The form and function of avian rictal bristles

Delaunay, Mariane G (2022) The form and function of avian rictal bristles. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Although mechanoreception is present throughout the animal kingdom, it is still relatively under-studied and poorly understood, especially in nocturnal birds. A particular type of facial feather, the rictal bristles, are thought to carry out a similar tactile function to mammalian whiskers, of which they superficially resemble. If they do, such a function could enhance foraging behaviour and facilitate obstacle avoidance, especially in dark, complex habitats. However, as rictal bristles are the least described of any feather, little is known about them. Therefore, this thesis characterises rictal bristle form and function by describing: i) bristle morphology and follicle anatomy, ii) the development of rictal bristles, and iii) the associated mechanosensory brain areas. It will also explore: iv) rictal bristle evolution and v) function. Results in this thesis demonstrated that rictal bristle morphology and the presence of mechanoreceptors around the follicle varied between species. Specifically, diurnal species did not have mechanoreceptors around their bristle follicles and had shorter bristles. Associated mechanosensory brain areas also varied between species, but there was no clear association between the neuroanatomy, rictal bristle morphology or foraging traits. Rictal bristles were absent in two species of altricial hatchlings, and only emerged after their eyes opened. Stimulation of the rictal region in these chicks led to behavioural feeding responses, especially coinciding with when chicks started to feed independently. Rictal bristle evolution underwent multiple events of disappearance and gain during avian evolution, and therefore, the presence and morphology of the rictal bristles also varied between orders, families and genera. Short rictal bristles with barbs at the base were likely to be present in the common ancestor of the phylogeny (108 mya). Rictal bristle presence and length were associated with nocturnality and foraging methods, and diet is also likely to be associated with rictal bristle length. Consequently, this thesis suggests that, in adult birds, rictal bristles are likely to act as facial tactile sensors in species that forage in low-light conditions in complex habitats. Rictal bristles are may, therefore, play a role in collision avoidance, foraging and eye protection. Species foraging in the daytime might have rictal bristles with a reduced tactile function. However, identifying rictal bristle function is challenging and demands further investigation. This thesis provides the first comparative description of avian rictal bristle form and function, and is an important foundation for further investigation of the sense of touch in birds.