The erection mechanism of the penis in most vertebrates is blood vascular. A major evolutionary transition occurred in birds, where the erection mechanism changed from blood vascular to lymphatic. Within birds, however, the erection mechanism of the ratite penis has remained unknown. Early work suggested that the erection mechanism in ostrich Struthio camelus was blood vascular while no description existed for the emu Dromaius novaehollandiae or the rhea Rhea americana. Because the penis in all other described birds has a lymphatic erection mechanism, clarifying that the erection mechanism of ratites is of great importance to understanding one of the major evolutionary transitions of penis morphology within amniotes. Here, we show that the erection mechanism of ratites is lymphatic, confirming that the evolutionary transition to lymphatic erection occurred in the last common ancestor of Aves.
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