DOI: 10.5061/dryad.g76n68h
title: Data from: Intraspecific variation and symmetry of the inner-ear labyrinth in a population of wild turkeys: implications for paleontological reconstructions

description: The cochlea and semicircular canals of the inner ear are vital
 neurosensory devices. There are associations between the anatomy of these
 sensorineural structures, their function, and the function of related
 biological systems, e.g., hearing ability, gaze stabilization, locomotor
 agility, and posture. The endosseous labyrinth is frequently used as a
 proxy to infer the performance of the hearing and vestibular systems,
 locomotor abilities, and ecology of extinct species. Such fossil
 inferences are often based on single specimens or even a single ear,
 representing an entire species. To address whether a single ear is
 representative of a population, we used geometric morphometrics to
 quantitatively assess the variation in shape and symmetry in a sample of
 endosseous labyrinths of wild turkeys Meleagris gallopavo of southern
 Ohio. We predicted that ears would be symmetrical both within individuals
 and across the sample; that labyrinth shape and size would covary; that
 labyrinth shape would vary with the size of the brain, measured as width
 of the endocranium at the cerebellum; and that labyrinths would be
 morphologically integrated. To test these predictions, we microCT-scanned
 the heads of 26 cadaveric turkeys, digitally segmented their endosseous
 labyrinths in Avizo, and assigned 15 manual landmarks and 20 sliding
 semilandmarks to each digital model. Following Procrustes alignment, we
 conducted an analysis of bilateral symmetry, a Procrustes regression
 analysis for allometry and other covariates including side and replicate,
 and analyses of global integration and modularity. Based on Procrustes
 distances, no individual’s left and right ears were clearly different from
 each other. When comparing the ears of different specimens, statistically
 clear differences in shape were found in only 66 of more than 1300
 contrasts. Moreover, effects of both directional and fluctuating asymmetry
 were very small—generally, two orders of magnitude smaller than the
 variance explained by individual variation. Statistical tests disagreed on
 whether these asymmetric effects crossed the threshold of significance,
 possibly due to non-isotropic variation among landmarks. Regardless,
 labyrinths appeared to primarily vary in shape symmetrically. Neither
 labyrinth size nor endocranial width was correlated with labyrinth shape,
 contrary to our expectations. Finally, labyrinths were found to be
 moderately integrated in a global sense, but four weakly separated
 modules—the three semicircular canals and cochlea—were recovered using a
 maximum-likelihood analysis. The results show that both fluctuating and
 directional asymmetry play a larger role in shape variation than
 expected—but nonetheless, endosseous labyrinths are symmetrical within
 individuals and at the level of the population, and their shape varies
 symmetrically. Thus, inferences about populations, and very possibly
 species, may be confidently made when only a single specimen, or even a
 single ear, is available for study.
description: Supplemental_Digital_Models S1ZIP file containing digital surface meshes of the inner-ear labyrinths of 26 cadaveric specimens of wild turkey. There are a total of 52 surface meshes in .PLY format, with each specimen represented by a left and a right labyrinth.