Ready to dive? Early constraints help juvenile southern elephant seals (Mirounga leonina) acclimatize to aquatic life

Breath-holding foraging implies different adaptations to limit oxygen (O₂) depletion and maximize foraging time. Physiological adjustments can be mediated through O₂ consumption, driven by muscle mitochondria, which can also produce reactive oxygen species during reoxygenation. Southern elephant seals spend months foraging at sea, diving for up to one hour. Pups transition abruptly to aquatic life after a post-weaning period, during which they fast and progressively increase their activity, making this period critical for the development of an adaptive response to oxygen restriction and oxidative stress. We compared the functional capacity of a swimming muscle in 5 recently weaned and 6 adult female southern elephant seals. High-resolution respirometry was employed to examine muscle mitochondrial respiratory capacity and differences in protein and gene expressions of the main regulatory pathways were determined using LC-MS/MS and RT-PCR, respectively. Oxidative damages were measured in the plasma. Juveniles have higher mitochondrial coupling efficiency compared to adults probably as a response to growth and important physicial activity reported during the post-weaning. There were no differences in oxidative damage, but the adults had a higher level of antioxidant defenses. Both hypoxia and oxidative response pathways appeared less activated in the juveniles. This study highlights the differences in muscle metabolism and the likely adaptive response to hypoxia and oxidative stress between juveniles and adult south elephant seals. It also suggests that early constraints such as fasting, physical activity, and short-term low O₂ partial pressure exposure could contribute to immediate and long-term responses and to acclimatize juveniles to aquatic life.