Researchers examined blood samples, hair samples and measurements collected from mantled howler monkeys and black howler monkeys that were live-captured and released in Mexico and Guatemala between 1998 and 2008. The two monkey species splintered off from a common ancestor about 3 million years ago; today they live in mostly separate habitats, except for a "hybrid zone" in the state of Tabasco in southeastern Mexico, where they coexist and interbreed.

Through an analysis of genetic markers, from both mitochondrial DNA (the DNA in the cells' energy-making structures that gets passed down by mothers) and nuclear DNA, the researchers identified 128 hybrid individuals that were likely the product of several generations of interbreeding. Even so, these hybrids shared most of their genome with either one of the two species and were physically indistinguishable from the pure individuals of that species, the team found.

"The implications of these results are that physical features are not always reliable for identifying individuals of hybrid ancestry," Liliana Cortés-Ortiz, an evolutionary biologist and primatologist at the University of Michigan, said in a statement. "Therefore, it is possible that hybridization has been underestimated in the human fossil record."