(A) Chlamydosaurus kingii, the frilled dragon, posturing with erect frill. (B) Chlamydosaurus with frill at rest. Each lobe of the frill has pleated into three folds. (C) During development of the cerebral cortex, grey matter grows more than white matter, leading to the formation of ridges and grooves called gyri and sulci. This is an example of frustrated growth at a surface. (D) The formation of the folds in the frill of Chlamydosaurus is an example of frustrated growth at a boundary: the frill is fixed at one edge by its attachment to the neck, so it buckles and forms folds as it grows. (E) Buckling due to boundary frustration illustrated by a physical analog experiment. Panels A, B and E are from Montandon et al., 2019.