While the previous sentence mentions the inherent difficulty of such studies, and the range of possible interpretations of results. The 'Language' article says "The term "animal languages" is often used for nonhuman languages. Linguists do not consider these to be language". Sounds like a lot of evidence could probably be found to support one view or the other, with suitable interpretation.
Even if it could be shown beyond reasonable doubt that verbal language exists among animals, you could ask why, in that case, there does not exist in any other species a cummulative culture, whereby each generation not only inherits knowledge and habits (memetic information, if you like) from the previous one, but builds on that knowledge and habits, giving rise to a culture which can grow, mutate and evolve in a manner somewhat analagous to biological evolution.