Testing the Terrain Hypothesis: Canada Geese See Their World Laterally and Obliquely

Abstract

The distribution of ganglion cells in the retina determines the specific regions of the visual field with high visual acuity, and thus reflects the perception of a species’ visual environment. The terrain hypothesis proposes that animals living in open areas should have a horizontal visual streak across the retina with high ganglion cell density to increase visual acuity along the horizon. We tested this hypothesis in Canada geese <i>(Branta canadensis)</i> by assessing retinal topography, visual field configuration, and scanning behavior. We found that geese have an oblique rather than a horizontal visual streak across the retina: from a dorsal-nasal to a ventral-temporal position. Geese showed narrow blind areas, which increased the range of their lateral vision, and a relatively large degree of eye movement. Canada geese have relatively wide binocular fields and can see their bill tips. Goose head movement rates were low compared to species with a single fovea, and head movement rates increased in visually obstructed habitats. Canada geese have high acuity across their retina, which would allow them to simultaneously scan the ground and the sky when the head is up and parallel to the ground, as well as align the visual streak with the horizon when the head is tilted downwards. Their visual streak, along with their large eye size, may reduce the need for large amplitude head movements during vigilance bouts in visually unobstructed habitats. Overall, the visual system of geese combines features related to the detection of predators/conspecifics in open areas (visual streak, large lateral field, reduced head movements) as well as visual specializations that would allow for monitoring both the ground and sky (oblique streak) and for extracting and handling of food items (wide binocular fields, visualization of the bill tip).

Oblique color vision in an open-habitat bird: spectral sensitivity, photoreceptor distribution and behavioral implications

“…The overall distribution of single and double cone photoreceptors overlapped with the orientation of the visual streak found at the retinal ganglion cell layer (Fernández-Juricic et al, 2011). This matching was more pronounced for the MWS and LWS single cones and the double cones, but less pronounced for the SWS and VS single cones (Fig.4).…”

“…To determine the degree to which our photoreceptor density data fit a horizontal or oblique orientation of the visual streak, we first measured the width of the visual streak found at the retinal ganglion cell layer (Fernández-Juricic et al, 2011). Using data from five individuals (Fernández-Juricic et al, 2011), we established that the proportional width of the visual streak at the ganglion cell layer was The Journal of Experimental Biology 215 (19) 13.84% of the whole retina.…”

“…Using data from five individuals (Fernández-Juricic et al, 2011), we established that the proportional width of the visual streak at the ganglion cell layer was The Journal of Experimental Biology 215 (19) 13.84% of the whole retina. We then used a layout of five sampling bands along the retina, each with the same proportional width as the visual streak: a central band, and two bands above and two below the central one.…”

“…We then used a layout of five sampling bands along the retina, each with the same proportional width as the visual streak: a central band, and two bands above and two below the central one. These bands had two orientations: horizontal (parallel to the ground) or oblique (following the inclination of the visual streak at the retinal ganglion cell layer: 30deg running from dorso-nasal to ventro-temporal) (Fernández-Juricic et al, 2011). We used general mixed linear models with the density of each single and double cone type as dependent variables.…”

“…) of retinal ganglion cells in each band [data from Fernández-Juricic et al (Fernández-Juricic et al, 2011)] and calculated the ratios of cone photoreceptors (single and double) to retinal ganglion cells. We acknowledge that these convergence ratios are coarse estimates as photoreceptor and ganglion cell density values came from different individuals; nevertheless, these ratios provide the first accounts of the variation in visual resolution across the goose retina.…”