I find it hard to believe that Zipf's law is at all meaningful for non-human species. We have evolved in a very specific way, such that we are limited in the bandwidth of our transmitted communications, as well as in our ability to memorize large vocabularies. There is no reason that this is true for other species, including dolphins; they may be able to distinguish and produce significantly more nuanced "words" (if that concept even exists); they may be born with, or be able to quickly learn and commit to memory, the "distance graph" between a huge number of distinct signals that represent nuanced concepts. Consider an organism that can quantize a dense signal, treat it as a hash table lookup, and reference a specific concept in their memory; these transmissions would look like pure noise to us, and any error correction might be written off as periodicity. We'd never know.