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The Journal of Experimental Biology 205, 2267-2273 (2002)
© 2002 The Company of Biologists Limited

Review

Adaptive thermogenesis in hummingbirds

José Eduardo P. W. Bicudo^1,*, Antonio C. Bianco² and Cláudia R. Vianna^1,3

¹ Department of Physiology, Biosciences Institute, University of São Paulo, SP, Brazil
² Thyroid Division, Department of Medicine, Brigham & Women's Hospital and Harvard Medical School, Boston, MA 02115, USA
³ Division of Endocrinology, Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, MA 02115, USA
^* Present address: Departmento de Fisiologia, Instituto de Biociências, Universidade de São Paulo, 05508-900 São Paulo, SP, Brazil

(e-mail: jebicudo{at}usp.br )

Accepted 13 May 2002

The occurrence of non-shivering thermogenesis in birds has long been a controversial issue. Although birds are endothermic vertebrates, sharing with mammals (placental mammals and marsupials) a common ancestor, they do not possess brown adipose tissue or a similar type of tissue, unlike their mammalian counterparts. Some bird species are, however, able to withstand very low ambient temperatures (-70 °C) or undergo periods of heterothermia, and there is now good experimental evidence showing that non-shivering thermogenesis may indeed occur in birds under such conditions. The skeletal muscles of birds, particularly the flight muscles, occupy a significant fraction (approximately 30 %) of the total body mass, and recent results have shown that they are likely to be the main sites for non-shivering thermogenesis. The precise mechanisms involved in adaptive thermogenesis in birds are still not fully understood. The translocation of Ca²⁺ between intracellular compartments and the cystosol mediated by the sarcoplasmic reticulum Ca²⁺-ATPase, uncoupled from ATP synthesis, is one mechanism whereby chemi-osmotic energy can be converted into heat, and it has been proposed as one of the possible mechanisms underlying non-shivering thermogenesis in birds on the basis of data obtained mainly from ducklings acclimatized to cold conditions. The recent characterization of an uncoupling protein homolog in avian skeletal muscle and the expression of its mRNA at different stages of the torpor/rewarming cycle of hummingbirds indicate that it has the potential to function as an uncoupling protein and could play a thermogenic role during rewarming in these birds.

Key words: non-shivering thermogenesis, brown adipose tissue, Ca²⁺-ATPase, uncoupling protein, bird, hummingbird

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