Unusual cone and rod properties in subterranean African mole‐rats (Rodentia, Bathyergidae)

We have determined the presence of spectral cone types, and the population densities of cones and rods, in subterranean mole‐rats of the rodent family Bathyergidae, for which light and vision seems of little importance. Most mammals have two spectral cone types, a majority of middle‐ to long‐wave‐sensitive (L‐) cones, and a minority of short‐wave‐sensitive (S‐)cones. We were interested to see whether the subterranean bathyergids show the same pattern. In three species, Ansell's mole‐rat Cryptomys anselli, the giant mole‐rat Cryptomys mechowi and the naked mole‐rat Heterocephalus glaber, spectral cone types and rods were assessed immunocytochemically with opsin‐specific antibodies. All three species had rod‐dominated retinae but possessed significant cone populations. A quantitative assessment in C. anselli and C. mechowi revealed surprisingly low photoreceptor densities of 100 000–150 000/mm2, and high cone proportions, ≈ 10% (8000–15 000/mm2). In all three species, the vast majority of the cones were strongly S‐opsin‐immunoreactive; L‐opsin immunoreactivity was much fainter. In C. anselli, ≈ 20% of the cones showed exclusive S‐opsin label, ≈ 10% exclusive L‐opsin label and ≈ 70% strong S‐opsin and faint L‐opsin double label (potential dual‐pigment cones). This is the first observation in any mammal of an S‐opsin dominance and low levels of L‐opsin across the entire retina. It contrasts starkly with the situation in the muroid blind mole‐rat Spalax ehrenbergi, which has been reported to possess L‐opsin but no S‐opsin. Evidently, within rodents an adaptation to subterranean life is compatible with very different spectral cone properties.