Light propagation in burrows of subterranean rodents: tunnel system architecture but not photoreceptor sensitivity limits light sensation range.

About 300 species of mammals have adapted to the dark underground ecotope. Despite a long history of underground existence, many strictly subterranean species have retained structurally normal eyes possessing the capability of image-forming vision. Moreover, their retinae often feature high cone proportions, an indication of conserved photopic (daylight) vision. Although it has been suggested that low acuity vision plays an important role in predator avoidance, not a single attempt to measure light conditions in burrows has been made so far. Here, we report the first measurements of light propagation in an illuminated artificial tunnel and in experimentally opened burrows of Ansell's mole-rat, F ukomys anselli in its natural habitat. Only about 0.2-2.5% of the ambient visible light entered the opened burrow. Light intensity attenuated quickly and reached mesopic light levels (at which both cones and rods contribute to vision) within a few centimetres from the burrow opening; scotopic light levels (at which only rods operate) were estimated to be reached at one to a few metres from the opening. Thus, although cones may hypothetically contribute to vision for up to a few metres, they play an indispensable role only in the immediate vicinity of a breach, where rods are fully saturated. Rod-mediated light sensation in straight tunnels seems to be possible over distances much longer than 100 m, implying that it is the burrow architecture (tortuosity and branching) what limits light sensation under natural conditions. These findings clearly show that light propagating within a breached burrow may serve as a reliable cue providing information about the site of potential predation risk. Both rods and cones contribute to this signalling. The fact that blue light propagated much less efficiently than longer wavelength light suggests that the short-wave-sensitive opsin dominance in the African mole-rats represents a non-adaptive feature that seems to be associated with arrested cone development. [ABSTRACT FROM AUTHOR]