What is it about?
We investigated how much middle ear function contributes to hearing in the elephant, the largest terrestrial mammal. It is not surprising that elephants have a lower frequency ear than humans, with an ear drum that is seven-times larger and ossicles that are ten times more massive. Given an anatomy geared toward low-frequency sensitivity, what is surprising is their sensitivity above 1 kHz. Our finding suggests an unexpected specialization in elephant middle ears for enhanced high-frequency hearing.
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Why is it important?
In order to hear in a frequency range that benefits communication in any particular species, it is understood that there are trade-offs in order for an ear to be specialized. To specialize in low-frequency hearing, it is expected that hearing at higher frequencies would be compromised--heavy ossicles being harder to move at high frequencies. This is a pattern across all mammals described to date. What is important in this study, is that elephants don't fit this metric. Somehow they are compensating for the handicap of their low-frequency specialization in order to hear at higher frequencies than expected.
Perspectives
Any time data does not fit an expected pattern, an opportunity presents itself. Why can elephants hear higher frequencies than expected? How are they compensating for their low-frequency specialization that in effect amounts to a high-frequency hearing handicap? Group delay across ossicles is thought to facilitate better high-frequency cochlear input. We show that the elephant stapes velocity phase exhibits a group delay that is double that of humans. One explanation for the elephant's higher than expected sensitivity to high frequencies may lie in the possibility of additional modes of motion that would contribute to a larger delay, and thus compensate for their high-frequency handicap. A further investigation into mechanisms driving this larger group delay in a low-frequency ear could lead to a better understanding of high frequency hearing sensitivity across mammals.
Caitlin OConnell-Rodwell
Harvard Medical School
Read the Original
This page is a summary of: The impact of size on middle-ear sound transmission in elephants, the largest terrestrial mammal, PLoS ONE, April 2024, PLOS,
DOI: 10.1371/journal.pone.0298535.
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