Data from: Lactation and resource limitation affect stress responses, thyroid hormones, immune function and antioxidant capacity of sea otters (Enhydra lutris)

1. Lactation is the most energetically demanding stage of reproduction in female mammals. Increased energetic allocation toward current reproduction may result in fitness costs, though the mechanisms underlying these trade-offs are not well understood. Trade-offs during lactation may include reduced energetic allocation to cellular maintenance, immune response and survival, and may be influenced by resource limitation. 2. As the smallest marine mammal, sea otters (Enhydra lutris) have the highest mass-specific metabolic rate necessitating substantial energetic requirements for survival. To provide the increased energy needed for lactation, female sea otters significantly increase foraging effort, especially during late-lactation. Caloric insufficiency during lactation is reflected in the high numbers of maternal deaths due to End-Lactation Syndrome in the California subpopulation. 3. We investigated the effects of lactation and resource limitation on maternal stress responses, metabolic regulation, immune function and antioxidant capacity in two subspecies of wild sea otters (northern: E. l. nereis and southern: E. l. kenyoni) within the California, Washington and Alaska subpopulations. 4. Lactation and resource limitation were associated with reduced glucocorticoid responses to acute capture stress. Corticosterone release was lower in lactating otters. Cortisol release was lower under resource limitation and suppression during lactation was only evident under resource limitation. Lactation and resource limitation were associated with alterations in thyroid hormones. Immune responses and total antioxidant capacity were not reduced by lactation or resource limitation. Southern sea otters exhibited higher concentrations of antioxidants, immunoglobulins and thyroid hormones than northern sea otters. 5. These data provide evidence for allocation trade-offs during reproduction and in response to nutrient limitation but suggest self-maintenance of immune function and antioxidant defenses despite energetic constraints. Income-breeding strategists may be especially vulnerable to the consequences of stress and modulation of thyroid function when food resources are insufficient to support successful reproduction and may come at a cost to survival, and thereby influence population trends.

1. 泌乳（Lactation）是雌性哺乳动物繁殖过程中能量消耗最高的阶段。尽管目前对这类权衡背后的机制尚不清楚，但将更多能量投入当前繁殖可能会带来适合度代价。泌乳期间的权衡可能包括减少用于细胞维持、免疫反应和生存的能量分配，且该过程可能受资源限制的影响。2. 作为体型最小的海洋哺乳动物，海獭（Enhydra lutris）拥有最高的单位质量代谢率，因此生存所需的能量投入极高。为满足泌乳所需的额外能量，雌性海獭会显著增加觅食强度，尤其是在泌乳后期。加州海獭亚种群中因泌乳末期综合征（End-Lactation Syndrome）导致的母体死亡数量居高不下，这正是泌乳期间能量摄入不足的直观体现。3. 本研究针对加州、华盛顿州与阿拉斯加的野生海獭两个亚种（北部亚种：E. l. nereis；南部亚种：E. l. kenyoni），探究了泌乳与资源限制对母体应激反应、代谢调控、免疫功能与抗氧化能力的影响。4. 泌乳与资源限制均会减弱机体对急性捕获应激的糖皮质激素（glucocorticoid）响应：泌乳个体的皮质酮（corticosterone）释放量更低；资源限制条件下皮质醇（cortisol）释放量降低，且泌乳期间的皮质醇抑制仅在资源限制状态下才会显现。此外，泌乳与资源限制会引发甲状腺激素水平的改变。但泌乳与资源限制并未对免疫反应与总抗氧化能力造成削弱。相较于北部海獭亚种，南部海獭亚种的抗氧化剂、免疫球蛋白（immunoglobulin）与甲状腺激素浓度均更高。5. 本数据集的相关数据为繁殖过程中以及应对营养限制时的能量分配权衡提供了实证支撑，同时也表明即便存在能量限制，机体仍可维持免疫功能与抗氧化防御系统的正常运转。当食物资源不足以支撑成功繁殖时，采用收入型繁殖策略的物种可能更容易受到应激与甲状腺功能调控异常的影响，进而付出生存代价，最终对种群动态产生影响。