How can we measure stress in whales? New research suggests a useful new tool.
Measuring stress in animals evolved to live stressful lives
I think we can all agree that 2020 was a tough year. Lockdowns, friends and family getting sick, international restrictions on movement. These things are continuing into 2021 for many of us and can cause a lot of stress. How do we know it causes stress? Well, because people can communicate easily: in fact, twitter is being used to judge the happiness of humanity.
But how do we know if wild animals are stressed? This is a particularly difficult question for baleen whales: they have evolved to cope with enormous stress. Southern right whales, for example, will swim thousands of kilometres from summer feeding grounds to winter nursery and socialising grounds, and won’t eat for perhaps 3-6 months over winter. How do you know when they’re being pushed beyond what they can endure? It’s not like they can tweet.
A new paper out by colleagues and I describes one tool, called stable isotopes. These are small microchemical markers that are found in an animal’s body and come from their food. We compared one particular stable isotope between mum and calf southern right whales using small samples of their skin: carbon (or d13C if you want to be technical).
We found that the difference this stable isotope in a mum and calf reflects how nutritionally stressed mum is, which has implications for population growth.
Looking more closely at mum – calf pairs
Southern right whales don’t eat over winter. Instead, they gorge in summer feeding sessions and lay down lots of blubber that provision themselves and to make milk to feed their calf over the cold months. This will mean the carbon offset between her and her calf is negative, since fats like blubber have low amounts of this carbon stable isotope. This is what we found in the New Zealand population, which has the fattest right whales in the world. Samples were also collected during a period when this population was doubling in size every 10 years – about as fast as right whale populations can grow.
However, if mum doesn’t have enough blubber, the milk in turn will be less rich in fat. So the calf will use more protein from its milk to grow and build its body. This will mean that the carbon offset between her and her calf is positive, since protein has more of this carbon stable isotope. This is what we found in the Argentinean southern right whale population in years that had mass mortality events, and this had previously been reported in an earlier paper by my co-author Luciano Valenzuela and his colleagues.
We also found cases where there was no significant difference between mum and calf in this carbon stable isotope. This was in Argentina in years without mass mortality events (as previously reported). It was also seen in Brazil, in years where modelling suggests there might have been less food available to mums. This suggests that these mums might still have been more nutritionally stressed than our fat New Zealand right whales, but not so stressed that they lost their calves like in the high mortality year in Argentina. More research needs to be done to understand where the tipping point is that means mums can’t support calves.
Linking stress to population recovery and climate change
Overall, this is another tool that we can use to understand how changing oceans impact recovering whale populations. What’s useful is that it is retrospective: we can use samples in archive to identify good and bad years for whale stress, and then see what good years and bad years look like on feeding grounds. Then we could predict how many good years the whales have in their future under climate change. As we’ve previously shown how changes in foraging is linked with poor population outcomes in South African southern right whales. Understanding where and on what these whales feed, and how these feeding areas will be impacted by climate change, is key to ensuring their continued recovery from whaling.