Unusual topographic specializations of retinal ganglion cell density and spatial resolution in a cliff‐dwelling artiodactyl, the Nubian ibex (Capra nubiana)

Abstract

The Nubian ibex (Capra nubiana) occurs in information‐rich visual habitats including the edges of cliffs and escarpments. In addition to needing enhanced spatial resolution to find food and detect predators, enhanced visual sampling of the lower visual field would be advantageous for the control of locomotion in such precarious terrains. Using retinal wholemounts and stereology, we sought to measure how the ganglion cell density varies across the retina of the Nubian ibex to reveal which portions of its surroundings are sampled with high resolution. We estimated a total of ~1 million ganglion cells in the Nubian ibex retinal ganglion cell layer. Topographic variations of ganglion cell density reveal a temporal area, a horizontal streak, and a dorsotemporal extension, which are topographic retinal features also found in other artiodactyls. In contrast to savannah‐dwelling artiodactyls, the horizontal streak of the Nubian ibex appears loosely organized possibly reflecting a reduced predation risk in mountainous habitats. Estimates of spatial resolving power (~17 cycles/degree) for the temporal area would be reasonable to facilitate foraging in the frontal visual field. Embedded in the dorsotemporal extension, we also found an unusual dorsotemporal area not yet reported in any other mammal. Given its location and spatial resolving power (~6 cycles/degree), this specialization enhances visual sampling toward the lower visual field, which would be advantageous for visually guided locomotion. This study expands our understanding of the retinal organization in artiodactyls and offers insights on the importance of vision for the Nubian ibex ecology.

Ganglion cells and displaced amacrine cells density in the retina of the collared peccary (Pecari tajacu)

“…The dorsotemporal extension forms a dorsal arch of ganglion cells increase Some authors reported that this specialization is closely related with demands required by the inferior visual field such as spotting predators, orientation during foraging or ambulation [32]. It was also found in other artiodactyl living in mountains and valleys [42], which points to a shared ecological niche.…”

“…We used morphological criteria to differentiate ganglion cells from neuroglial cells and displaced amacrine cells according to Hughes [41] The same criteria was used in other studies for artiodactyls species [32,42,43] and other species [17,40,44]. Briefly, ganglion cells have a large soma with an oval pale nucleus and a prominent nucleolus with abundant Nissl substance visible in the cytoplasm.…”

“…These visual specializations (visual streak and area temporalis) were described in many artiodactyls such as giraffes [32], goats [41], cattle, pigs, and sheep [32,33], animals that inhabit a variety of environments, from the savannah to dense rainforests. Whereas the dorsal extension was found in artiodactyls such as sheep [35], giraffes [32], and the Nubian ibex [42].…”

A Review of Cervidae Visual Ecology

“…However, medium-and large-sized cervid species (Table 1) might not have sufficient visual resolution for nearby ground objects due to an increased ground-to-eye distance [31]. In both the domestic sheep (Ovis aries) and the Nubian ibex (Capra nubiana), the dorsotemporal extension of their M-cone topography corresponds with an increase in ganglion cell density at the temporal region of the visual streak and the dorsotemporal region [78,79]. Increased ganglion cell and M-cone density near the temporal and dorsotemporal regions of midand large-size cervids might improve the visual resolution in the frontal and lower visual fields.…”

Retinal Ganglion Cell Topography and Spatial Resolution in Three Indian Pteropodid Bats

“…Retinal ganglion cell (RGC) density maps have been used to estimate visual spatial resolution of a wide range of terrestrial vertebrates including birds (e.g., Wathey and Pettigrew, 1989;Hart, 2002;Coimbra et al, 2012;Mitkus et al, 2014;Lisney et al, 2020), reptiles (e.g., Hart et al, 2012;Hauzman et al, 2014), and mammals (e.g., Coimbra et al, 2013;Coimbra et al, 2017a;Coimbra et al, 2019). Spatial resolution has been estimated from RGC density in more than 25 bat species including 12 pteropodids (online Suppl.…”