Fig 1.
Tree of vocal learners with a focus on behavior.
Differences between this tree and the canonical tree are reflected in the presence of primates and rodents. The inclusion of whole families or orders is made under the assumption that all members thereof are at least worth studying and is not a claim about their actual vocal learning abilities, which are an empirical question. Common and scientific names from NCBI (https://www.ncbi.nlm.nih.gov/taxonomy). Tree built with taxize R package [82]. NCBI, National Center for Biotechnology Information.
Fig 2.
Comparison of simplified phylogenies of vocal learning: The canonical list with the addition of the orangutan (Pongo) for contrast.
Key: green indicates species or orders considered to conform to the criterion on each tree, and red indicates species or orders that are considered not to. Left: list of accepted vocal learners. Center: list of species for which there is evidence of a direct connection between forebrain and phonatory muscles. Right: list of species for which there is (some) evidence of vocal imitation.
Fig 3.
Outline of a vocal learning contiguum.
Three main dimensions are considered: learned vocal behavior (green), functional pressures (red), and specialized neural implementation (blue). Examples are given to different factors that can be included in each of these dimensions. Black dots represent hypothetical species placed in the morphospace according to evidence. Species A displays vocal learning abilities and is thought to have a particular neuronal implementation in place, as well as subject to considerable functional pressures that help shape the behavior. Species B is a vocal learner, but evidence for a particular specialized implementation is scarce. For species C, there is no behavioral evidence of vocal learning, and no specialized neural implementation for vocal learning is thought to be present (and concomitantly no functional pressures).