ECR Spotlight – Julia Watson and Chloé Souques Free

Julia Watson (left) and Chloé Souques (right)

How did you become interested in biology?

Julia: Since I was small, I have always been interested in living things, especially animals, and wanted to be a vet (who didn't?). And ever since I have been aware of climate change, I have wanted to play a part in helping the world face it and understand how organisms are dealing with it. I remember when I was about 10, with a friend we made posters on climate change and tried putting them up around our town – sweet but very naive! At that time, I was also reading a lot about animals and living things and my interest in biology has remained uninterrupted since then.

Chloé: What first drew me to biology was my fascination with the living world – especially animals, their diversity of forms, and strategies they use to interact with and adapt to their environments. More specifically, I was particularly interested in animal behaviour, whether through wildlife documentaries or by observing animals around me. At the same time, I was deeply concerned about the various threats that humans, whether directly or indirectly, are imposing on biodiversity and this fuelled my strong desire to raise awareness about species conservation. When I read My Life with the Chimpanzees by Jane Goodall, I realized that I wanted to become a zoologist – to be specialist of one group of species and contribute to a better understanding of how these species live, communicate and interact with both their conspecifics and their environment. This motivation led me to study ecology to uncover the ‘why’ and ‘how’ behind animal behaviour. I remained strongly drawn to conservation, particularly in the light of the increasing environmental challenges that organisms face today. This raised a crucial question for me: how will species respond to environmental changes? Over courses at university, I have become fascinated by the evolutionary perspective we can apply to animal behaviour. Then, through my early research experiences, I discovered the important role that physiological phenotypes play in shaping these behaviours and this introduced me to the field of ecophysiology. This marked a major turning point in my scientific interests and thinking and now I am also deeply interested in what energetics can reveal about how animals cope with ecological constraints. Today, I may not be working in the jungle studying primate behaviour as I once imagined, but my experiences have shown me how exciting it is to take a cross-disciplinary, integrative approach applicable to a variety of biological models, and investigate if they are ‘principles’ that govern the living world.

Describe your scientific journey and your current research focus

Chloé: I started university with the idea of studying zoology and specializing in a particular group of species, hoping to better understand their ecology, allowing better protection of them. During my studies, I discovered the fascinating mechanistic and evolutionary dimensions behind each trait at every level of life, and that really captured my interest. My first research experience, on metabolism and personality in freshwater fish, introduced me to the field of ecophysiology through the links we can make with behaviour. This project showed me how combining different disciplines can open up exciting new perspectives, and it confirmed that research was the path I wanted to follow. Later on, during a longer master's internship and throughout my PhD, I focused on how animals respond – through their physiology and behaviour – to a growing environmental challenge: thermal variability. More specifically, I investigate unpredictable variability, or thermal stochasticity. Indeed, beyond regular daily and seasonal cycles, animals often face unpredictable temperature changes because they move in their environment, and because of the weather. Despite this ecological reality, the effect of thermal stochasticity on organisms is often underexplored in experimental research, even though it is likely to increase with climate change and more frequent extreme weather events.

I am studying how this kind of variability affects energy metabolism, personality traits, feeding behaviour and growth in freshwater fish. One of the key questions I'm exploring is whether unpredictable changes trigger unique physiological mechanisms or behavioural responses, compared with the more regular variations typically used in lab studies. To do so, it is crucial to finely understand how a single variable event – like a heat stress – impacts whole-animal and subcellular energetics, which is what we explore in this paper. I believe that combining ecophysiology and animal behaviour offers a particularly insightful perspective – both for examining how organisms’ lifestyle are affected by environmental changes and for uncovering the underlying mechanisms that drive these responses. As I continue investigating these questions, my interest in better understanding these patterns keeps increasing scientifically.

Julia: At high school I specialized in science and did a biology degree during which I did a first internship in a lab working on a certain gene that had a key role in obesity in mice. This made me realize that I wanted to do research but not clinically; that world was not for me. During the degree, we had a course on energetics and how organisms absorb nutrients and oxygen to produce energy. This course made me wonder how on earth all these metabolic pathways could have been designed to work so well, and more; they can adapt when the environment changes! This raised my interest in metabolism and different adaptations organisms can have when facing different environmental stresses. Then during my master’s, I did internships on metabolism, stress and ageing in birds that I enjoyed and confirmed the fact I wanted to stay in research. The opportunity to do a PhD on the effects of temperature on freshwater fish metabolism – I had never worked on fish but I am very happy now. Currently, my aim is to try and understand the links between cellular and whole-body metabolism in fish exposed to high temperatures. At the cellular level, I study mitochondria, organelles that consume oxygen and nutrients to produce ATP, the energy that is used by cells. As mitochondria are said to consume most of the oxygen taken in by an organism, I also study whole-body oxygen consumption in fish (I basically put them in Tupperware and measure their oxygen consumption) and try and link both… which can be tricky at times! More particularly, I study freshwater fish on a thermal gradient along the Rhône river and try to see how each population is adapted to different thermal ranges.

