Ontogeny of Carbon Monoxide-Related Gene Expression in a Deep-Diving Marine Mammal

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.

Contrasting offspring dependence periods and diving development rates in two closely related marine mammal species

Understanding the ontogeny of diving behaviour in marine megafauna is crucial owing to its influence on foraging success, energy budgets, and mortality. We compared the ontogeny of diving behaviour in two closely related species-northern elephant seals (Mirounga angustirostris, n = 4) and southern elephant seals (Mirounga leonina, n = 9)-to shed light on the ecological processes underlying migration. Although both species have similar sizes and behaviours as adults, we discovered that juvenile northern elephant seals have superior diving development, reaching 260 m diving depth in just 30 days, while southern elephant seals require 160 days. Similarly, northern elephant seals achieve dive durations of approximately 11 min on their first day of migration, while southern elephant seals take 125 days. The faster physiological maturation of northern elephant seals could be related to longer offspring dependency and post-weaning fast durations, allowing them to develop their endogenous oxygen stores. Comparison across both species suggests that weaned seal pups face a trade-off between leaving early with higher energy stores but poorer physiological abilities or leaving later with improved physiology but reduced fat stores. This trade-off might be influenced by their evolutionary history, which shapes their migration behaviours in changing environments over time.

Intracellular negative feedback mechanisms in blubber and muscle moderate acute stress responses in fasting seals

Animals may limit the cost of stress responses during key life history stages such as breeding and molting by reducing tissue sensitivity to energy-mobilizing stress hormones (e.g. cortisol). We measured expression of genes encoding glucocorticoid receptor (GR, NR3C1), GR inhibitor (FKBP5) and cortisol-inactivating enzyme (HSD11B2) in blubber and muscle of northern elephant seals before and after stress axis stimulation by adrenocorticotropic hormone (ACTH) early and late in a fasting period associated with molting. ACTH elevated cortisol levels for >24 h and increased FKBP5 and HSD11B2 expression while downregulating NR3C1 expression in blubber and muscle, suggesting robust intracellular negative feedback in peripheral tissues. This feedback was maintained over prolonged fasting, despite differences in baseline cortisol and gene expression levels between early and late molt, suggesting that fasting-adapted animals use multiple tissue-specific, intracellular negative feedback mechanisms to modulate downstream impacts of acute stress responses during key life history stages.

Elevated antioxidant defence in the brain of deep-diving pinnipeds

While foraging, marine mammals undertake repetitive diving bouts. When the animal surfaces, reperfusion makes oxygen readily available for the electron transport chain, which leads to increased production of reactive oxygen species and risk of oxidative damage. In blood and several tissues, such as heart, lung, muscle and kidney, marine mammals generally exhibit an elevated antioxidant defence. However, the brain, whose functional integrity is critical to survival, has received little attention. We previously observed an enhanced expression of several antioxidant genes in cortical neurons of hooded seals (Cystophora cristata). Here, we studied antioxidant gene expression and enzymatic activity in the visual cortex, cerebellum and hippocampus of harp seals (Pagophilus groenlandicus) and hooded seals. Moreover, we tested several genes for positive selection. We found that antioxidants in the first line of defence, such as superoxide dismutase (SOD), glutathione peroxidase (GPX) and glutathione (GSH) were constitutively enhanced in the seal brain compared to mice (Mus musculus), whereas the glutaredoxin and thioredoxin systems were not. Possibly, the activity of the latter systems is stress-induced rather than constitutively elevated. Further, some, but not all members, of the glutathione-s-transferase (GST) family appear more highly expressed. We found no signatures of positive selection, indicating that sequence and function of the studied antioxidants are conserved in pinnipeds.