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

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          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 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. 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. 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. 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. 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.

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          Most cited references 84

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              Physiological stress in ecology: lessons from biomedical research.

               L. Romero (2004)
              Increasingly, levels of the 'stress hormones' cortisol and corticosterone are being used by ecologists as indicators of physiological stress in wild vertebrates. The amplitude of hormonal response is assumed to correlate with the overall health of an animal and, by extension, the health of the population. However, much of what is known about the physiology of stress has been elucidated by the biomedical research community. I summarize five physiological mechanisms that regulate hormone release during stress that should be useful to ecologists and conservationists. Incorporating these physiological mechanisms into the design and interpretation of ecological studies will make these increasingly popular studies of stress in ecological settings more rigorous.
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                Author and article information

                Contributors
                sarahchinn@gmail.com
                Journal
                Ecol Evol
                Ecol Evol
                10.1002/(ISSN)2045-7758
                ECE3
                Ecology and Evolution
                John Wiley and Sons Inc. (Hoboken )
                2045-7758
                25 July 2018
                August 2018
                : 8
                : 16 ( doiID: 10.1002/ece3.2018.8.issue-16 )
                : 8433-8447
                Affiliations
                [ 1 ] Department of Biology Sonoma State University Rohnert Park California
                [ 2 ] U.S. Geological Survey Alaska Science Center Anchorage Alaska
                [ 3 ] U.S. Geological Survey Western Ecological Research Center Long Marine Laboratory Santa Cruz California
                [ 4 ] Monterey Bay Aquarium Monterey California
                Author notes
                [* ] Correspondence

                Sarah M. Chinn, Department of Biology, Sonoma State University, Rohnert Park, California.

                Email: sarahchinn@ 123456gmail.com

                Article
                ECE34280
                10.1002/ece3.4280
                6145021
                © 2018 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd.

                This is an open access article under the terms of the http://creativecommons.org/licenses/by/4.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

                Page count
                Figures: 6, Tables: 2, Pages: 15, Words: 11307
                Product
                Funding
                Funded by: CSU Council on Ocean Affairs, Science & Technology
                Funded by: American Cetacean Society – Monterey Bay
                Funded by: Sea Otter Foundation & Trust
                Funded by: Office of Naval Research
                Award ID: N000141410393
                Funded by: California State University's Council on Ocean Affairs, Science & Technology
                Categories
                Original Research
                Original Research
                Custom metadata
                2.0
                ece34280
                August 2018
                Converter:WILEY_ML3GV2_TO_NLMPMC version:version=5.4.9 mode:remove_FC converted:19.09.2018

                Evolutionary Biology

                lactation, resource limitation, sea otter, trade‐offs

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