HEARING IN PRIMITIVE MAMMALS - MONODELPHIS-DOMESTICA AND MARMOSA ELEGANS

Although opossums of the Family Didelphidae usually serve as a parsimo nious starting point for tracing the otological and neurological evolu tion of modern mammals, audiological data for Didelphid opossums is av ailable only for the North American opossum (Didelphis virginiana) whi ch because of its large size, may be one of the least representative g enera of the family. The present report extends the audiological data to two other species of Didelphid opossums, Monodelphis domestica, and Marmosa elegans. At 60 dB SPL, the hearing of Monodelphis extends fro m 3.6 kHz to 77 kHz, with a range of best sensitivity from 8 to 64 kHz while the hearing of Marmosa extends from 3.8 kHz to 80 kHz, with a r ange of best sensitivity from 8 to 64 kHz. Neither species was found t o be particularly sensitive to tones, with the average lowest threshol d near 20 dB SPL for Monodelphis and 33 dB SPL for Marmosa. These resu lts indicate that like the North American opossum both genera are sens itive to high frequencies yet relatively insensitive to sound. Because the hearing of the three genera of Didelphids agree in several respec ts, it can be concluded that sensitivity to high frequencies almost ce rtainly was present in ancient mammals, probably following quickly aft er the acquisition of a 3 ossicle middle ear linkage. It is not unlike ly that the utility value of high frequency hearing, rather than highl y sensitive hearing, may have been a primary source of selective press ure for this morphological transformation.