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

Climate change is often seen as a gradual rise in average temperatures, but it also brings increased variability that can lead to thermal shocks – like during heat stresses. And this affects fish too. Just because they live in water does not mean they stay cool in the summer during a heat stress! They also experience large temperature fluctuations. In this paper, we focused on an endangered freshwater fish from France, the Rhône streber. We aimed to better understand the effects of a heat stress on their energetic needs – both at the whole-animal level and at the subcellular level. Our results show that there is no additional metabolic cost associated with the heat stress, and that fish quickly return to their pre-heat-stress state, but there is a temporary decrease in mitochondrial activity – which uses oxygen and nutrients to produce energy that is used by the cells – just after the temperature rise. This suggests that, within the temperature range tested, the heat stress doesn't represent a major physiological stress for the species. This highlights the value of using a multi-scale approach to better understand how animals cope with environmental fluctuations.

What do you enjoy most about research, and why?

Julia: I think I enjoy the people most. Indeed, going to work with people who are interested in the same topics as you and working together to answer the same questions is great. Moreover, meeting people at conferences and reading new papers from them is really encouraging as there is a whole community out there trying to answer the same questions as you and you get the feeling that everyone is working towards the same goal and that together we are slowly but steadily moving forwards. And of course, my PhD journey definitely wouldn't be the same without Chloé!

Chloé: I really enjoy the possibility of multidisciplinary approaches and the resulting diversity of activities. Experimentally speaking, I might conduct personality tests, count out worms for predation tests, or place fish in a swim tunnel or a respirometer – sometimes all during the same day! Consequently, the following data analysis involves a wide range of methods and concepts. Beyond the experimental work, stages of data manipulation and analysis, writing and communication of results, each challenges different skills and at the end, there are no two days or months that are ever the same.

I also greatly appreciate the ‘without borders’ aspect of scientific research, which places teamwork and knowledge sharing at the heart of daily activities. This provides the opportunity to meet and collaborate with inspiring people from diverse backgrounds and areas of expertise, both daily and during fieldwork, conferences and collaborations. I really enjoy this opportunity to do science with people I choose and get along with – including Julia my inseparable PhD partner!

Finally, what is for me a major source of fulfilment lies in the literal sense of the word ‘research’. I really enjoy this scientific approach: investigating, asking questions and seeking solutions to problems, which I find both stimulating and rewarding. I initially came to focus on behaviour, and now I find myself sometimes exploring mitochondrial questions. I find it exciting that, through encounters, theoretical exploration and scientific inquiry, research often leads us to unexpected paths, opening the way to new questions and discoveries.

What is the hardest challenge you have faced in the course of your research and how did you overcome it?

Chloé: I think the most challenging part of my PhD so far has been to plan a preliminary experiment that was essential for designing the core experiment of my thesis and involved documenting thermal performance curves. From the start, designing this experiment was a theoretical and conceptual challenge, and because I am working with three supervisors from very different backgrounds: behavioural ecology, comparative physiology and ecophysiology. This interdisciplinary approach is incredibly enriching, generating original ideas and offering complementary perspectives but here I realized that aligning everyone on the same experimental design was kind of challenging because each brought different assumptions and ways of thinking. That's when I truly became aware of the importance of building bridges between disciplines, by realizing that linking responses across different organization levels – physiology, behaviour and ecology – was not as straightforward as it might seem. Then came the practical side of running the experiment, which was logistically demanding, required a lot of planning and coordination… and had to be postponed across two seasons. First, because of the weather: high river flows delayed fishing for several months. Then, when we were finally able to catch the fish, individuals were in poor condition, and most of them didn't survive the transfer to the animal facility. It was the first time I really experienced the inherent unpredictability of working with living – and wild – models. Fortunately, we did not give up but instead designed another experiment, which is precisely the one with strebers described in our recent paper, and it worked!

Julia: I think the biggest challenge is how unpredictable research can be at times. Funnily enough, without this challenge this paper would not have existed! Indeed, in October 2023, I had just started my PhD and we were supposed to fish along the Rhône River for my first experiment. However, from October until February, fishing was not possible because the river was flooded, and this was very frustrating. As we had some Rhône strebers in our animal care facility, we designed an experiment with them, and it led to this paper. Moreover, when we went fishing in February, there was a problem with the fish and most of them did not make it so that was quite hard as well but thanks to the strebers, I had something to do and some data to work on.

What is your favourite animal, and why?

Chloé: One of my favourite animals is the deep-sea anglerfish. I think it illustrates how life can adapt to even the most extreme conditions, with fascinating innovations like a kind of lantern organ producing bioluminescence. Living in depths that are rarely explored, it also represents the unknown – what remains largely out of reach – inviting both humility and curiosity about the diversity of animal adaptations and all that we have yet to uncover. It highlights the need to protect, as much as possible, habitats that have not yet been impacted by human activity.

Julia: Spoiler alert: my favourite animal is not a fish (but it is an ectotherm)! I really like octopuses; the way they look, the way they are made and the way they work. They are rather fascinating creatures and I would love to have the opportunity to work on them one day.