Abstract
To what extent can the mammalian visual system be shaped by visual behavior? Here we analyze the shape of the visual fields, the densities and distribution of cells in the retinal ganglion-cell layer and the organization of the visual projections in two species of facultative non-strictly subterranean rodents, Spalacopus cyanus and Ctenomys talarum, aiming to compare these traits with those of phylogenetically closely related species possessing contrasting diurnal/nocturnal visual habits. S. cyanus shows a definite zone of frontal binocular overlap and a corresponding area centralis, but a highly reduced amount of ipsilateral retinal projections. The situation in C. talarum is more extreme as it lacks of a fronto-ventral area of binocular superposition, has no recognizable area centralis and shows no ipsilateral retinal projections except to the suprachiasmatic nucleus. In both species, the extension of the monocular visual field and of the dorsal region of binocular overlap as well as the whole set of contralateral visual projections, appear well-developed. We conclude that these subterranean rodents exhibit, paradoxically, diurnal instead of nocturnal visual specializations, but at the same time suffer a specific regression of the anatomical substrate for stereopsis. We discuss these findings in light of the visual ecology of subterranean lifestyles.
Highlights
In the case of facultative non-strictly subterranean species, no study so far has considered whether and to what extent underground behavior affects different features of the visual system, such as visual field organization, retinal specializations and central projections
We show the results obtained for one representative animal for each species (Fig. 1a,b)
In both species we found that all major visual projections were recognizable; namely, the superior colliculus (SC), n. geniculatus lateralis pars dorsalis (GLd), n. geniculatus lateralis pars ventralis (GLv), suprachiasmatic nucleus (SCN), pretectal complex (PRT), and accessory optic system (AOS, e.g., medial terminal nucleus (MTN)) (Fig. 3)
Summary
In the case of facultative non-strictly subterranean species, no study so far has considered whether and to what extent underground behavior affects different features of the visual system, such as visual field organization, retinal specializations and central projections. Visual conditions faced by nocturnal animals may well greatly differ from those encountered by subterranean animals: on the one hand the burrowing environment seems not to be scotopic but almost lightless; on the other, facultative non-strictly subterranean species behave in a dual fashion between the underground ecotope and the photopic conditions around burrow entrances[1,2]. In this regard, interestingly most studies so far have shown that facultative non-strictly subterranean animals exhibit retinal features more close to diurnal than to nocturnal mammals (see discussion). We discuss these results in the context of the visual ecology of subterranean animals
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