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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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          Ecological immunology: costly parasite defences and trade-offs in evolutionary ecology

          In the face of continuous threats from parasites, hosts have evolved an elaborate series of preventative and controlling measures - the immune system - in order to reduce the fitness costs of parasitism. However, these measures do have associated costs. Viewing an individual's immune response to parasites as being subject to optimization in the face of other demands offers potential insights into mechanisms of life history trade-offs, sexual selection, parasite-mediated selection and population dynamics. We discuss some recent results that have been obtained by practitioners of this approach in natural and semi-natural populations, and suggest some ways in which this field may progress in the near future.
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            Physiological stress in ecology: lessons from biomedical research.

            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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              Oxidative stress as a mediator of life history trade-offs: mechanisms, measurements and interpretation.

              The concept of trade-offs is central to our understanding of life-history evolution. The underlying mechanisms, however, have been little studied. Oxidative stress results from a mismatch between the production of damaging reactive oxygen species (ROS) and the organism's capacity to mitigate their damaging effects. Managing oxidative stress is likely to be a major determinant of life histories, as virtually all activities generate ROS. There is a recent burgeoning of interest in how oxidative stress is related to different components of animal performance. The emphasis to date has been on immediate or short-term effects, but there is an increasing realization that oxidative stress will influence life histories over longer time scales. The concept of oxidative stress is currently used somewhat loosely by many ecologists, and the erroneous assumption often made that dietary antioxidants are necessarily the major line of defence against ROS-induced damage. We summarize current knowledge on how oxidative stress occurs and the different methods for measuring it, and highlight where ecologists can be too simplistic in their approach. We critically review the potential role of oxidative stress in mediating life-history trade-offs, and present a framework for formulating appropriate hypotheses and guiding experimental design. We indicate throughout potentially fruitful areas for further research.
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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

                Author information
                http://orcid.org/0000-0001-9155-5359
                Article
                ECE34280
                10.1002/ece3.4280
                6145021
                fda00c21-031e-4972-ad98-e0e3cd9390fe
                © 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.

                History
                : 15 January 2018
                : 22 April 2018
                : 19 May 2018
                Page count
                Figures: 6, Tables: 2, Pages: 15, Words: 11307
                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
                Evolutionary Biology
                lactation, resource limitation, sea otter, trade‐offs

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