Editing Oreopithecus (section)

====Semicircular canals====
The [[semicircular canal]]s of the inner ear serves as a sense organ for balance and controls the reflex for gaze stabilization. The inner ear has three canals on each side of the head, and each of the six canals encloses a membranous duct that forms an [[endolymph]]-filled circuit. Hair cells in the duct's [[auditory ampulla]] pick up endolymph disturbances caused by movement, which register as rotatory head movement. They respond to body sway of frequencies greater than 0.1&nbsp;Hz and trigger the vestibulocollic (neck) reflex and vestibuloocular (eye) reflex to recover balance and gaze stability. The bony semicircular canals allow estimates of duct arc length and orientation with respect to the sagittal plane.

Across species, the semicircular canals of agile animals have larger arcs than those of slower ones. For example, the rapid leaper ''[[Tarsius bancanus]]'' has semicircular canals much bigger than the slow-climbing ''[[Nycticebus coucang]]''. The semicircular canals of brachiating [[gibbon]]s are bigger than those of arboreal and terrestrial quadrupedal [[great ape]]s. As a rule of thumb, arc size of the ducts ''decreases'' with body mass and consequently slower angular head motions. Arc size ''increases'' with greater agility and thus more rapid head motions. Modern [[human]]s have bigger arcs on their anterior and posterior canals, which reflect greater angular motion along the sagittal plane. The lateral canal has a smaller arc size, corresponding to reduced head movement from side to side.
<ref name="Spoor-2003-p96">{{harvnb|Spoor|2003|pp=96–97}}</ref><!-- ref for para -->

[[Allometry|Allometric]] measurements on the [[bony labyrinth]] of BAC-208, a fragmentary cranium that preserves a complete, undeformed [[Petrous part of the temporal bone|petrosal bone]] suggest that ''Oreopithecus'' moved with agility comparable to extant [[great apes]]. Its anterior and lateral semicircular canal sizes fall within the range for great apes.
<ref name="Rook2004-p355">{{harvnb|Rook|Bondioli|Casali|Rossi|2004| p=355}}</ref>
Its relatively large posterior arc implies that ''Oreopithecus'' was more proficient at stabilizing angular head motion along the sagittal plane.