1. Consonant–vowel ratio;Maddieson,2011
2. Phonetic universals in consonant systems;Lindblom,1988
3. The Evolution of Language;Fitch,2010
4. The contribution of source–filter theory to mammal vocal communication research;Taylor;J. Zool.,2010
5. A description of the orangutan's vocal and sound repertoire, with a focus on geographic variation;Hardus,2009