Several different types of rodents are found among the specialized mammals exploiting the subterranean ecological niche. Tunnels dug by them can often be detected through the mounds of earth thrown up during the burrowing. Striking anatomical and behavioral adaptations are associated with their fossorial mode of life (Ellerman, 1956; Pearson, 1959). The mole rats of the genus Spalax are an extreme example of such rodents; they are largely confined to a subterranean existence. Their adaptations include the great reduction in the development of their eyes, which are not visible externally.