mammals produce specific hormones as a response to stress, one of these stress hormones is Cortisol. It can be difficult to measure cortisol levels in wild animals such as seals as it often requires the animal to be caught and restrained in order to collect samples (usually blood). But the act of catching and restraining the animal causes stress, which can confound the results so that they are reflective of capture stress not baseline levels.
This new study from our colleagues at SMRU shows that blubber cortisol levels could be a better indication of baseline levels. The study investigated plasma (blood) and blubber samples from 85 live captured adult harbour seals. The length of time a seal is restrained during a capture event varies (often on how many seals you catch in a net!) and in this sample the capture times ranged from 50 to 281 minutes. The study showed that plasma cortisol concentrations were positively correlated with capture duration, which suggests that the blood cortisol levels are largely driven by a stress response to the capture rather than being representative of baseline levels. In contrast, blubber cortisol concentrations were not significantly affected by capture duration and so are more likely to indicate normal baseline levels. The full study, including an assessment of the use of blubber cortisol to be used as a physiological state indicator, can be read here.
A previous study has also shown that blubber tissue obtained from dart biopsies can provide stress level information with minimal effects of sampling. This study measured cortisol from blubber sample from stranded and bycaught common dolphins. Interestingly the study showed that dolphins that had been stranded had on average 6.1-fold higher cortisol levels than those dolphins that were bycaught. The authors suggest that this is because there is a much longer time between the start of the stress response and death in stranded animals, which allows time for blubber cortisol levels to rise to higher levels. The full study can be read here.
At SMRU Consulting we are really keen to know what levels of stress hormones such as cortisol are produced in relation to various man-man disturbances and how stress can impact on vital rates such as survival and fertility. This relates to the PCoD project where we are trying to understand how different kinds of disturbance impact marine mammals: you can check that out here.