Body temperature depression and peripheral heat loss accompany the metabolic and ventilatory responses to hypoxia in low and high altitude birds
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SUMMARYThe objectives of this study were to compare the thermoregulatory,metabolic and ventilatory responses to hypoxia of the high altitude bar-headed goose with low altitude waterfowl. All birds were found to reduce body temperature (Tb) during hypoxia, by up to 1–1.5°C in severe hypoxia. During prolonged hypoxia, Tb stabilized at a new lower temperature. A regulated increase in heat loss contributed to Tb depression as reflected by increases in bill surface temperatures (up to 5°C) during hypoxia. Bill warming required peripheral chemoreceptor inputs, since vagotomy abolished this response to hypoxia. Tb depression could still occur without bill warming, however, because vagotomized birds reduced Tb as much as intact birds. Compared to both greylag geese and pekin ducks, bar-headed geese required more severe hypoxia to initiate Tb depression and heat loss from the bill. However, when Tb depression or bill warming were expressed relative to arterial O2 concentration (rather than inspired O2) all species were similar; this suggests that enhanced O2 loading,rather than differences in thermoregulatory control centres, reduces Tb depression during hypoxia in bar-headed geese. Correspondingly, bar-headed geese maintained higher rates of metabolism during severe hypoxia (7% inspired O2), but this was only partly due to differences in Tb. Time domains of the hypoxic ventilatory response also appeared to differ between bar-headed geese and low altitude species. Overall, our results suggest that birds can adjust peripheral heat dissipation to facilitate Tb depression during hypoxia,and that bar-headed geese minimize Tb and metabolic depression as a result of evolutionary adaptations that enhance O2transport.
