Based on mitochondrial DNA analysis, black tamarins were found to be closer related to populations on the same side of the Tocantins River than on the other, showing that the river constitutes an effective gene flow barrier.[10] As a consequence of the genetic divergence, as well as minor differences in pelage color, some recent authorities have argued for recognizing the population east of the Tocantins river as S. ursula, leaving the "true" S. niger for the population west of this river.[11]