English: Hypothetical distributions of two behavioral phenotypes: vocal learning and sensory (auditory) sequence learning. We hypothesize that the behavioral phenotypes of vocal learning and auditory learning are distributed along several categories. (A) Vocal learning complexity phenotype and (B) auditory sequence learning phenotype. The left axis (blue) illustrates the hypothetical distribution of species along the behavioral phenotype dimensions. The right axis (black step functions) illustrates different types of transitions along the hypothesized vocal-learning (A) or auditory-learning (B) complexity dimensions. Whether the actual distributions are continuous functions (blue curves), will need to be tested, in relation to the alternatives that there are several categories with gradual transitions or step functions (black curves). Although auditory learning is a prerequisite for vocal learning and there can be a correlation between the two phenotypes (A–B), the two need not be interdependent. A theoretical Turing machine (Turing, 1968) is illustrated [G∗], which can outperform humans on memory for digitized auditory input but is not a vocal learner. From Petkov, CI; Jarvis ED (2012). "Birds, primates, and spoken language origins: behavioral phenotypes and neurobiological substrates". Front. Evol. Neurosci. 4:12.