21 December, 2017
The inner ear is a rather beautiful structure. It is formed of two main parts, the cochlear canal (the part responsible for hearing) and three semicircular canals, which detect movements of the head and are involved in our sense of balance. Given the inner ear’s similarity to the network of passages in which Theseus slew the Minotaur in Greek myth, it’s easy to see why these convoluted structures are usually referred to as the ‘labyrinth’.
The semicircular canals of birds have long been known to be slender and long compared with those of reptiles and most mammals. The enlargement of the semicircular canals of birds is generally accepted to be a result of the need for fine balance control in flight. The canals also vary strongly in shape between different bird species, leading scientists to suppose that these differences relate to how a bird moves, or to other aspects of behaviour. Today, micro X-ray computed tomography (µCT) can be used to investigate labyrinth shape in any animal, and also in extinct fossil animals. It has become quite normal to attempt to infer the agility or manoeuvrability of a fossil animal from the shape of its labyrinth in these investigations, but is this justified?
A paper in the current issue of the Journal of Anatomy describes how staff and a student at the universities of Oxford and Birmingham, and Stig Walsh at National Museums Scotland, tested this assumption. The team used µCT to reconstruct the labyrinths of 64 bird skulls held in the collections of the National Museums Scotland, and also the Natural History Museum in London. The species were chosen based on their flying ability, including highly agile fliers such as swifts at one end of the spectrum, and weak fliers and flightless birds at the other end. We examined the range of labyrinth shape using a technique known as 3D geometric morphometrics, which analyses labyrinth shape as a series of mathematical coordinates.
It turns out that both large and more agile birds possess long semicircular canals, but none of the variability in labyrinth shape appears to be related to how a bird flies. We instead found that labyrinth shape is actually strongly related to the size of the bird, suggesting that most of the shape variation is a way of fitting a large labyrinth and large eyes into a small skull. Complex labyrinth shapes found in fossil birds probably have more to do with changes in body size over evolutionary time than the flying ability of their owners.