Southern elephant seals (Mirounga leonina) are faced with contrasting periods in terms of energy requirements. They alternate foraging periods at sea, where they feed to replenish their body fuels, and periods on land where they fast and complete their breeding cycle and moult. The moult is an energetically costly phase of their lifecycle during which Southern elephant seals aggregate or huddle more or less closely depending on local climate. Huddling is a powerful energy saving strategy widely used by mammals and birds facing high energetic demands. However, huddling behaviour and its energetics in Southern elephant seals have not yet been extensively studied. This project therefore focuses on this energy saving strategy used by Southern elephant seals during their moult on land. We hypothesise that behavioural and physiological adaptations linked to huddling during the moult, may be influenced by the organism’s body condition and the environmental constraints while fasting. Huddling would thus allow individuals to minimise the time and energy required to complete the necessary replacement of skin and hair. Our main objectives are to determine how Southern elephant seals behave during the moulting period (huddling, posture, haul-out sites and changes of location), and how they cope with the energy demands of the moult (body composition, core, and skin temperature) according to weather conditions. Our preliminary results seem to confirm the above hypotheses. Elephant seals indeed aggregated as a function of meteorological conditions and their stage of moult, using different habitats (grass, wallows and beach). They aggregated more densely on days of bad weather (depending on habitat type) and individuals with a poorer body condition (lower body mass index) tended to aggregate more frequently. Moreover, elephant seals gained a thermal benefit from aggregations (surface temperature of aggregated individuals being higher when in the centre and more constant within aggregations). These thermal benefits would permit them to accelerate their moult, and thus optimise the energy allocated to their moult. This study will permit us to examine the thermal ecology of a species facing challenging energetic constraints, in order to better understand how organisms are able to cope behaviourally and physiologically with critical periods in their lifecycle in relation to changing environmental conditions.