Behavioral measures of vowel sensitivity in Mongolian gerbils (Meriones unguiculatus): effects of age and genetic origin.

Absolute thresholds for complex vowel stimuli were compared in Mongolian gerbils (Meriones unguiculatus) as a function of age and genetic origin. For a group of 12-month-old 'domestic' gerbils obtained from Tumblebrook Farms, lowest thresholds averaging 14 dB SPL occurred for the vowel /alpha/, which had its most intense formant (F1) at 730 Hz. Thresholds increased to 22 dB SPL for /i/, which had its two most intense formants (F1 and F3) at 270 and 3000 Hz, respectively. Highest thresholds of 30 dB SPL occurred for /u/, which had its most intense formant (F1) at 300 Hz. Thresholds increased by about 10 dB per year through the ages of 12-36 months, with most of the loss occurring for /alpha/ and /u/. The domestic gerbils' /alpha/ thresholds corresponded well to those measured in aging gerbils in electrophysiological studies. Vowel thresholds were also measured in a group of first-generation offspring of 'wild' gerbils imported from Asia, first tested at the ages of 18-24 months. Thresholds were similar to those of the 12-month-old domestic gerbils, and showed no hearing loss with age up to 36 months. The wild gerbils were also free of ear impactions, which commonly occurred in the domestic gerbils. The hearing loss with age in the domestic gerbils may have a genetic basis, and might be due to inbreeding in the domestic strain, in contrast to the hybrid vigor of the wild gerbils.