The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

What do primate auditory ossicles tell us about hearing patterns in living and extinct taxa?


Anatomy, Midwestern University

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The auditory ossicles have been used for functional and phylogenetic comparisons in various groups of mammals including primates. However, the implications of variation in ossicular morphology remains poorly understood and few studies have looked at the influence of different measurement protocols. In this study we examined a sample of primate earbones and took measurements on both isolated and articulated mallei and incudes. Our sample consisted of over 1000 ossicles representing 61 genera of primates. The measurements we took included the functional lengths (lever arms) of the malleus and incus, the angles of the manubrium and long process relative to the axis of rotation, and various other measures capturing shape characteristics of each bone. Measurements were made on scaled digital photographs of the ossicles taken under low magnification.

Many of the measurements differed depending on whether they were taken on isolated or articulated specimens. For example, although the two sets of measurements were correlated, the lever arm lengths for both the malleus and incus could differ by up to 18%. Similarly, the angles of the lever arms could show more than 20 degrees of difference depending on whether the measurements were taken on articulated pairs or on isolated bones. This is likely related to differences in interpreting the axis of rotation. However, regardless of which approach was used, strepsirhines demonstrated a lower angle of the manubrium when compared with haplorhines. This pattern may be related to the increased high frequency sensitivity characteristic of most strepsirhine taxa.

Funding for this project was provided by Midwestern University

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