Table 2.

The question of why a particular behavioral trait, such as vocal production learning, exists in a species can be answered taking ultimate and proximate causes into account (Tinbergen, 1963).

	Humans	Pinnipeds
Mechanism	In humans the mechanisms underlying speech production are increasingly well understood by studying brain areas e.g., auditory and motor cortices, basal ganglia (Kung et al., 2013), vocal folds' dynamics, and sound articulation in the upper vocal tract (Fitch, 2000).	Likewise, an increasingly compelling hypothesis is that harbor seals may produce flexible vocalizations via human-like laryngeal vibrations and finely controlled vocal tract filtering (Schneider, 1962; Schneider et al., 1964; Spasikova et al., 2008; Table 1A–C).
Ontogeny	The ontogeny of human speech production is studied by tracking how the linguistic input infants receive from birth influences and shapes the uttering of first words.	The ontogeny of vocal production in harbor seals is quite complex: early developmental influences due to mother-infant communication (Sauvé et al., 2015b) seem to complement later social interactions (Riedman, 1990; Table 1D,E).
Function	Contrasting hypotheses on the original function of human speech abound, ranging from a primate-like lip-smacking social display, later exapted for communication, to mate attraction via production of complex vocalization, as in songbirds (see Fitch, 2010).	Vocal behavior in harbor seals is involved in male-male competition (Hanggi and Schusterman, 1994), mother-infant interaction (Sauvé et al., 2015a,b), individual recognition, sexual and territorial advertisement, or lek (i.e., group competitive) displays (Hayes et al., 2004); VPL may have evolved under functional pressure for one of these functions (Table 1E,I,M).
Phylogeny	Current evidence suggests that humans were the only ones who acquired speech (Fitch, 2000, 2010) among the ancestors of living apes, instead of the alternative possibility that all apes but humans have lost an ancestral proto-speech.	Phylogeny of VPL in seals is more uncertain: phocids and walruses (Odobenus rosmarus) are vocal learners but sea lions seem not to be (Schusterman, 2008; Schusterman and Reichmuth, 2008; Reichmuth and Casey, 2014; Stansbury et al., 2015).