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

Breath-holding foraging implies adaptations to limit oxygen depletion and maximize foraging time. Physiological adjustments can be mediated through oxygen consumption and muscle mitochondria, which also produce reactive oxygen species. 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 increase their activity, making this period critical for the development of adaptive response to hypoxia and oxidative stress. We investigated the functional capacity of a swimming muscle in recently weaned and adult female southern elephant seals. High-resolution respirometry was employed to examine muscle respiratory capacity, and we assessed differences in protein and gene expressions using LC-MS/MS and RT-PCR. Lipid and protein oxidation was measured in the plasma. We found higher mitochondrial coupling efficiency in juveniles, probably in response to higher activity. There were no differences in oxidative damage, but the adults had higher levels of antioxidant defenses. Both hypoxia and oxidative response pathways appeared down-regulated in juveniles. This study highlitghts the effect of regular exposure to oxygen restriction in triggering the adaptive response to hypoxia and oxidative stress, but also how on-land constraints contribute to immediate and long-term responses to physiological stresses.