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To support the high rates of oxygen consumption associated with being warm-blooded, the hearts of birds and mammals must pump rapidly and frequently. These high heart rates are made possible by a network of conductive tissue that spreads across the heart, controlling its contraction. The evolutionary origin of this conductive tissue has long been a mystery; it has never been identified in reptiles, the shared ancestors of birds and mammals, leading scientists to wonder whether the conductive network evolved independently in the two taxa.
Lab Anim. (NY) 41, 300 (2012).
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