Production, usage, and comprehension in animal vocalizations.

In this review, we place equal emphasis on production, usage, and comprehension because these components of communication may exhibit different developmental trajectories and be affected by different neural mechanisms. In the animal kingdom generally, learned, flexible vocal production is rare, appearing in only a few orders of birds and few species of mammals. Compared with humans, the majority of species produce a limited repertoire of calls that show little modification during development. Call usage is also highly constrained. Unlike humans, most animals use specific call types only in a limited range of contexts. In marked contrast to production and usage, animals' comprehension of vocalizations, as measured by their responses, are highly flexible, modifiable as a result of experience, and show the most parallels with human language. The differences among vocal production, usage, and comprehension create an oddly asymmetric system of communication in which a small repertoire of relatively fixed calls, each linked to a particular context, can nonetheless give rise to an open-ended, highly modifiable, and cognitively rich set of meanings. Recent studies of baboons and eavesdropping songbirds provide two examples.