Mongolian Gerbil psychoacoustics have been under investigation for several years now - particularly by Dr. Joan Sinnott, Endowed Professor of Research at the  Psychology Department, University of South Alabama, US.

And now, for the first time (?) an experiment has quantified gerbils’ ability to recognise vowel sounds in human speech. The study finds that

“Gerbils easily learn to differentiate the spectrally-dissimilar vowels /u/ versus /i/ …“

And, with suitable training, gerbils can even differentiate between similar-sounding vowels such as /a/ versus /ae/.

See : A vowel identification procedure for gerbils published in J. Acoust. Soc. Am. Volume 122, Issue 5, pp. 2947-2947