Extreme fieldwork: flame-sealed capillaries versus frozen serums to estimate body composition of southern elephant seals (Mirounga leonina) using isotopic dilution

Body composition is an important parameter in diving animals intermittently fasting on land, such as southern elephant seals, as it is known to impact their time of departure at sea, swimming effort, foraging success, and survival. Deuterium labeled water is commonly used to assess total body water, but a precise indication of equilibration time in this species and reliable sample storage are lacking. We injected newly weaned southern elephant seal female pups with deuterium-labeled water. We thereafter blood sampled the seals several times to assess the equilibration time and body composition. We implemented two storage methods, flame-sealed capillaries and frozen serums stored at − 20 °C in a gas freezer. The frozen serums method was reliable as capillaries and safer to bring undamaged samples from the field. This method of determining body composition in wild animals is reliable and easily applicable for isolated and difficult-to-access fieldwork.