ECR Spotlight is a series of interviews with early-career authors from a selection of papers published in Journal of Experimental Biology and aims to promote not only the diversity of early-career researchers (ECRs) working in experimental biology during our centenary year but also the huge variety of animals and physiological systems that are essential for the ‘comparative’ approach. Nicolas Séon is an author on ‘ Determination of water balance maintenance in Orcinus orca and Tursiops truncatus using oxygen isotopes’, published in JEB. Nicolas conducted the research described in this article while a PhD student in Dr Peggy Vincent, Dr Romain Amiot and Prof. Sylvain Charbonnier's lab at Centre de Recherche en Paléontologie - Paris (CR2P), France. He begins a post-doc in February 2024 in the lab of Dr Mia Wege and Prof. Trevor McIntyre at University of South Africa (UNISA)/University of Pretoria (UP), South Africa, investigating the ecology and physiology of extant and fossil marine vertebrates using isotope geochemistry.

Nicolas Séon

Describe your scientific journey and your current research focus

I have just finished my PhD thesis on the determination of thermoregulatory strategies in extant and fossil marine vertebrates using oxygen isotopes. My thesis work focused on two issues related to marine vertebrate physiology: thermoregulation and water balance. Using oxygen isotopes, we demonstrated that intra-skeletal variability in δ18Op (the oxygen isotope composition of the phosphate from vertebrate bones and teeth) could be used to locate regional heterothermies in marine vertebrates, based on their skeletons. This main result paves the way to infer thermoregulatory strategies of extinct marine vertebrates. Then, to study the maintenance of water balance, we used an approach combining experimental data and modelling to define how Cetacea, and more specifically killer whales and common bottlenose dolphins, maintain their balance and determine the contribution of the environmental water, the food and the metabolic water produced by the organism. This work on modern marine vertebrates has enabled me to develop hypotheses concerning thermoregulation and the maintenance of water balance in marine reptiles living in the time of the dinosaurs. I will soon be starting a post-doc in South Africa on the intra- and inter-individual variability of the diet of Ross's seals in the Weddell Sea.

How would you explain the main finding of your paper to a member of the public?

Early 20th century studies on cetacean water balance were mainly carried out on fasting dolphin and porpoise pups, and it was assumed that cetaceans did not drink salt water. Later, it was demonstrated that the body water of fasting dolphins was a combination of ingested salt water and metabolic water, while some authors considered that surrounding salt water was the main source of water in fasting cetaceans. Until the beginning of the 21st century, therefore, there was considerable controversy over the maintenance of water balance in Cetacea. Currently, prey body water and water derived from their metabolism (i.e. metabolic water) are considered to be the two main sources of water for cetaceans, but their respective contributions remain unknown, particularly in fed animals. In our study, we measured and used the isotopic composition of oxygen as a tracer of water sources in cetaceans to determine the contribution of each source using a physiological model. The results of the model indicate a high contribution of prey water (61–69%) and metabolic water (26–35%) to the body water of orcas and common bottlenose dolphins; the differences in metabolic water production between the two species are mainly due to the composition of their diet, the contributions of inhaled water vapour and surrounding salt water being low (<5%). This demonstrated that Cetacea hydrate mainly via the water contained in their prey and not with surrounding salt water.

What are the potential implications of this finding for your field of research?

In addition to the physiological implications, this study opens up new perspectives on the conservation biology of Cetacea. Indeed, showing that the quantity of fish ingested by Cetacea, as well as their nutritional value (protein and lipid composition), are important parameters for maintaining the water balance in these organisms, could help to guide fishing policies in certain regions where Cetacea populations live. Going even further, this study raises questions about the future of Cetacea in the face of global warming. Rising ocean temperatures mean that certain species of fish will migrate to greater depths than cetaceans are able to dive to. So what impact will this scarcity of prey have on Cetacea populations? Will they migrate, at the risk of upsetting the food chains in certain regions of the globe, or will they disappear?

Which part of this research project was the most rewarding/challenging?

I really liked the multidisciplinary nature of this study. It involved physiology, isotope geochemistry and modelling and gave me the opportunity to discuss these subjects with specialists. Moreover, as a palaeontologist and geochemist by training, this study enabled me to immerse myself in the field of animal physiology.

Nicolas prepares samples of killer whale and common bottlenose dolphin body fluids for measurement of their oxygen isotope composition. Photo by Romain Amiot.

Nicolas prepares samples of killer whale and common bottlenose dolphin body fluids for measurement of their oxygen isotope composition. Photo by Romain Amiot.

Why did you choose JEB to publish your paper?

My co-authors and I chose to publish our article in JEB because we acquired unique experimental data (oxygen isotope composition of cetacean body fluids). JEB's vocation is to promote studies based on the experimental approach. It was therefore natural for us to turn to this journal. In addition, JEB's recognised impact within the biology and physiology community provides an exceptional window of exposure for our study.

What do you think experimental biology will look like 50 years from now?

It's difficult to predict what will happen to experimental biology in the future, but I hope that experimental work will have a bright future ahead of it. In isotopic geochemistry, and even more so when we are interested in extinct organisms as I had the opportunity to do during my thesis, we need a well-constrained framework in order to define the influence of each parameter on isotopic data, which can sometimes be the result of several factors.

What changes do you think could improve the lives of early-career researchers, and what would make you want to continue in a research career?

In my opinion, the most challenging aspect lies in the scarcity of permanent job positions available in research, even after having held one or two post-doc positions. It can be difficult for some young researchers to envisage a stable family life when they have to be mobile to take advantage of the post-doc opportunities available to them. Increasing the number of research positions available after one or two post-doc positions would greatly enhance the prospects of early-career researchers.

What's next for you?

At the beginning of next year, I'll be starting a post-doc on the trophic ecology of the Ross seal. I'm really looking forward to starting a new project with a new team. It will be an opportunity for me to discover new infrastructures, new ways of working, a new subject of study – the Ross seal – and to learn new techniques that I'm convinced will be beneficial for the rest of my research career.

Nicolas Séon's contact details: Centre de Recherche en Paléontologie - Paris (CR2P), Muséum national d'Histoire naturelle, 75005 Paris, France and ‘Laboratoire de Géologie de Lyon, Terre, Planètes et Environnement’, Université Claude Bernard Lyon 1, Ecole Normale Supérieure de Lyon et Université Jean Monnet, 69622 Villeurbanne Cedex, France

E-mail: nicolasseon517@gmail.com

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et al. 
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2023
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Determination of water balance maintenance in Orcinus orca and Tursiops truncatus using oxygen isotopes
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J. Exp. Biol.
226
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jeb245648
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