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      Energy Homeostasis in Monotremes

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          Abstract

          In 1803, the French anatomist Étienne Geoffroy Saint-Hilaire decided that the newly described echidna and platypus should be placed in a separate order, the monotremes, intermediate between reptiles and mammals. The first physiological observations showed monotremes had low body temperatures and metabolic rates, and the consensus was that they were at a stage of physiological development intermediate between “higher mammals” and “lower vertebrates.” Subsequent studies demonstrated that platypuses and echidnas are capable of close thermoregulation in the cold although less so under hot conditions. Because the short-beaked echidna Tachyglossus aculeatus, may show very large daily variations in body temperature, as well as seasonal hibernation, it has been suggested that it may provide a useful model of protoendotherm physiology. Such analysis is complicated by the very significant differences in thermal relations between echidnas from different climates. In all areas female echidnas regulate T b within 1°C during egg incubation. The lactation period is considered to be the most energetically expensive time for most female mammals but lactating echidnas showed no measurable difference in field metabolic rate from non-lactating females, while the lactation period is more than 200 days for Kangaroo Island echidnas but only 150 days in Tasmania. In areas with mild winters echidnas show reduced activity and shallow torpor in autumn and early winter, but in areas with cold winters echidnas enter true hibernation with T b falling as low as 4.5°C. Monotremes do not possess brown adipose tissue and maximum rates of rewarming from hibernation in echidnas were only half those of marmots of the same mass. Although echidnas show very large seasonal variations in fat stores associated with hibernation there is no relationship between plasma leptin and adiposity. Leptin levels are lowest during post-reproductive fattening, supporting suggestions that in evolutionary terms the anorectic effects of leptin preceded the adiposity signal. BMR of platypuses is twice that of echidnas although maximum metabolism is similar. High levels of thyroid hormones in platypuses may be driving metabolism limited by low body temperature. Monotremes show a mosaic of plesiomorphic and derived features but can still inform our understanding of the evolution of endothermy.

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          The Expensive-Tissue Hypothesis: The Brain and the Digestive System in Human and Primate Evolution

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              Ratio of central nervous system to body metabolism in vertebrates: its constancy and functional basis.

              We present and document an hypothesis that healthy adults of most vertebrate species use 2-8% of their basal metabolism for the central nervous system (CNS). This relationship is constant across all classes of vertebrates, as we found by examining data from 42 species, including 3 fish, 3 amphibia, 2 reptiles, 6 birds, and 28 mammals. To explain its constancy, we hypothesize that an optimal functional relationship between the energy requirements of an animal's executor system (muscle metabolism) and its control system (CNS metabolism) was established early in vertebrate evolution. Three types of exceptional cases are discussed in terms of the hypothesis: very large animals, domesticated animals, and primates.
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                Author and article information

                Contributors
                Journal
                Front Neurosci
                Front Neurosci
                Front. Neurosci.
                Frontiers in Neuroscience
                Frontiers Media S.A.
                1662-4548
                1662-453X
                21 April 2017
                2017
                : 11
                Affiliations
                Biological Sciences, University of Tasmania Hobart, TAS, Australia
                Author notes

                Edited by: Maximilian Michel, University of Michigan, USA

                Reviewed by: Fritz Geiser, University of New England, Australia; Frank Grutzner, University of Adelaide, Australia

                *Correspondence: Stewart C. Nicol s.c.nicol@ 123456utas.edu.au

                This article was submitted to Neuroendocrine Science, a section of the journal Frontiers in Neuroscience

                Article
                10.3389/fnins.2017.00195
                5399094
                7576a0db-46d8-46fe-b7ae-2db9092d997d
                Copyright © 2017 Nicol.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

                Page count
                Figures: 6, Tables: 1, Equations: 1, References: 177, Pages: 17, Words: 15274
                Funding
                Funded by: Australian Research Council 10.13039/501100000923
                Award ID: A19602801
                Funded by: National Geographic
                Award ID: 9171-12
                Categories
                Neuroscience
                Review

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