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

Abstract

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