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Avian cerebellar flocculus size and flying ability.



From: Ben Creisler
bcreisler@gmail.com


New in PLoS ONE:


Stig A. Walsh, Andrew N. Iwaniuk, Monja A. Knoll, Estelle Bourdon,
Paul M. Barrett, Angela C. Milner, Robert L. Nudds, Richard L. Abel &
Patricia Dello Sterpaio (2013)
Avian Cerebellar Floccular Fossa Size Is Not a Proxy for Flying
Ability in Birds.
PLoS ONE 8(6): e67176.
doi:10.1371/journal.pone.0067176
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0067176

Extinct animal behavior has often been inferred from qualitative
assessments of relative brain region size in fossil endocranial casts.
For instance, flight capability in pterosaurs and early birds has been
inferred from the relative size of the cerebellar flocculus, which in
life protrudes from the lateral surface of the cerebellum. A primary
role of the flocculus is to integrate sensory information about head
rotation and translation to stabilize visual gaze via the
vestibulo-occular reflex (VOR). Because gaze stabilization is a
critical aspect of flight, some authors have suggested that the
flocculus is enlarged in flying species. Whether this can be further
extended to a floccular expansion in highly maneuverable flying
species or floccular reduction in flightless species is unknown. Here,
we used micro computed-tomography to reconstruct “virtual” endocranial
casts of 60 extant bird species, to extract the same level of
anatomical information offered by fossils. Volumes of the floccular
fossa and entire brain cavity were measured and these values
correlated with four indices of flying behavior. Although a weak
positive relationship was found between floccular fossa size and
brachial index, no significant relationship was found between
floccular fossa size and any other flight mode classification. These
findings could be the result of the bony endocranium inaccurately
reflecting the size of the neural flocculus, but might also reflect
the importance of the flocculus for all modes of locomotion in birds.
We therefore conclude that the relative size of the flocculus of
endocranial casts is an unreliable predictor of locomotor behavior in
extinct birds, and probably also pterosaurs and non-avian dinosaurs.