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 1 h. 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 expression of the main regulatory pathways were determined using LC-MS/MS and RT-qPCR, respectively. Oxidative damage was measured in the plasma. We found that juveniles have higher mitochondrial coupling efficiency compared with adults, probably as a response to growth and significant physical activity reported during the post-weaning period. There were no differences in oxidative damage, but adults had a higher level of antioxidant defenses. Both hypoxia and oxidative response pathways appeared less activated in juveniles. This study highlights the differences in muscle metabolism and the likely adaptive response to hypoxia and oxidative stress between juvenile 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 help to acclimatize juveniles to aquatic life.