Relative Size of Auditory Pathways in Symmetrically and Asymmetrically Eared Owls

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The eyes of vertebrates show adaptations to the visual environments in which they evolve. For example, eye shape is associated with activity pattern, while retinal topography is related to the symmetry or ‘openness’ of the habitat of a species. Although these relationships are well documented in many vertebrates including birds, the extent to which they hold true for species within the same avian order is not well understood. Owls (Strigiformes) represent an ideal group for the study of interspecific variation in the avian visual system because they are one of very few avian orders to contain species that vary in both activity pattern and habitat preference. Here, we examined interspecific variation in eye shape and retinal topography in nine species of owl. Eye shape (the ratio of corneal diameter to eye axial length) differed among species, with nocturnal species having relatively larger corneal diameters than diurnal species. All the owl species have an area of high retinal ganglion cell (RGC) density in the temporal retina and a visual streak of increased cell density extending across the central retina from temporal to nasal. However, the organization and degree of elongation of the visual streak varied considerably among species and this variation was quantified using H:V ratios. Species that live in open habitats and/or that are more diurnally active have well-defined, elongated visual streaks and high H:V ratios (3.88–2.33). In contrast, most nocturnal and/or forest-dwelling owls have a poorly defined visual streak, a more radially symmetrical arrangement of RGCs and lower H:V ratios (1.77–1.27). The results of a hierarchical cluster analysis indicate that the apparent interspecific variation is associated with activity pattern and habitat as opposed to the phylogenetic relationships among species. In seven species, the presence of a fovea was confirmed and it is suggested that all strigid owls may possess a fovea, whereas the tytonid barn owl (Tyto alba) does not. A size-frequency analysis of cell soma area indicates that a number of different RGC classes are represented in owls, including a population of large RGCs (cell soma area >150 µm²) that resemble the giant RGCs reported in other vertebrates. In conclusion, eye shape and retinal topography in owls vary among species and this variation is associated with different activity patterns and habitat preferences, thereby supporting similar observations in other vertebrates.

Section: Cell Soma Areamentioning

confidence: 99%

Section: Retinal Topography and Activity Pattern And Habitatmentioning

confidence: 99%

Eye Shape and Retinal Topography in Owls (Aves: Strigiformes)

Lisney¹,

Iwaniuk

Bandet

et al. 2012

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“…On the basis of Bravo and Pettigrew's [1981] results, we expect differences in the relative size of the thalamofugal pathway, between diurnal and nocturnal species and/or between strigid and tytonid owls. Finally, we have recently shown that asymmetrically eared owls have enlarged auditory pathways when compared to symmetrically eared ones [Gutiérrez-Ibáñez et al, 2011]. Several studies in mammals [Eisenberg, 1981;Baron et al, 1996;Barton, 1998;Catania, 2005] have shown that trade-offs can occur between different sensory systems, and species that rely heavily on one sensory modality (with a corresponding enlargement of associated brain areas) have relatively smaller brain regions dedicated to other sensory modalities.…”

Section: Introductionmentioning

confidence: 83%

Comparative Study of Visual Pathways in Owls (Aves: Strigiformes)

Gutiérrez-Ibáñez

Iwaniuk²,

Lisney³

et al. 2012

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Although they are usually regarded as nocturnal, owls exhibit a wide range of activity patterns, from strictly nocturnal, to crepuscular or cathemeral, to diurnal. Several studies have shown that these differences in the activity pattern are reflected in differences in eye morphology and retinal organization. Despite the evidence that differences in activity pattern among owl species are reflected in the peripheral visual system, there has been no attempt to correlate these differences with changes in the visual regions in the brain. In this study, we compare the relative size of nuclei in the main visual pathways in nine species of owl that exhibit a wide range of activity patterns. We found marked differences in the relative size of all visual structures among the species studied, both in the tectofugal and the thalamofugal pathway, as well in other retinorecipient nuclei, including the nucleus lentiformis mesencephali, the nucleus of the basal optic root and the nucleus geniculatus lateralis, pars ventralis. We show that the barn owl (Tyto alba), a species widely used in the study of the integration of visual and auditory processing, has reduced visual pathways compared to strigid owls. Our results also suggest there could be a trade-off between the relative size of visual pathways and auditory pathways, similar to that reported in mammals. Finally, our results show that although there is no relationship between activity pattern and the relative size of either the tectofugal or the thalamofugal pathway, there is a positive correlation between the relative size of both visual pathways and the relative number of cells in the retinal ganglion layer.

“…Beach et al [1987] suggested that formaldehyde can only penetrate 1-2 mm in tissue blocks, but formaldehyde can, in fact, penetrate much deeper than that, provided that the specimens are left in the fixative for an extended period of time [Fox et al, 1985]. Indeed, immersion fixation can yield high-quality specimens for measuring brain region volume, cytoarchitecture and immunohistochemistry [Iwaniuk and Wylie, 2007;Iwaniuk et al, 2009;Gutiérrez-Ibáñez et al, 2011;Corfield et al, 2012] even for large brains [Hof et al, 2000;Manger et al, 2003;Wang et al, 2008;Raghanti et al, 2009].…”

Section: Brain Specimens and Histologymentioning

confidence: 99%

“…The proportional sizes of the brain regions were also examined through the use of hierarchical cluster analysis [Rehkamper et al, 2003;Iwaniuk and Hurd, 2005;Gutiérrez-Ibáñez et al, 2011]. The proportional sizes of the four main brain regions, telencephalon, optic lobes, cerebellum and brainstem, or the telencephalic regions (see above) were included.…”

Section: Statistical Analysesmentioning

confidence: 99%

Brain Size and Morphology of the Brood-Parasitic and Cerophagous Honeyguides (Aves: Piciformes)

Corfield¹,

Birkhead²,

Spottiswoode³

et al. 2013

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Honeyguides (Indicatoridae, Piciformes) are unique among birds in several respects. All subsist primarily on wax, are obligatory brood parasites and one species engages in ‘guiding' behavior in which it leads human honey hunters to bees' nests. This unique life history has likely shaped the evolution of their brain size and morphology. Here, we test that hypothesis using comparative data on relative brain and brain region size of honeyguides and their relatives: woodpeckers, barbets and toucans. Honeyguides have significantly smaller relative brain volumes than all other piciform taxa. Volumetric measurements of the brain indicate that honeyguides have a significantly larger cerebellum and hippocampal formation (HF) than woodpeckers, the sister clade of the honeyguides, although the HF enlargement was not significant across all of our analyses. Cluster analyses also revealed that the overall composition of the brain and telencephalon differs greatly between honeyguides and woodpeckers. The relatively smaller brains of the honeyguides may be a consequence of brood parasitism and cerophagy (‘wax eating'), both of which place energetic constraints on brain development and maintenance. The inconclusive results of our analyses of relative HF volume highlight some of the problems associated with comparative studies of the HF that require further study.