<b>Ready to dive? Early constraints help juvenile southern elephant seals (</b><b><i>Mirounga leonina</i></b><b>) acclimatize to aquatic life</b><b><i>Mirounga leonina</i></b>

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.屏息觅食需要机体具备适应性机制，以限制氧耗并最大化觅食时长。此类生理调节可通过耗氧量与肌肉线粒体介导实现，而线粒体同时也会产生活性氧（reactive oxygen species）。南象海豹（southern elephant seals）会在海上觅食数月，单次潜水时长可达一小时。幼崽在断奶后期会突然转向水生生活，此阶段它们会禁食并提升活动量，因此该时期对其建立缺氧（hypoxia）与氧化应激（oxidative stress）的适应性反应至关重要。本研究针对近期断奶的幼崽与成年雌性南象海豹的游泳肌肉功能展开探究。我们采用高分辨率呼吸测定法（high-resolution respirometry）检测肌肉呼吸能力，并通过液相色谱-串联质谱法（LC-MS/MS）与逆转录聚合酶链式反应（RT-PCR）评估蛋白质与基因表达的差异。此外，我们还检测了血浆中的脂质与蛋白质氧化水平。研究结果显示，幼崽的线粒体偶联效率更高，这可能是对更高活动量的适应性响应。两组间的氧化损伤水平并无显著差异，但成年个体的抗氧化防御系统活性更高。幼崽体内的缺氧应答与氧化应激应答通路均呈现下调趋势。本研究不仅阐明了周期性暴露于氧限制环境如何触发机体对缺氧与氧化应激的适应性反应，同时揭示了陆地生存约束如何影响机体对生理应激的即时与长期响应。