Bonobos make themselves appear smaller than they actually are

In general, one cannot only see how tall an animal or a person is, one can also hear it, because with increasing body size the pitch decreases. Although bonobos and chimpanzees are similar in size, bonobo calls sound an octave higher than chimpanzee calls. Researchers at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, explain this discrepancy with the fact that the vocal folds of bonobos are only half as long as those of chimpanzees of the same age. Whether or not other factors have contributed to this discrepancy is subject to further research.