Semicircular canals and agility: the influence of size and shape measures

Abstract

The semicircular canals of the inner ear sense angular accelerations and decelerations of the head and enable co-ordination of posture and body movement, as well as visual stability. Differences of agility and spatial sensitivity among species have been linked to interspecific differences in the relative size of the canals, particularly the radius of curvature (R) and the ratio of the canal plane area to streamline length (P ⁄ L). Here we investigate the scaling relationships of these two size variables and also out-of-plane torsion in the three semicircular canals (anterior, posterior and lateral), in order to assess which is more closely correlated with body size and locomotor agility. Measurements were computed from 3D landmarks taken from magnetic resonance images of a diverse sample of placental mammals encompassing 16 eutherian orders. Body masses were collected from the literature and an agility score was assigned to each species. The R and P ⁄ L of all three semicircular canals were found to have highly significant positive correlations with each other and no statistical difference was found between the slope of 2P ⁄ L against R and 1. This indicated that, contrary to initial hypotheses, there is little difference between 2P ⁄ L and R as measures of semicircular canal size. A measure of the in-plane circularity of the canal was obtained by dividing 2P ⁄ L by R and out-of-plane torsion was measured as angular deviation from a plane of best fit. It was predicted that deviations from in-plane and out-of-plane circularity would increase at small body size due to the constraints of fitting a proportionately larger canal into a smaller petrous bone. However, neither measurement was found to have a significant correlation with body mass, indicating that deviations from circularity (both in-plane and out-of-plane) are not sufficient to alter P ⁄ L to an extent that would impact the sensitivity of the canals. 2P ⁄ L and R were both shown to be significantly correlated with locomotor agility. The posterior canal was the least correlated with agility, suggesting that it may be generally less closely aligned to the direction of movement than the anterior canal. Of the three canals, the lateral canal was the most highly correlated with agility. In particular, it could be used to distinguish between species that move in a largely 2D environment and those that locomote in 3D space (aerial, arboreal and aquatic species). This complements previous work suggesting that the lateral canal primarily commands navigation, whereas the vertical canals control reflex adjustments. It was also found that 2P ⁄ L is substantially better correlated with agility than is R in the lateral canal. This result is intriguing given the above finding that there is no statistical difference between 2P ⁄ L and R, and requires further investigation.

Comparative study of notoungulate (Placentalia, Mammalia) bony labyrinths and new phylogenetically informative inner ear characters

“…The cochlea functions primarily in hearing, whereas the other structures are associated with spatial orientation and balance. The semicircular ducts detect angular acceleration of the head, and aid in stabilizing vision during motion (summarized by Spoor, 2003;Spoor et al 2007;Cox & Jeffery, 2010).…”

“…Spoor et al 2007;Cox & Jeffery, 2010), and thus can be used to infer agility capabilities in extinct mammals (e.g. Silcox et al 2009).…”

Digital reconstruction of the inner ear of Leptictidium auderiense (Leptictida, Mammalia) and North American leptictids reveals new insight into leptictidan locomotor agility

“…Leptictidium is derived from the leptictidan groundplan in having proportionally larger semicircular canals, as evidenced from the morphometric analysis, and its lateral semicircular canal is proportionally larger (relative to other canals) than in the two leptictid species under study (Table 1). This suggests adaptation to faster horizontal head movements and higher agility (e.g., Schmelzle et al 2007;Cox and Jeffery 2010; see discussion below), and may reflect the different hypothesized locomotor adaptations in the postcranial skeleton of European and North American leptictidans. Maier et al (1986) first suggested that Leptictidium was a bipedal runner, not a saltator, because its unfused tibia and fibula and the weak sacroiliac joint (involving only one sacral vertebra) could cause weakness or instability of these joints, which would in turn be inconsistent with saltation.…”

“…As Macroscelidea were not included in the original analysis by Spoor et al (2007), we virtually reconstructed the bony labyrinth of an extant specimen of Macroscelides proboscideus (SZ 7397), which has a mean body mass ranging from 38 to 70 g (Silva and Downing 1995); we used the approximate average of 54 g for comparison (Table 1). We also included inner ear measurements by Cox and Jeffery (2010) of the largest living macroscelidean, Rhynchocyon cirnei, in our study.…”

“…In Macroscelides the mean radius agility score is 4.4 and the lateral semicircular canal reveals the highest value (4.9), followed by the anterior semicircular canal (4.4) and the posterior semicircular canal (4.0). Rhynchocyon has a mean radius agility score of 4 based on field observations, but a score of 5.3 based on the given measurements in Cox and Jeffery (2010) and the equations by Silcox et al (2009). The lateral semicircular canal also yields an agility score of 5.3, whereas the anterior and posterior semicircular canals have smaller values (4.7 and 4.6, respectively).…”