Evolution of the Avian Vision

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Book Chapter

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This chapter provides an overview of the properties of the avian visual system and what selective pressures are thought to have operated on its evolution. The evolutionary perspective is invaluable in approaching the questions of structure and function in avian vision. Such a perspective provides insights on the capabilities and limitations of avian vision, and how different species utilize information to navigate, forage, mate, et cetera. A brief review of amniote (reptiles, birds, and mammals) evolution is outlined, with descriptions of the many similarities in brain organization and basic visual pathways common to all amniotes. Such similarities include: the importance of the tectum as a visual structure for maintaining topographic representation of sensory space, and the possession of dorsal thalamic zones in both birds and mammals which receive ascending visual input from the retina either directly (lemnothalamic pathways) or indirectly (via the tectum, as in the collothalamic pathways). Consideration of these similarities, however, must coincide with appreciation for obvious differences in brain structure between birds and mammals (i.e. dorsal ventricular ridge versus the six-layered isocortex of mammals). The chapter includes general ideas regarding brain evolution, such as forebrain expansion and changes in the ratio of midbrain and forebrain structures.

Dinosaur and avian evolution are considered, along with a discussion of the two major competing hypotheses regarding avian evolution. The two major competing hypotheses are the "basal archosaur hypothesis" (which proposes that the first birds descended directly from ancestral reptiles about 230 million years ago) and the "theropod dinosaur hypothesis" (which advocates a much later entry of birds, with derivation from dinosaurs some 100 million years after the time proposed by the basal archosaur hypothesis). Debates regarding the origin of flight (arboreal vs cursorial theories) and the metabolism of the dinosaurs from which birds arose (ecto- vs endo-thermy) are also mentioned.

The avian retina contains interesting differences from that of primates. Such variations include retinal morphology (e.g., avian double cones), more numerous photopigments, more horizontal and amacrine cells, richer intraretinal connections, and more complex ganglion cell response properties. Comparisons of avian and primate retinae are discussed in terms of their differing evolutionary paths from common amniotic ancestry (e.g., periods of nocturnality during the reign of dinosaurs; Bottleneck theory). Visual processing beyond the retina is discussed with comparisons between the collo- and lemno-thalamic visual pathways in amniote brains. Issues related to lateral versus frontal eyes and binocular vision are addressed. The development of flight undoubtedly had an important effect on the transition from ancient reptile to avian brains. Useful clues regarding this are derived from comparisons of early flying reptiles like pterosaurs and the brains of modern birds in terms of optic lobe, cerebellar, and forebrain developments.

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Citation / Publisher Attribution

Evolution of the Avian Vision, in R. Cook (Ed.), Avian Visual Cognition